A contribution to the understanding of the oxydation mechanism in mineral oils of low aromatic content

A CONTRIBUTION TO THE

UNDERSTANDING OF THE OXIDATION

MECHANISM IN MINERAL OILS OF LOW

AROMATIC CONTENT

P k O K F S C H k I F T

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT, KRACHTENS ARTIKEL 2 VAN HET KONINKLIJK BESLUIT VAN 16 SEPTEMBER 1927, STAATSBLAD NO. 310. OP GEZAG VAN DE RECTOR MAGNIFICUS DR. O. BOTTEMA, HOOGLERAAR IN DE AFDELING DER ALGEMENE WETENSCHAPPEN. VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP

WOENSDAG H NOVEMBER 1956 DES NAMIDDAGS TE 2 UUR

DOOR

WILLRM JOHAN TAAT

Dit proefschrift is goedgekeurd door de P r o m o t o r e n : P r o f . d r . i r . H . I . W a t e r m a n

^n

Van deze plaats wil ik gaarne mijn dank betuigen aan a l -len, die aan de totstandkoming van dit proefschrift hebben bijgedragen, in het bijzonder aan de Heer D. J . W. Kreulen voor de wijze waarop hij mij in de gelegenheid stelde de r e s u l t a t e n van het e e r s t e gedeelte van dit onderzoek, dat in zijn laboratoriumi werd v e r r i c h t , hierin te p u b l i c e r e n .

Introduction Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Tables I Graphs I I II III IV V VI V I I VIII -- XX - XX C O N T E N T S • - Survey of l i t e r a t u r e on the p r o p e r t i e s of t r a n s f o r m e r oils and their behav-iour in p r a c t i c e

Survey of l i t e r a t u r e on the relation between c h e m i c a l s t r u c t u r e and c h a r -a c t e r i s t i c o x i d -a t i o n - p r o p e r t i e s of hydrocarbons in a r e l a t i v e l y pure s t a t e and of n a r r o w - r a n g e fractions of m i n e r a l oils

Résumé of the work, d e s c r i b e d in Chapters I and II, and some general conclusions drawn from it

E x p e r i m e n t a l investigations into the m e c h a n is m of oxidation of a white oil with low a r o m a t i c content

Mathematical evaluation of the oxi-dation e x p e r i m e n t s on a white oil and on its m i x t u r e s with oxidized o i l s . Short t i m e s of oxidation

Mathematical evaluation of the oxi-dation e x p e r i m e n t s on a mixture of white oil and oxidized white oil. The attainment of s t a t i o n a r y concentra-tions of p r i m a r y hydroperoxides by the addition of higher p e r c e n t a g e s of oxidized o i l s . The influence of syn-thetic inhibitors

The i n c r e a s e of the concentration of carbonyl compounds during the p r o -c e s s of oxidation of a white oil and of its m i x t u r e s with oxidized white oil S u m m a r y . Conclusions drawn from the investigation . 9 11 19 40 50 60 75 85 87 93 95 105

I N T R O D U C T I O N

In the course of the last 25 y e a r s quite a number of in-vestigations have been published on the oxidation of min-e r a l oils, p a r t i c u l a r l y of t r a n s f o r m min-e r o i l s . In spitmin-e, how-ever, of all these investigations, no g e n e r a l theory on the m ec h a n i sm of the chemical r e a c t i o n s in question has been a g r e e d upon yet. One of the consequences of this state of affairs is that no uniform and efficient method for the evaluation of t r a n s f o r m e r oils has r e s u l t e d so f a r .

F o r instance, in 1954 the Technical Commission Nr 10 of the International E l e c t r o t e c h n i c a l Commission proposed a test for the evaluation of t r a n s f o r m e r oils whereby the conditions to which these oils a r e submitted in p r a c t i c a l use a r e seemingly imitated. However Kreulen and Taat have shown that this test can be no m o r e adequate than other national t e s t s of its kind, accepted as s t a n d a r d s in v a r i o u s countries, since it is not based on fundamen-tal knowledge of the r e a c t i o n mechanism in question. The available l i t e r a t u r e on the mechanism of oxidation of hydrocarbons r e v e a l s a g r e a t number of contradictory v i e w s . Some of the investigators hold the idea that the first stage of the oxidation p r o c e s s is the formation of p e r o x i d e s . These a r e believed to f o r m r a d i c a l s which in t h e i r t u r n a r e supposed to be the i n i t i a t o r s of chain r e -actions, leading to the consumption of m o r e oxygen and to the generation of such oxidation products as ketones, aldehydes, acids and a l c o h o l s . Other investigators a r e of the opinion that the peroxides t h e m s e l v e s a r e products of chain r e a c t i o n s . Still o t h e r s believe that the peroxides a r e not the r e a l i n i t i a t o r s of the chain r e a c t i o n s r e f e r r e d to above.

Likewise there is much divergence between various e x -planations of the action of synthetic and n a t u r a l inhibitors, and the s a m e goes for the relation between the formation of acids and the generation of insoluble r e s i n l i k e c o m -pounds (technically called sludge) during the p r o c e s s of oxidation.

b e t t e r understanding of the m e c h an i s m of oxidation of m i -n e r a l o i l s . Worki-ng with a-n oil of the „white o i l " or ,,pa" raffinum liquidum" type, which is practically free from a r o m a t i c hydrocarbons, c e r t a i n r e g u l a r i t i e s were found, and these can be explained by p r e s u m i n g a mechanism which has much in common with the one proposed by Wi-baut and Strang for the oxidation of a number of s a t u r a t e d octane i s o m e r s and naphthenes with eight carbon a t o m s . Undoubtedly substituted a r o m a t i c s and condensed a r o matics play an important p a r t during the oxidation p r o c e s s of t r a n s f o r m e r and lubricating o i l s . In the l i t e r a -t u r e deal-t wi-th in -the f i r s -t -two c h a p -t e r s -t h e r e a r e indic-ations of the probability of a r o m a t i c hydrocarbons being the cause of complications, and this is the r e a s o n why our own investigation was c a r r i e d out with m i n e r a l oil of low a r o m a t i c content. F u r t h e r m o r e it is highly probable that the oxidation p r o c e s s s t a r t s in aliphatic molecules o r in aliphatic s i d e - c h a i n s of substituted a r o m a t i c s .

The reviews of l i t e r a t u r e a r e mostly r e s t r i c t e d to those publications dealing with t r a n s f o r m e r o i l s , mainly b e -cause these a r e the oils the behaviour of which during oxidation has been the object of the m o s t detailed study. It is the conviction of the author that the investigation of the role of different a r o m a t i c compounds, such as n o r mally found in oils used for the p r e p a r a t i o n of t r a n s f o r -m e r - and lubricating oils, will be greatly facilitated by a complete understanding of the oxidation of aliphatic and naphtenic h y d r o c a r b o n s .

CHAPTER I

SURVEY O F LITERATURE ON THE PROPERTIES O F TRANSFORMER OILS AND THEIR BEHAVIOUR IN

PRACTICE

1.1. General properties of transformer oils

The function of a m i n e r a l oil used in a t r a n s f o r m e r is of a dual n a t u r e :

1, as a d i e l e c t r i c medium and insulator; 2. as a medium for t r a n s p o r t a t i o n of the heat

generated in the c o n d u c t o r s .

The r e q u i r e m e n t s to be met by a t r a n s f o r m e r oil a r e of c o u r s e closely r e l a t e d to this dual function. They have been discussed in detail by Wood-Mallock (1). Briefly stated, the oil should have:

1. high e l e c t r i c r e s i s t i v i t y and d i e l e c t r i 9 strength;

2. a high specific inductive capacity; 3. a low power-factor;

4. high heat-conductivity and specific heat; 5. a low solidification-point;

6. a low volatility; 7. a low viscosity;

8. a low t o l e r a n c e for water;

9. high r e s i s t a n c e to chemical and physical ageing, especially under conditions fa-vouring oxidation and hydrolysis;

10. considerable i n e r t n e s s towards the other m a t e r i a l s used in the construction of t r a n s f o r m e r s , such as copper, iron and c e l l u l o s e .

Some of the above mentioned r e q u i r e m e n t s , like those specified under 1, 2 and 3, will pr a ct ic al ly limit the choice of the medium to m i n e r a l o i l s , notwithstanding the fact that some other p r o p e r t i e s of m i n e r a l oils do not ideally meet c e r t a i n other r e q u i r e m e n t s — for instance number 9. In other r e s p e c t s too one has to find a comp r o m i s e if the choice is to be limited to m i n e r a l oils -so for instance r e q u i r e m e n t s number 6 and number 7 a r e contradictory in that c a s e .

Within the scope of this t h e s i s we shall in p a r t i c u l a r occupy o u r s e l v e s with p r o p e r t y number 9: the r e s i s t a n c e of the oil to oxidation at t e m p e r a t u r e s of the o r d e r of 100 d e g r e e s c e n t i g r a d e . This p r o p e r t y also has been the ob-j e c t of nunaerous published investigations.

I. 2. The evaluation of transformer oils in practice

Up to the p r e s e n t day the r e s i s t a n c e against oxidation of t r a n s f o r m e r oils is being determined in p r a c t i c e by means of v a r i o u s s t a n d a r d t e s t s . In different countries different methods have been accepted. Ham and Thomp-son (2) have made a c r i t i c a l comparative study of these m e t h o d s .

In their review the authors point out that most of these s t a n d a r d methods have the following c h a r a c t e r i s t i c s in common:

1. They a r e of the „fixed-time" type, which involves quantitative determination of those oxidation products which a r e considered most noxious for the quality of the oil, after the t e s t has been run for a p r e d e t e r -mined and fixed span of t i m e .

2. In most t e s t s the p r e s c r i b e d t e m p e r a t u r e is 120 d e g r e e s centigrade, which is far higher than the operating t e m p e r a t u r e in t r a n s f o r m e r s . The British method (BS-148) even p r e s c r i b e s a testing t e m p e r a t u r e of 150OC.

Ham and Thompson r e m a r k that c h a r a c t e r i s t i c number 1 p re v e n ts us from learning anything about the actual rate at which the oxidation proceeds with time, whilst number 2 is rejectable because of our uncertainty about the question whether ageing at the p r e s c r i b e d t e s t t e m p e r a t u r e s follows the sanme pattern as at lower t e m p e r -a t u r e s .

Several y e a r s e a r l i e r Kreulen and T e r Horst (3) had repeatedly r a i s e d the s a m e objections against the „fixedt i m e " „fixedt e s „fixedt s . They s „fixedt r e s s e d „fixedthe fac„fixedt „fixedtha„fixedt for a p r o p e r a p -p r a i s a l of the oxidation -p r o -p e r t i e s of oils the -period of induction a s well as the r a t e of oxidation t h e r e a f t e r have to be d e t e r m i n e d .

They further e m p h a s i z e d the d e s ir a bi li ty of a method w a r r a n t i n g the even distribution of an e x c e s s supply of oxygen, through the whole bulk of the tested quantity, as this is the only way to preclude invalidation of the m e a s -u r e m e n t s by diff-usion phenomena (4).

Massey (5) objects to the evaluation t e s t s so far used on the ground of his e x p e r i e n c e in extensive long-time t e s t s in the p r a c t i c a l field between 1926 and 1940, which r e v e a l e d g e n e r a l discordance between his p r a c t i c a l r e -sults and the findings in BS sludge t e s t s .

Barton (6) c r i t i c i z e s the BS sludge test because of its poor reproducibility and its high t e m p e r a t u r e . The s a m e disadvantages prompted Pollitt (7) to review other n a -tionally accepted s t a n d a r d t e s t m e t h o d s .

In spite of all this c r i t i c i s m Technical Commission Nr 10 of the International E l e c t r o t e c h n i c a l Commission in 1954 proposed a s i m i l a r t e s t a s an international s t a n d a r d m e -thod. The disadvantages of the proposed method have been d i s c u s s e d in e v e r y aspect by Kreulen and Taat (8). The conclusion of t h e i r review is that fundamental knowledge of the m e ch a n is m of the oxidation r e a c t i o n s is i n d i s p e n s able for the e s t a b l i s h m e n t of a justified method of e v a -luation. Only in the light of such fundamental knowledge a sound judgment of p r a c t i c a l r e s u l t s could be r e a c h e d . Rigidly s t a n d a r d i z e d t e s t s , such as the one now p r o -posed, cannot put us on the right t r a c k .

1.3. Products of oxidation which cause deterioration of

transformer oils

Acids and sludge deposits a r e the products generally considered a s being the most undesirable chemical c o m -pounds formed in the oil during use in t r a n s f o r m e r s , s i n c e :

Acids will have a harmful effect on the e l e c

-t r i c a l p r o p e r -t i e s of -the oil and will m o r e o v e r c o r r o d e the m e t a l s of conductors and casing, whilst

Sludge deposits will diminish the t r a n s f e r of

heat from conductors to oil and m o r e o v e r h a m p e r the circulation of the oil itself, thus

in two ways impeding the dissipation of heat from the t r a n s f o r m e r .

Great is the number of published investigations as to the factors which influence the generation of acids and sludge by the oxidation of t r a n s f o r m e r o i l s . As such we may mention:

1. t e m p e r a t u r e 2. time

3. nature and surface of metal p a r t s , in contact with the oil, which may e x e r t a c a t a -lytic action

4. the r a t e of a c c e s s of oxygen to the oil 5. the t e m p e r a t u r e of the ventilation ducts 6. the refining method applied to the oil and

the intensity with which it was c a r r i e d out 7. the p r e s e n c e or absence of different types

of i n h i b i t o r s .

It is generally understood that there a r e two main types of inhibitors, both counteracting oxidation of the oil. The first type tends to extend the induction period, the second to slow down the r a t e of oxidation after the induction p e r i o d . It should be borne in mind that some inhibitors may combine these effects.

The investigations with r e g a r d to factors 1 through 5 have been d i s c u s s e d in detail by Kreulen and Taat (8), and we shall therefore confine o u r s e l v e s to a review of the work concerning factors 6 amd 7.

1,3.1. The influence of the refining method applied to the

oil

Wood-Mallock has shown (1) that the method a s well a s the degree of refining, applied to the oil, can greatly in-fluence its oxidation p r o p e r t i e s .

1.3.1.1. Refining and the formation of acids

The tendency of the oil to form acids turned out to in-c r e a s e with the perin-centage of sulphuriin-c ain-cid used in the refining p r o c e s s , up to a c e r t a i n l i m i t . F u r t h e r i n c r e a s e of the a c i d - t o - o i l r a t i o above this limit will not lead to an

i n c r e a s e d tendency of the oil to form acids, this tendency r e m a i n i n g p r a c t i c a l l y constant.

Gossling and Michie (9), investigating a range of oils refined to i n c r e a s i n g d e g r e e s , o b s e r v e d the s a m e p h e -nomenon.

The maximum tendency to form acids was found by Wood-Mallock to be p r a c t i c a l l y independent of the r e l a t i v e contents of paraffinic and naphthenic carbon a t o m s in the oil.

Wood-Mallock a t t r i b u t e s the low acid-forming tendency of low- and m e d i u m - d e g r e e refined oils to an inhibiting effect of sulphonatable components in those o i l s . These components a r e classified as follows:

a. a r o m a t i c hydrocarbons;

b . a r y l and alkyl compounds, containing s u l -p h u r .

Refinement of the oil with liquified sulphur dioxide r e -s u l t -s in a -strong diminution of it-s tendency to form acid-s, the l a t t e r becoming p r a c t i c a l l y independent of the r e l a -tive quantity of SO2 employed in the extraction p r o c e s s .

Addition of SO2extracts to oils which a r e highly r e fined with s u l p h u r i c acid, and which, as said above, n o r -mally have a s t r o n g tendency to form acids, r e s u l t s in a considerable diminution of this tendency. Wood-Mallock explains this phenomenon, as well as the one mentioned in the previous p a r a g r a p h , by the fact that the sulphur compounds of the oil a r e only p a r t i a l l y r e m o v e d by e x -t r a c -t i o n wi-th sulphur dioxide. I-t would be -these sulphur compounds, p r e s e n t in the S 0 2 - e x t r a c t as well as in the S 0 2 - e x t r a c t e d oil, and not the a r o m a t i c hydrocarbons, which would be r e s p o n s i b l e for the inhibition of the acid f o r m a t i o n . In o r d e r to prove this idea WoodMallock a d -ded to a c e r t a i n oil the following o r g a n i c compounds of known chemical s t r u c t u r e :

1. Sulphur and dibenzyl disnlphide: t h e s e caused a s t r o n g reduction of the acid for-mation.

2. Naphthalene, diphenyl and a n t h r a c e n e : these had no influence on the tendency to' form a c i d s .

3 . Substituted amines and phenols: these had but little effect on the acidforming p r o p -e r t i -e s .

1.3.1.2. Refining and the formation of sludge

Wood-Mallock concluded and Gossling and Michie (9) confirmed that the formation of sludge is diminished as the refinement with sulphuric acid is intensified.

Extractiqn with SO2 r e s u l t s in a lower tendency to form sludge in some oils; in other oils it does not.

In accordance with these findings oils of medium to low tendency to form sludge a r e not obtained by m e r e extraction with SO2. Wood-Mallock s t a t e s that as a rule S 0 2 - e x t r a c t i o n is followed by a light sulphuric acid t r e a t m e n t to produce oils of satisfactory sludge c h a r a c t e r i s t i c s , but such oils s t i l l have a r a t h e r s t r o n g t e n -dency to form a c i d s . Addition of a r o m a t i c a c i d - i n h i b i t o r s to these oils r e s u l t s in a lower tendency to form apids without i n c r e a s i n g the tendency to form sludge.

1.3.2. Inhibitors

Numerous compounds a r e known to have an inhibiting effect on the oxidation of h y d r o c a r b o n s .

Evans (10) has given a detailed review of the v a r i o u s types of oxidation inhibitors used in p r a c t i c e .

Several a u t h o r s have pointed out that the conventional methods used for the evaluation of the r e s i s t a n c e to oxidation of m i n e r a l oils a r e of no use in the case of oils to which inhibitors have been added. This is one m o r e s e r i o u s disadvantage of these methods, the m o r e so since the application of inhibitors is constantly being extended.

P r a c t i c a l experience has shown that the „ s u s c e p t i b i l i ty" of an oil for inhibitors depends on the degree of r e -fining. „Susceptibility" h e r e means the extension of the induction period r e s u l t i n g from the addition of a c e r t a i n percentage of the inhibitor.

Beavan, Irving and Thompson (11) have indeed p r o -duced evidence of the fact that the susceptibility inc r e a s e s with the intensity of refinement. Their e x p e r i

-ments were c a r r i e d out on various t r a n s f o r m e r oils in combination with two phenolic-type and two amino-type i n h i b i t o r s .

Von Fuchs and Diamond (12) found that the s a m e rule applies to lubricating o i l s .

I. 4. Résumé

The function of a t r a n s f o r m e r oil in p r a c t i c e is a double o n e : it acts a s a d i e l e c t r i c medium and as a c o o l -ing medium.

The p r o g r e s s of the oxidation of oil in a t r a n s f o r m e r is d et er min e d by a number of f a c t o r s , of which the r e -s i -s t a n c e of the oil again-st oxidation i-s only one, though it is an important one.

Other f a c t o r s , r e l a t e d to the construction of the t r a n s -f o r m e r , a r e in p r a c t i c e doubtlessly equally decisive -for the actual p r o g r e s s of oxidation. This g r e a t l y compli-cates a thorough understanding (and a p r o p e r definition) of ageing of oils in p r a c t i c e , and provides another p r e s s -ing argument in favour of the development of a funda-mentally justified method for the evaluation of the oxida-tion stability of o i l s .

With a view to the p r o p e r appreciation of the v a r i o u s n a t i o n a l l y - s t a n d a r d i z e d oxidation r e s i s t a n c e t e s t s a g r e a t many investigations have been conducted on the influence of:

1. t e m p e r a t u r e 2. time

3. nature and surface of m e t a l c a t a l y s t s 4. the intensity of oxygen supply

5. the t e m p e r a t u r e of the ventilation d u c t s .

These investigations have been c r i t i c a l l y d i s c u s s e d by Kreulen and Taat (8). In this discussion it was argued that no wholly or p a r t l y a r b i t r a r i l y standardized method, established without any knowledge of the r e a c t i o n m e c h -anism in question, is likely to yield s a t i s f a c t o r y in-formation. The object of any s e r i o u s work in this field should be the establishment of a method enabling us to

select the most a p p r o p r i a t e type of oil for well-defined purposes in a reliable way. This aim can only be r e a l ized if we can complete our understanding of the m e c h -a n i s m s of the r e -a c t i o n s involved.

Only then the quality of both oils and t r a n s f o r m e r s can be improved in an efficient, purposeful way. As long as the r e s i s t a n c e to oxidation cannot be defined a s a funda-mental and independent property, the t r i a l - a n d - e r r o r method will lead to contradictory conclusions.

CHAPTER II

SURVEY OF LITERATURE ON THE RELATION BETWEEN CHEMICAL STRUCTURE AND

CHARAC-TERISTIC OXIDATION-PROPERTIES OF HYDRO-CARBONS IN A RELATIVELY PURE STATE AND O F

NARROW-RANGE FRACTIONS OF MINERAL OILS

Stephens (13) was one of the first i n v e s t i g a t o r s to examine the oxidation of pure hydrocarbons between the t e m p e r a t u r e s of 85 and 100 d e g r e e s c e n t i g r a d e . He studied a number of alkyl-substituted b e n z e n e s .

Toluene, xylene and mesitylene, in this o r d e r , proved to be increasingly a c t i v e . The oxidation products con-s i con-s t e d mainly of mono-aldehydecon-s and a l e con-s con-s e r quantity of mono-carboxylic a c i d s .

Cymene was oxidized much more e a s i l y than the above-mentioned compounds. The p r e s e n c e of water proved to have a hampering effect on the r a t e of oxida-tion of xylene, mesitylene and cymene.

In a subsequent investigation (14) Stephens found that upon oxidation ethyl-benzene yields acetophenone, whilst npropylbenzene yields propiophenone at low t e m p e r a t u r e s , and a black r e s i n o u s product at higher t e m p e r a -t u r e s . Cumene yielded ace-tophenone, w h e r e a s p - c y m e n e was principally converted into tolyl-methyl-ketone and to a v e r y s m a l l extent into cumene-aldehyde and cumenic acid.

Water, while inhibiting the oxidation of ethyl-benzene and n - p r o p y l - b e n z e n e , a c c e l e r a t e s the oxidation of cu-m e n e .

F r o m his e x p e r i m e n t s Stephens draws the conclusion that the speed of oxidation i n c r e a s e s with the degree of substitution in the benzene ring and that it s t a r t s p r e -dominantly in the a - c a r b o n - a t o m of substituted b e n z e n e s , provided hydrogen atoms a r e tied to this carbon a t o m .

If t h e r e is no hydrogen attached to the a - C - a t o m , such as is the case in t e r t i a r y butylbenzene, little or no oxidation o c c u r s .

Water inhibits oxidation, except in those c a s e s where the a - C - a t o m c a r r i e s only one hydrogen atom (cumene). It then e x e r t s a catalytic action.

Stephens a r g u e s that the hydroxylation theory, put forward by Bone for the oxidation of hydrocarbons at high t e m p e r a t u r e s in the gaseous phase, is not valid for these oxidations at lower t e m p e r a t u r e s in the liquid p h a s e .

Bone (15) a s s u m e s alcohols a s i n t e r m e d i a t e products in the p r o c e s s of oxidation:

O O

CH4 • CH3OH » CH2(OH)2 F = ^ CH2O +H2O Four objections were r a i s e d by Stephens against such a m e c h a n i s m :

1. If the hypothesis of Bone were to be valid, water should inhibit the oxidation in all c a s e s , which, how-e v how-e r , is not in accordanchow-e with o b s how-e r v a t i o n .

2. Upon oxidation at 100 d e g r e e s centigrade a r o m a t i c alcohols yield mainly high-boiling e t h e r s and benzoic acid, w h e r e a s hydrocarbons yield carbonyl com-pounds .

3. Assuming the validity of Bone's r e a c t i o n p a t t e r n one would expect that upon oxidation of xylene or ethyl-benzene in the p r e s e n c e of acetic anhydride the acetic e s t e r s of the alcoholic i n t e r m e d i a t e s would be formed; experimentally, however, no e s t e r s could be obtained in this way.

4. Oxidation of cumene in the p r e s e n c e of water should r e s u l t in the production of the r e s p e c t i v e carbinol, since it has experimentally been e s t a b l i s h e d that further oxidation of this carbinol is inhibited by wa-t e r . The oxydawa-tion of cumene on wa-the p r e s e n c e of wawa-ter,, however, r e s u l t s in the production of acetophenone. Stephens (16) put f o r w a r d the following r e a c t i o n p a t t e r n :

HYDROCARBON + O2 ? = ^ COMPLEX ^=^: H2O + Oo

+ unsaturated r e s t — ' ^ ALDEHYDE or KETONE This pattern would r e a s o n a b l y explain the inhibiting action of w a t e r . Upon oxidation of cumene no water would be formed in the second stage, but t h e r e would be a d i s c h a r g e of methyl alcohol; hence t h e r e would be no inhibition by the p r e s e n c e of w a t e r . Methyl alcohol, however, could not be t r a c e d during the oxidation of cumene, but Stephens detected the p r e s e n c e of fornnic acid.

Stephens and Roduta (17) also went into the oxidation of s e v e r a l substituted benzenes with only one hydrogen atom attached to the a - c a r b o n - a t o m , this carbon atom being, in these c a s e s , a s y m m e t r i c a l .

It turned out that

A r - C - H I R2

upon oxidation is invariably converted into

A r - C = 0 +R2OH,

in which formulae R2 is to be understood to be l a r g e r than R j .

George and Robertson further developed and extended the t h e o r y on the oxidation of hydrocarbons (18).

They consider the generation of hydroperoxides as the first s t e p . These hydroperoxides have been isolated in some c a s e s ; in other c a s e s t h e i r o c c u r r e n c e Qould not be substantiated.

According to George and Robertson the r a t e at which the hydroperoxides a r e formed depends on the s t r e n g t h of the weakest carbon-hydrogen link in the m o l e c u l e . In methane the energy in question is of the o r d e r of 102 Kcal/mol; in higher hydrocarbons it is sonaewhat l e s s ,

owing to the resonance of the v a l e n c e - e l e c t r o n s . So for instance in a methylene group, attached to the benzene ring, the energy is lower by about 10 K c al /m ol , and in a methylene group linked to an unsaturated carbon atom the energy is by about 14 Kcal/mol l e s s than in CH4. It is understandable why hydroperoxides a r e m o r e readily formed in the l a t t e r two c a s e s .

By m e a s u r i n g the energy of pyrolysis for various alkyl iodides, Polanyi (19) has shown that p r i m a r y , secondary and t e r t i a r y C-H links p o s s e s s an i n c r e a s i n g energy of resonance in this o r d e r . Therefore the r a t e of formation of hydroperoxides should i n c r e a s e in the s a m e o r d e r . In concordance with this conclusion Chavanne (20) found that the oxidation of alkyl-eyelohexanes s t a r t s at the t e r t i a r y carbon atoms in the r i n g s .

George and Robertson i l l u s t r a t e d their t h e o r e t i c a l considerations by giving the figures for the following s e r i e s of hydrocarbons, e x p r e s s i n g the r a t e of oxidation as the number of m i l l i l i t r e s of oxygen absorbed by one m i l l i l i t r e of hydrocarbon in one hour at 110 d e g r e e s centigrade, using copper s t e a r a t e as a catalyst:

naphtalene 0 (only a r o m a t i c carbon atoms)

tetraline 125 (two a-CH2 groups attached to the benzene ring)

(no benzene ring)

(one a-CH2 group attached to the benzene ring)

(no benzene ring)

(oneot-CH group attached to the benzene ring)

(no benzene ring)

(no hydrogen atom attached to the tt-carbon atom) (no benzene ring but one t e r t i a r y carbon atom in the ring)

(a-CH3 groups at the ben-zene ring only; no secondary or t e r t i a r y carbon atoms) (three t e r t i a r y carbon atoms in the ring)

decaline 4.3 ethyl-benzene 7.5 ethyl-cyclohexane 1.5 isopropyl-benzene 6.0 isopropyl-cyclohexane 2.2 t e r t i a r y butyl-benzene 0.02 t. butyl-cyclohexane 0.35 mesitylene 4.8 trimethyl-cyclohexane 11.2

paraffinic fractions

with n o r m a l chains 0.25

The influence of a r o m a t i c r i n g s and t e r t i a r y carbon atoms shows up c l e a r l y in this s e r i e s , and the figures p e r m i t the deduction of the following r u l e s :

1. Aromatic r i n g s i n c r e a s e the liability to oxidation of a-CH2 and a-CH g r o u p s .

2. Aliphatic groups, attached to a-CH2- and a-CH groups, r a i s e the r e a c t i v i t y of the l a t t e r g r o u p s .

3. T e r t i a r y CH groups in cyclohexane r i n g s a r e m o r e reactive than aliphatic CH2 groups, but much l e s s reactive than CH2- and CH groups which a r e in the a-position to a benzene r i n g .

4. Substituted a r o m a t i c compounds having no hydrogen atoms linked to a-C atoms on the benzene ring have a minimal r e a c t i v i t y .

5. Hatoms linked to a r o m a t i c Catoms show no r e a c -tivity.

George and Robertson have mathematically t r e a t e d the oxidation as a chain r e a c t i o n . For the o v e r a l l r a t e of oxidation they used the formula:

1 Q /L _ r a t e of initiation x rate of propagation ^' r a t e of t e r m i n a t i o n

In this formula the r a t e of initiation stands for the r a t e at which c h a i n - s t a r t e r s a r e formed. The r a t e of propagation is the r a t e at which r e a c t i o n - c h a i n s grow, once they have been s t a r t e d , and the r a t e of iermination is the r a t e at which the growth of r e a c t i o n - c h a i n s is stopped in the r e a c t i o n - m i x t u r e .

There can be two r e a s o n s for low r e a c t i v i t y in a hydrocarbon:

a. The r a t e of initiation may be small;

b . The average number of links of the r e a c t i o n - c h a i n may be s m a l l as a consequence of a high r a t e of t e r m i n a t i o n .

1. If two hydrocarbons, having about the s a m e r a t e s of termination, but p o s s e s s i n g widely different products of the r a t e s of initiation and propagation, a r e mixed, the r e a c t i v i t y of the r e s u l t i n g mixture will be the a r i t h m e t i c m e a n of the r a t e s of oxidation of the com-ponents. In other w o r d s : in this case the addition of

the hydrocarbon of low reactivity has a „diluting" effect on the one with the higher r e a c t i v i t y .

2. If a hydrocarbon of high reactivity, having a low rate of termination, is mixed with another hydrocarbon, the individual r e a c t i v i t y of which is low because of a high r a t e of termination, the r a t e of oxidation of the r e s u l t i n g mixture will be lower than the a r i t h m e t i c mean of the r a t e s of oxidation of the individual c o m -ponents. The effect of the admixture on the m o r e highly r e a c t i v e hydrocarbon here a p p e a r s a s an inhi-bition of the oxidation.

3. If a hydrocarbon with a high r a t e of initiation is mixed with another hydrocarbon having a low r a t e of initia-tion but a high r a t e of propagainitia-tion, the r e a c t i v i t y of the resulting mixture will be higher than the a r i t h -metic mean of the individual r a t e s of oxidation, and therefore it would appear a s if the reactivity of the l e s s r e a c t i v e component is promoted by the m o r e r e a c t i v e component.

The expected effects have indeed been experimentally observed, as shown by the following e x a m p l e s :

Mixtures of ethylbenzene with naphtalene and a n t h r a -cene have r e a c t i v i t i e s below the a r i t h m e t i c m e a n . Anthracene and naphtalene seemingly have an inhibit-ing effect on the oxidation of e t h y l - b e n z e n e .

On the other hand phenanthrene, t e r t i a r y butyl-benzene and ethyl-cyclohexane just have a „diluting" effect on the reactivity of e t h y l - b e n z e n e .

F u r t h e r investigation l e a r n e d that the above effects only a p p e a r e d in c o p p e r - c a t a l y z e d oxidations, but w e r e absent in uncatalyzed oxidations. This phenomenon led George and Robertson to the conclusion that the r e a c t i o n pattern for catalyzed oxidation must be different from the pattern for uncatalyzed oxidation, in spite of the fact that in both c a s e s the s a m e products a r e formed.

The findings of George and Robertson a r e , on the whole, in a g r e e m e n t with those of Booser and Fenske (21). These investigators studied a number of pure hy-dr o ca r bo ns , a few m i x t u r e s , a concentrated fraction of condensed-ring a r o m a t i c s and a light fraction obtained from a technical paraffin-base oil.

Moreover from the l a t t e r fraction a paraffinic-naph-tenic fraction and an a r o m a t i c one were obtained by percolation through s i l i c a g e l , and these sub-fractions were also studied s e p a r a t e l y .

Booser and Fenske e x p r e s s e d t h e i r findings as the number of hours r e q u i r e d for the absorbtion of 100 millimol O2 by one mol of hydrocarbon, and thus ob-tained the following r e l a t i v e f i g u r e s :

hexadecene 0.35 1. m e t h y l - 4 . isopropyl benzene 0.55 c i s - d e c a l i n e 1.1 t e t r a - i s o b u t a n e 1.5 hexadecane 2.3 1. methyl-naphthalene 124 phenanthrene over 440 More insight was gained by m e a s u r e m e n t of the r e l -ative contents of the following specific groups in the oxidized product:

acid groups, saponifiable groups, peroxides, carbonyl groups, hydroxyl groups, pentane-- insoluble m a t t e r and sludge.

It was found that aliphatic hydrocarbons yield r e l a -tively high quantities of carbonyl compounds, somewhat l e s s e r quantities of acids, and no sludge.

A r o m a t i c hydrocarbons, like m e t h y l i s o p r o p y l b e n -zene and 1. methyl-naphtalene, yield a r e m a r k a b l y high proportion of hydroxyl g r o u p s . Methyl-naphthalene in p a r t i c u l a r produced much sludge, w h e r e a s phenanthrene almost exclusively yielded water and carbon dioxide.

The next two c a s e s show that the oxidation of m i x t u r e s of a r o m a t i c and n o n a r o m a t i c hydrocarbons i n -variably r e s u l t s in a product in which much m o r e oxygen is found back in hydroxyl groups than in other g r o u p s .

Upon oxidation of t h e i r concentrated fraction of con-d e n s e con-d - r i n g a r o m a t i c hycon-drocarbons, Booser ancon-d Fenske found that they obtained v e r y much sludge and hiydroxyl compounds, w h e r e a s v e r y little acids and carbonyl conn-pounds were g e n e r a t e d . These findings a r e in a g r e e m e n t

with the r e s u l t s obtained by the oxidation of a mixture of hexadecane and an equal part of methyl-naphthalene, as mentioned h e r e u n d e r :

The stability of this mixture is pr act i cal l y equal to that of methyl-naphthalene itself, which s e e m s to in-dicate that the oxidation of the relatively reactive hexadecane is inhibited by methyl-naphthalene.

In the r e s u l t i n g mixture of oxidation products the r a t i o of oxygen in hydroxyl groups to oxygen in other groups is found to be much higher than the corresponding r a t i o in the oxidation products of the individual components. For these phenomena Booser and Fenske proposed the following explanation:

In the hydrocarbon mixture a r o m a t i c hydroxyl c o m -pounds would be formed from methyl-naphthalene. These a r y l - h y d r o x y l compounds would act as inhibi-t o r s for inhibi-the oxidainhibi-tion of hexadecane.

An example of stimulated oxidation is found in the mixture of equal p a r t s of hexadecane and c i s - d e c a l i n e . At the onset of oxidation of this mixture the r e a c t i o n is of the n o r m a l autocatalytic type with a r a t e pr a ct i c al l y equal to the a r i t h m e t i c mean of the oxidation r a t e s of the components. In the next stage, however, the r a t e of oxidation of the mixture r i s e s to a considerable extent above the mean r a t e . This must be attributed to the stimulating action of cis-decaline on the oxidation of hexadecane.

The light paraffine-base oil fraction proved to have a much g r e a t e r stability than e i t h e r the a r o m a t i c or the paraffinic-naphthenic sub-fraction obtained from this cut.

A s i m i l a r effect had a l r e a d y been r e p o r t e d by Fenske et alea (22), who worked with fractions they had obtained from a light m i n e r a l oil by distillation and e x t r a c t i o n .

Booser and Fenske (21) have shown that their original m i n e r a l oil, upon oxidation, showed the s m a l l e s t in-c r e a s e of visin-cosity but the g r e a t e s t produin-ction of sludge, as compared to the two fractions obtained from it.

The a r o m a t i c fraction showed the g r e a t e s t i n c r e a s e of viscosity and yielded a black, tacky product.

The paraffinic-naphthenic fraction yielded the g r e a t e s t proportion of acids, showed the s m a l l e s t degree of d i s -coloration, and formed p r a c t i c a l l y no sludge.

The investigators obtained definite indications to the effect that during the oxidation of the original oil its a r o m a t i c components were much m o r e heavily attacked than its paraffinic-naphthenic components.

The oxidation r a t e of the a r o m a t i c fraction itself was much lower than that of the paraffinicnaphthenic f r a c -tion. This s e e m s to indicate that in the original oil the oxidation of the paraffinic-naphthenic components has a promoting effect on the oxidation of the a r o m a t i c com-ponents. By this induced oxidation the a r o m a t i c compo-nents yield a r y l - h y d r o x y l compounds which, in t h e i r turn, a r e capable of inhibiting the oxidation of the p a r -affinic-naphthenic components. In the light of this hypo-t h e s i s hypo-the r a l a hypo-t i v e l y high shypo-tabilihypo-ty of hypo-the original oil, as compared to its fractions, would be understandable. The high r a t e of oxidation of the paraffinic-naphthenic fraction justifies the conclusion that it must contain m o l -e c u l -e s with a high d-egr-e-e of branching, and cons-equ-ently many t e r t i a r y carbon atoms (compare e . g . l . m e t h y l -4 . i s o p r o p y l - b e n z e n e ) .

This is in a g r e e m e n t with the high r e a c t i v i t y r e p o r t e d by L a r s e n , Thorpe and Armfield (23), working with di-i s o p r o p y l - p e r h y d r o - a n t h r a c e n e , whdi-ich di-is a hdi-ighly branched compound.

The fraction containing isolated a r o m a t i c r i n g s had a lower stability towards oxidation than the fraction con-taining condensed-ring a r o m a t i c s which was studied by Booser and Fenske, but n e v e r t h e l e s s it proved to be m o r e stable than 1. m e t h y l - 4 . isopropyl-benzene.

Similar inhibiting effects of c o n d e n s e d a r o m a t i c s y s -t e m s have of-ten been r e p o r -t e d — amongs-t o -t h e r s by Von

Fuchs and Diamond (12), George and Robertson (18) and L a r s e n , Thorpe and Armfield (23).

Of a few pure hydrocarbons, notably t e t r a l i n e , the pattern of oxidation was investigated through kinetic studies by George, Robertson and Rideal (24). For t e t r a l i n e in the absence of inhibiting compounds it was e x -p e r i m e n t a l l y found that:

dt ^o- C R H • ^=02

In this equation ko stands for the r a t e - c o n s t a n t of the uninhibited reaction, C R H for the concentration of the hydrocarbon, and CQ2 for that of oxygen.

As for the oxidation of t e t r a l i n e in the p r e s e n c e of in-hibiting compounds it was found that the duration of the induction period shows a linear r e l a t i o n to the initial concentration of the inhibitor ( r e p r e s e n t e d in the equa-tion by cjo):

cio

On the assumption that each molecule of the inhibitor is capable of interrupting one r e a c t i o n - c h a i n , George and Robertson proceed to the m a t h e m a t i c a l t r e a t m e n t of the inhibited r e a c t i o n and r e a c h the conclusion that the constant a in the above equation is identical to the r a t e of initiation.

F r o m this m a t h e m a t i c a l t r e a t m e n t it further t r a n s p i r e s that the length of the r e a c t i o n c h a i n can be c a l culated from the m e a s u r a b l e r a t e of the uninhibited r e -action and the duration of the induction period, which a r e r e p r e s e n t e d in the formula given below by RQ and t j r e s p e c t i v e l y : „ ,

«o H Chain length

cio

With the aid of this formula it was found from e x p e r i -m e n t s that the 9hain length at 100 d e g r e e s centigrade is of the o r d e r of a few hundred units. It d e c r e a s e s as the t e m p e r a t u r e r i s e s . F o r chain r e a c t i o n s this r e l a t i o n was to be expected, since for such r e a c t i o n s

chain length = r a t e of propagation ^ r a t e of t e r m i n a t i o n

In g e n e r a l the energy r e q u i r e d for the activation of the propagation r e a c t i o n s is low, and therefore the r a t e of propagation v a r i e s but slightly with t e m p e r a t u r e , w h e r e as the r a t e s of the s t o p r e a c t i o n s , the energy of a c t i v a -tion of the l a t t e r being much higher, i n c r e a s e strongly with t e m p e r a t u r e .

It was further shown e xp er i m e nt al ly that the r a t e of r e a c t i o n v a r i e s with t i m e . The logarithm of this r a t e , plotted against t i m e , proved to be a straight line for about 10% of the total oxidation. Thus:

Rate = A . e "^^ in which A and cp a r e c o n s t a n t s .

Such a relation was deduced by Semenoff (25) for the case he defined as „degenerate chain b r a n c h i n g " . Here a p r i m a r y chain r e a c t i o n leads to the formation of a r e l a -tively stable i n t e r m e d i a t e product. This i n t e r m e d i a t e product is converted into tlic end-product by a s e c o n d a r y chain r e a c t i o n . Now if in this c a s e , at the expense of the secondary r e a c t i o n s , nuclei a r e formed which in t h e i r turn a r e capable of generating p r i m a r y r e a c t i o n - c h a i n s , phenomena s i m i l a r to those o b s e r v e d in the case of t e -t r a l i n e will o c c u r .

George and Robertson suppose that in the c a s e of t e -t r a l i n e a p r i m a r y r e a c -t i o n - c h a i n leads -to -the forma-tion of a hydroperoxide acting as a r e l a t i v e l y stable i n t e r -mediate product. This i n t e r m e d i a t e hydroperoxide would then be converted through a secondary chain r e a c t i o n into tetralone — which indeed can be detected e x p e r i m e n -tally. The fact that with t e t r a l i n e the l o g - r a t e / t i m e curve is s t r a i g h t for no m o r e than about 10% of the total oxida-tion points to the conclusion that in this case the degen-e r a t i o n of thdegen-e sdegen-econdary chain r degen-e a c t i o n s only takdegen-es placdegen-e to a s m a l l extent.

The outcome of the m e a s u r e m e n t s of the initial r a t e of the inhibited oxidation by George and Robertson can be mathematically c o m p r i s e d in the equation:

1

= a . Cxr,

and the induction p e r i o d s of inhibited m i x t u r e s in s o l u -tion by:

-1 0 ^1 = C R H - ^ 0 2

in which v^ r e p r e s e n t s the initial rate of oxidation. According to these equations, both the initial r a t e of oxidation and the inducjtion period are independent of the concentration of the oxygen.

F r o m the equations, derived from the e x p e r i m e n t a l r e s u l t s exposed in the preceding, George and Robertson have concluded that

the r a t e of initiation a ~ Cj^jj . c ^

and this means that the initiationre action is m o n o m o -lecular in r e l a t i o n to the hydrocarbon and that it is in-dependent of the concentration of the oxygen.

The formation of r a d i c a l s R* and H* according to the hypothetical p r i m a r y r e a c t i o n

RH » R* + H*

could make these conclusions eligible. The energy of activation for an initiation along this pattern would, how-ever, be of the o r d e r of 100 Kcal/mol, according to what is said on page 21, w h e r e a s George, Rideal and Robertson, using the equation of A r r h e n i u s , found the energy of activation to be 28.5 ± 2 . 5 K c a l / m o l .

In the s a m e way Booser and Fenske (21) obtained for other hydrocarbons values of 19.5 to 28 K c a l / m o l . This level of the energy of activation being established beyond doubt, George and Robertson proposed a way out by a s s u m i n g an „energy c h a i n - r e a c t i o n " :

Initiation: RH » RH' Propagation: RH' + O2 * p^

px + RH » ROOH + RH'

Termination: pX —» O2 + inactive molecules pX + inhibitor —* inactive molecules

Here RH' denotes an e n e r g y r i c h hydrocarbon m o l e -cule, and pX stands for a hydrocarbon-oxygen complex the nature of which is not d i s c u s s e d . We shall r e t u r n l a t e r on to these considerations of George and Robertson.

Wibaut and Strang (26, 27) investigated the oxidation of a number of oqtane i s o m e r s and a few dim e t h y l c y c l o -hexane s .

They o b s e r v e d that t h e s e h y d r o c a r b o n s , after careful elimination of peroxides, p o s s e s s a relatively high r e -s i -s t a n c e to oxidation. Exception-s to thi-s rule w e r e tho-se i s o m e r s p o s s e s s i n g one o r m o r e t e r t i a r y carbon a t o m s , provided these were not s t e r i c a l l y shielded. (Example: 2.5. dimethyl - hexane).

When oxygen is blown for ten hours through n-octane at 125°C, m e a s u r a b l e quantities of peroxides a r e f o r m -ed, but no rapid oxidation takes place as long as no cat-alyst is added. Upon addition of t r a c e s of cobalt s t e a r a t e to hydrocarbon containing p e r o x i d e s , a v e r y rapid oxida-tion s e t s in at onpe. The s a m e phenomenon is o b s e r v e d when t r a c e s of 2.5. d i m e t h y l - h e x a n e - d i h y d r o p e r o x i d e - 2 . 5 and t r a c e s of cobalt s t e a r a t e a r e added simultaneously to a hydrocarbon which is free from p e r o x i d e s .

A highly r e a c t i v e hydrocarbon like 2.5. dimethyl-hexane quite rapidly produces m e a s u r a b l e quantities of p e r o x i d e s . It was even o b s e r v e d that the 2.5. dimethyl-hexane-dihydroperoxide, g e n e r a t e d at room t e m p e r a t u r e during p r o t r a c t e d s t o r a g e in a g l a s s bottle, spontaneous-ly s e g r e g a t e d from the liquid.

Wibaut and Strang found that the lowering of the p a r -t i a l - p r e s s u r e of -the oxygen in -the gas mix-ture c a r r i e d through the liquid did not affect the r a t e of oxidation un-til the value of 0.5 a t m o s p h e r e was r e a c h e d . F u r t h e r reduction of the oxygen p r e s s u r e r e s u l t s in a drop of the r a t e of oxidation.

It was further found that the r a t e of oxidation of s t r a i g h t - c h a i n paraffines is directly proportional to the number of carbon atoms per molecule of hydrocarbon.

The maximum r a t e of oxygen absorbtion by 2.5. di-methyl-hexane diluted with i n e r t solvents turned out to be l i n e a r l y proportional to the concentration of the

hyd r o c a r b o n . With 2. methylheptane, however, the m a x i -mum r a t e of oxygen absorption is proportional to the s q u a r e of the concentration.

It was o b s e r v e d that soon after the onset of oxidation the originally c o l o u r l e s s solution of cobalt s t e a r a t e turned green, owing to the conversion of Co* " -ions into

Co" • • . If the oxidation is c a r r i e d on for longer p e r i o d s , the g r e e n colour vanishes and t h e r e a p p e a r s a pink p r e -cipitate in which Co' " -ions can be detected.

The g r e e n Co* ' ' -colour also a p p e a r s when, in an a t m o s p h e r e of nitrogen, a t r a c e of 2.5. dimethyl-hexane-dihydroperoxide-2.5 is added to the benzene solution of cobalt s t e a r a t e . In this experiment the peroxide is d i s -integrated, and it is found that a certain quantity of cobalt s t e a r a t e is capable of disintegrating much m o r e than the equivalent quantity of peroxide. It depends on the nature of the solvent whether the peroxide yields acetone or 2 . 5 . d i m e t h y l h e x a n e d i o l 2 . 5 . If the solvent m o l e cules contain active hydrogen a t o m s , mainly 2 . 5 d i m e t h y l h e x a n e d i o l 2 . 5 is formed. In a nonactive s o l -vent the generation of acetone is predominant.

How s t r o n g a catalytic action cobalt s t e a r a t e e x e r t s on the disintegration of the dihydroperoxide is d e m o n s t r a t e d by the fact that this peroxide itself is an e x t r e m e l y stable compound. After heating a solution of it in benzene for t h r e e hours at 100 d e g r e e s centigrade, it can be r e -covered p r a c t i c a l l y quantitatively.

Wibaut and Strang recognized the influence of the chemical s t r u c t u r e of hydrocarbons upon t h e i r r a t e s of oxidation.

a. Straight-chain hydrocarbons a r e not v e r y r e a c t i v e ; t h e i r r a t e of oxidation i n c r e a s e s with the length of the carbon chain.

b . Hydrocarbons, p o s s e s s i n g t e r t i a r y carbon a t o m s , have much higher r a t e s of oxidation than s t r a i g h t chain hydrocarbons, the latter containing only s e c -ondary and p r i m a r y carbon a t o m s .

Steric hindrance is of importance, as for instance 3.4. dimethyl-hexane is appreciably l e s s r e a c t i v e than 2.5. dimethyl-hexane.

It h a s b e e n s h o w n t h a t o x i d a t i o n a l w a y s s t a r t s at t h e t e r t i a r y c a r b o n a t o m , if p r e s e n t : 2 . 5 . d i m e t h y l - h e x a n e y i e l d s a c e t o n e 3 . 4 . d i m e t h y l - h e x a n e i s c o n v e r t e d i n t o m e t h y l - e t h y l - k e t o n e . Wibaut a n d S t r a n g , g u i d e d by the r e s u l t s of t h e i r e x -p e r i m e n t s -p r o -p o s e d a r e a c t i o n m e c h a n i s m b a s e d u-pon t h e t h e o r y of H a b e r a n d W e i s s (28), a s s u m i n g t h e f o l -l o w i n g p a t t e r n : I. I n i t i a t i o n : R i 1) R2 — CH R 3 0 2 , R i « 2 R 3 — COOH f o r m a t i o n of h y d r o p e r o x i d e s w i t h m o l e c u l a r o x y g e n , not c a t a l y z e d b y m e t a l i o n s II. P r o p a g a t i o n : 2) ^COOB + Co* • 3) T C - O * + :7C-H 4) 7 C * + 0 2 *• 5) 7COO* + -^C-E Co* * • + O H ' + 7 \ : - 0 ^ ' :?C-OH + ^ C * • 7 0 0 0 ^ -7 COOH + -7C* In t h i s p a t t e r n t h e a l k o x y r a d i c a l g e n e r a t e d by r e a c -t i o n 2) c a n a l s o d i s i n -t e g r a -t e in -t h i s w a y : 6) ^ ^ : : : C - C H 2 - R 3 R2 ^ * C O i ; R 2 ^ C = 0 + R 3 - C H 2 ^ ' 7) R 3 - C H 2 + O 2 * R 3 - C H 2 O O ' ' 8) R 3 - C H 2 O O * + :^CH — R 3 - C H 2 O O H + T C *

R e a c t i o n 2) e x p l a i n s how Co* * i s c o n v e r t e d into"Co* * *, w h i l s t t h e c h a i n - r e a c t i o n s 3) t h r o u g h 5) a n d 6) t h r o u g h 8)

explain the rapid oxidation which o c c u r s in the p r e s e n c e of preformed hydroperoxide, oxygen and a metal c a t a -lyst.

Wibaut and Strang pointed out that in r e a c t i o n s 3) 5) and 8) hydrocarbon molecules a r e among the compon e compon t s . As it is kcomponowcompon from the c h e m i s t r y of r a d i c a l -polymerization that r e a c t i o n s like 4) and 7) proceed at a very high r a t e , it is easy to see why the overall r a t e of the above r e a c t i o n - c h a i n is de te r m i n ed by 3) 5) and 8) and therefore depends on the concentra'tion of t h e ' h y d r o -carbon.

It also can be understood why at oxygen p r e s s u r e s above 0.5 a t m o s p h e r e the r a t e of oxidation is independ-ent of the oxygen concindepend-entration. The concindepend-entration of the r a d i c a l s R-CH2* is v e r y s m a l l and therefore decisive for the r a t e of the r e a c t i o n .

In the range of low oxygen concentrations the concen-tration of this element becomes a determining factor, as the r a d i c a l s among t h e m s e l v e s can enter into a num-ber of t e r m i n a t i o n r e a c t i o n s , as there a r e :

III. Termination:

9) 2 ^COO* >• >COOC^ +O2 10) ^COO* + R-CH2 » ^GOOCH2R 11) ^ C " + •7C''~ > inactive hydrocarbon

molecule

These t e r m i n a t i o n r e a c t i o n s grow m o r e important as soon as the r a d i c a l s get a longer life span through lack of oxygen for r e a c t i o n s 4) and 7).

The r e g e n e r a t i o n of C o ' ' - i o n s out of C o * ' ' - i o n s is explained by Wibaut and Strang by assuming the follow-ing r e a c t i o n s :

12) R-COOH ? = ^ R-COO' + H*

13) R-COO' + C o * * > R-COO* + Co*' 14) H' +OH' * H2O

abled Wibaut and Strang to e s t a b l i s h the — e x p e r i m e n t ally proved — r e l a t i o n s for the r e a c t i o n r a t e s t h e o r e t i -cally.

It is quite n a t u r a l that the p a t t e r n of the c h a i n r e a c -tions is v e r y susceptible to the environment in which they have to take p l a c e . This is i l l u s t r a t e d by the d i s -integration of 2.5. d ime th yl- h ex a ne - di hyd r ope r ox i de - 2. 5, j u s t to mention one example, in different s o l v e n t s .

Wibaut and Strang found that in a benzene solution the main product of the r e a c t i o n is acetone, w h e r e a s in a diphenyl-methane solution mainly 2.5. dimethyl-hexane-diol-2.5 is f or me d .

A further instance of this effect is found in the work of Stephens (14) and Fortuin (29). Stephens, working with cumene, found that upon n o r m a l oxidation of this c o m -pound, when the p r i m a r y hydroperoxide is disintegrated t h e r m a l l y , mainly acetophenone is obtained. Fortuin, who investigated the disintegration of the s a m e h y d r o p e r -oxide in an a t m o s p h e r e of sulphur di-oxide, found that the principal compounds he obtained were phenol and acetone.

Apart from these investigations of pure hydrocarbons, l i t e r a t u r e also deals with the oxidation of hydrocarbon m i x t u r e s obtained as fractions from technical oil p r o d -u c t s . The a r t i c l e s in q-uestion, being of i n t e r e s t beca-use of t h e i r value for our understanding of the influence of chemical s t r u c t u r e upon oxidation c h a r a c t e r i s t i c s , de-s e r v e to be mentioned h e r e .

Kapff, Bowman and Lowy (30) have split up a l u b r i c a t -ing oil into fractions of different mean molecular weights by m e a n s of distillation. By m e a n s of extraction these fractions were in t h e i r turn split up into the definite testing fractions, not only differing in molecular weights, but also, because of the extraction methods employed, in t h e i r p e r c e n t a g e s of a r o m a t i c , naphthenic and aliphatic carbon a t o m s . These p e r c e n t a g e s were d et er m i ned a c -cording to a g r a p h i c a l - s t a t i s t i c a l method.

Unfortunately the t e s t s were c a r r i e d out in such a way that no complete s a t u r a t i o n of the mixture with" oxygen was w a r r a n t e d . As, however, the e x p e r i m e n t s were

made with very s m a l l quantities of the testing fractions, s p r e a d out in v e r y thin l a y e r s , it is probable that the state of s a t u r a t i o n was v e r y closely approached.

As c r i t e r i a for the p r o g r e s s of oxidation a. the formation of acids

b . the i n c r e a s e of viscosity were m e a s u r e d .

The following c h a r a c t e r i s t i c s were determined:

a. The acid formation was lowest in the fraction which contained the highest proportion of naphthenes. It in-c r e a s e d with the a r o m a t i in-c in-content, but proved to be more susceptible to a r i s e in the aliphatic content. The influence of the molecular weight turned out to be s m a l l .

b. The i n c r e a s e of viscosity is most m a r k e d in those fractions which contain a high percentage of a r o m a t i c carbon a t o m s .

Kreulen (31) (32) (33) investigated the oxidation of a technical ,,white oil" which contained no a r o m a t i c carbon a t o m s .

In the investigation mentioned under (31) attention was chiefly paid to the influence of copper. In his subsequent work Kreulen, basing himself on the r e s u l t s of his f i r s t investigations, only used s m a l l quantities of copper filings during the period of induction, in o r d e r to s h o r t e n the duration of this period.

Plotting the l o g a r i t h m s of the peroxide number, of the carbonyl number and of the acid number of the oil, a s d e t e r m i n e d after various s h o r t spans of time following the conclusion of the induction period, against the log-arithm of the time of oxidation, straight curves were ob-tained.

Although t h e r e was s o m e s c a t t e r i n g of the points in the g r a p h s , and although the spans of time were of ne-c e s s i t y short bene-cause Kreulen wanted to study the e a r l y stages of oxidation, he ventured the conclusion that the log-log-plots had tangents of 1.1, 2.0 and ^.0 for the peroxides, the carbonyl compounds and the acids r e -spectively.

On the b a s i s of the m a t h e m a t i c a l derivations a s given by Kreulen and T e r Horst (33) Kreulen concluded that the p e r o x i d e s , as determined by iodometric titration, were the first intermediate product, from which as a second i n t e r m e d i a t e the carbonyl compounds were formed. These in t h e i r t u r n would be converted into acids in the third s t a g e .

The work dealt with in this t h e s i s has provided e v i dence that K r e u l e n ' s conclusion with r e g a r d to the f o r -mation of peroxides is e r r o n e o u s . Kreulen, a s said above, worked with v e r y s h o r t oxidation tiines, and did not take the tolerance of the peroxide determination suf-ficiently into account.

The consequence of K r e u l e n ' s s h o r t t i m e s of oxidation was that the oil was still in the t r a n s i t i o n stage between the induction period and the full development of the oxi-dation p r o c e s s p r o p e r . A meticulous inspection of the points in K r e u l e n ' s graphs r e v e a l s that a curved line, such a s obtained in our own e x p e r i m e n t s , would be n e a r e r the t r u t h than a s t r a i g h t line.

We want to emphasize here that the peroxides g e n e r -ated in the white oil must be of another type than those formed in solid paraffin wax. This has been indicated by the fact that the peroxides formed in hard paraffin during oxidation a r e instantaneously disintegrated when some copper filings a r e added, w h e r e a s the white oil peroxides do not respond to the addition of copper (34).

Hibbard (35) fractionated a lubricating oil of high v i s -cosity by a chromatographic p r o c e d u r e . The fractions thus obtained had widely divergent chemical s t r u c t u r e s , which is obvious from their p e r c e n t a g e s of a r o m a t i c , naphthenic and aliphatic carbon. These p e r c e n t a g e s were determined by a g r a p h i c a l - s t a t i s t i c a l method according to Deansly and Carleton (36).

The fractions in question were submitted to oxidation according to a method, developed by Burk, Hughes, Scovill and Bartleson (37), and which s e e m s to e n s u r e a p r a c t i c a l l y complete s a t u r a t i o n with oxygen. Unfortu-nately Hibbard worked with a fixed testing time (36 hours) and a fixed t e m p e r a t u r e ( 2 8 0 O F ) .

After the oxidation test, the following c h a r a c t e r i s t i c s were determined:

1. the quantity of oil-insoluble sludge 2. the quantity of pentane-insoluble m a t t e r 3. the i n c r e a s e of the viscosity

4. the corroding action upon c o p p e r - l e a d test panels 5. the quantity of acids fo rm e d.

The r e s u l t s of these m e a s u r e m e n t s led to the follow-ing general conclusions:

1. The saturated fraction does not yield any oil-insoluble sludge, but much pentane-insoluble m a t t e r and a great quantity of aoids of a highly c o r r o s i v e n a t u r e .

A 90:10 mixture by weight of this fraction and the fraction with isolated a r o m a t i c r i n g s behaves in a much s i m i l a r way.

A 90:10 mixture by weight of this fraction and the fraction with condensed-ring a r o m a t i c s yields a quite noticeable quantity of sludge.

2. The fraction with condensed-ring aromatics, which contains much sulphur, has a very s m a l l capacity to produce acids and pentane-insoluble m a t t e r , and shows a v e r y low c o r r o s i v i t y and a slight i n c r e a s e of the v i s c o s i t y .

On the other hand it yields a great amount of o i l -insoluble sludge.

3. Very little acid, pentane-inso-luble m a t t e r and oil-insoluble sludge a r e formed by that fraction of the oil which could only be dislodged from the s i l i c a gel (through which the oil had been percolated) by polar solvents (alcohol and chloroform), and which has a r a t h e r m a r k e d sulphur content.

Admixture of this fraction to the other fractions 10:90 produces a s i m i l a r effect.

Thus this third fraction can be considered as an ef-fective n a t u r a l inhibitor. This is in a g r e e m e n t with the findings of Denison (38), who showed that the n a t u r a l inhibitors in lubricating oils a r e sulphur compounds. Denison believes their inhibiting action to consist in the disintegration of p e r o x i d e s .

George and Robertson (18) have given a general outline of the method by which the behaviour of a t r a n s -f o r m e r oil in p r a c t i c e could be predicted. In-formation should be collected about the following p a r t i c u l a r s :

a. The conditions prevailing during oxidation, i . e . during use in the t r a n s f o r m e r .

b. The nature of the undesirable reaction products, notably acids and sludge, and the mechanism of their formation.

C. The chemical constitution of the components of the oil in question and the influence of their constitu-tion upon the r a t e of oxidaconstitu-tion.

d. The metals with which the oil e n t e r s into contact and which a r e capable of e n t e r i n g into solution, as well as the m e c h a nis m of this e n t r a n c e .

Regarding p a r t i c u l a r c. details should be known about: 1. what part of the oil is n o n - a r o m a t i c ;

2. which a r e the proportions of p r i m a r y , secondary, t e r t i a r y and naphthenic carbon atoms in the non-a r o m non-a t i c pnon-art;

3. whether the a r o m a t i c r i n g s a r e linked to p r i m a r y , secondary o r t e r t i a r y carbon atoms;

4. whether the oil contains inhibitors, for instance a n -thracene and compounds of nitrogen or s u l p h u r .

It is obvious that it would be a v e r y extensive work to find out all these details for a given oil. And even in the case of an oil about which all above p a r t i c u l a r s a r e known there is a fair possibility that still m o r e informa-tion would be r e q u i r e d .

So for instance the hydroperoxides formed by the most r e a c t i v e components of the oil, and the r a d i c a l s r e s u l t -ing from the disintegration of these hydroperoxides, could well influence or e n t e r into r e a c t i o n with other, by t h e m s e l v e s n o n - r e a c t i v e components of the oil, and thus activate them o r convert them into inhibiting com-pounds (compare the formation of a r o m a t i c hydroxyl compounds in m i x t u r e s also containing aliphatic hydro-c a r b o n s ) .

T h e r e f o r e , in our opinion, it is open to question whether the way, indicated by George and Robertson, could e v e r be followed in p r a c t i c e .

C h a p t e r I I I

RESUME O F THE WORK DESCRIBED IN CHAPTERS I AND II, AND SOME GENERAL CONCLUSIONS DRAWN

FROM IT

Summarizing the investigations discussed in the first two c h a p t e r s , the following points and conclusions come to the fore:

III. 1. The products generated by oxidation of m i n e r a l oil can be classified a s :

a. peroxidic compounds of g r e a t e r or l e s s e r stability; b. carbonyl compounds (ketones and aldehydes);

c. acids;

d. products insoluble in oil or in pentane, given the g e n e r a l name of sludge;

e. compounds which, under p r a c t i c a l working conditions of the oil, escape from the latter as g a s e s -notably carbon dioxide, carbon monoxide and water vapour.

III. 2. Acids and sludge a r e generally considered a s b e -ing the most undesirable products of oxidation in both t r a n s f o r m e r - and lubricating o i l s .

III. 3. It is evident that t h e r e a r e great differences in the r a t e of oxygen absorbtion between the t h r e e main chemical types of h y d r o c a r b o n s . Such differences also exist in the r e l a t i v e proportions of the oxidation products a s listed sub III. 1. It is possible to formulate the following e m p i r i c r u l e s :

a. The relation between structure and rate of oxidation

a. 1. In the group of the paraffins the r a t e of oxidation in g e n e r a l i n c r e a s e s with the degree of branching. Steric f a c t o r s , especially s t e r i c shielding of t e r t i a r y carbon a t o m s , play an important part, according to the findings of Wibaut and Strang.

a. 2. Naphthenes and substituted naphthenes behave in a s i m i l a r way a s paraffins. Thus in alkyl-naphthenes oxidation s t a r t s at t e r t i a r y carbon atoms of the r i n g . Naphthenes g e n e r a l l y show a lower reactivity than a l i -phatic hydrocarbons with t e r t i a r y carbon a t o m s , and also a lower r e a c t i v i t y than a l k y l - a r o m a t i c s in which the a-carbon atom is secondary or t e r t i a r y .

a. 3. Aromatic h y d r o c a r b o n s :

A r o m a t i c carbon rings a r e , by t h e m s e l v e s , v e r y stable to oxidation.

In a l k y l - a r o m a t i c s , according to the findings of Stephens, George and Robertson t h e r e is always a c t i -vation of the a -c a rb on a t o m s .

Evidence has been obtained as to the fact that the secondary o r t e r t i a r y c h a r a c t e r of this carbon atom r e s u l t s in an activation which is s u p e r i m p o s e d upon the activation by the a r o m a t i c r i n g .

Consequently t h e r e is a gradual i n c r e a s e of r e a c t i v -ity in the sequence:

s t r a i g h t - c h a i n aliphatics alkyl-substituted naphthenes

aliphatics with t e r t i a r y carbon atoms (provided t h e r e is no s t e r i c hindrance)

a r o m a t i c s with s t r a i g h t - c h a i n alkyl groups (thus p o s s e s s i n g secondary a-carbon atoms)

a r o m a t i c s with t e r t i a r y a - c a r b o n a t o m s .

F r o m the investigations of Stephens, George and Rob e r t s o n and of Booser and Fenske it has Robecome e v i -dent that an i n c r e a s e in the number of a-carbon a t o m s in substituted a r o m a t i c s r e s u l t s in an i n c r e a s e of r e -activity.

a. 4. The investigations of George and Robertson as well as the work of Booser and Fenske throw a c l e a r light upon the fact that m i x t u r e s of various groups of hydrocarbons can show different c h a r a c t e r i s t i c s as far as the r a t e of oxidation is concerned.

A. The r a t e of oxidation may be the a r i t h m e t i c mean of the r a t e s of oxidation of the components.

B. The r a t e of oxidation may drop appreciably below the a r i t h m e t i c mean, thus revealing an inhibiting effect of one of the components upon the o t h e r . C. The r a t e may be higher than the a r i t h m e t i c mean;