Vo l.
27. No. 210.
Ap r i l1941.
O C T A N E R A T IN G R E L A T IO N S H IP S O F A L IP H A T IC , A L IC Y C L IC , M O N O N U C LE A R A R O M A T IC H Y D R O C A R B O N S, A L C O H O L S, E T H E R S , A N D K E T O N E S *
B y
Gt j s t a v E g l o f f a n dP. M.
Va n Ar s d e l l.T
hemodem internal-combustion engine owes the general smoothness of its performance to the quality of motor fuels used. There are many organic compounds which have boiling points within gasoline range which will run an automobile engine. The fuels operating to-day’s engines are hydrocarbons derived from petroleum and natural gas. These gasolines have varying efficiencies that are expressed as octane ratings. The detonation properties of motor fuels are due to the type of organic mole
cules present, and their octane rating is a function of the structure and of the oxidation characteristics under high-temperature and pressure con
ditions. ' It should be pointed out that variation in engine conditions markedly alters the octane ratings of organic substances. Studies of the octane ratings, oxidation characteristics, and thermal stabilities of alkanes, alkenes, alkynes, cyclanes, cyclenes, mononuclear aromatics, alcohols, ethers, and ketones have been made in order to correlate the octane ratings of these compounds. Since modem synthetic processes have made in
dividual hydrocarbons available in large volumes, their chemical structures have become increasingly important from a motor-fuel standpoint.
The relation of chemical structure to motor-fuel efficiency was first brought out in 1931, when Lovell, Campbell, and Boyd 1 published then- work on the combustion properties of aliphatic hydrocarbons. These properties were reported as “ aniline equivalents.” In 1932 the octane- rating scale was officially established, following the work of Edgar 2 on synthesis of “ iso-octane ” and evaluation of mixtures of “ iso-octane ” in w-heptane.
The octane ratings of many pure hydrocarbons, alcohols, ethers, and ketones have been determined. The most extensive study so far attempted has been the work of the American Petroleum Institute, now starting the third year of its work in synthesis and determination of fuel qualities of pure hydrocarbons 3 boiling in the gasoline range. There are a number of pure hydrocarbons, alcohols, ethers, and ketones that have a higher anti
knock value than iso-octane and could not be rated directly on the octane scale; hence they were extrapolated.
The research, motor, and four other proposed test methods for evaluating octane numbers of gasoline are shown in Table I.
Other factors, not mentioned in test-method specifications on which octane ratings are dependent, are valve and spark timing, manifold distri-
K
* P a p e r r e c e i v e d 1 4 t h J a n u a r y , 1 9 4 1 .
T a b l e I .
Comparison of Operating Conditions fo r the Gasoline Knock Test Methods using the C .F .R . Engine a s a B a s i s *
A l l m e t h o d s u s e 3 J - i n . x 41 - in . v a r i a b l e c o m p r e s s i o n c y l i n d e r , u n l e s s o t h e r w i s e n o t e d .
M o t o r M e t h o d . L - 3 M e t h o d . 1 9 3 9 R e s e a r c h M e t h o d .
O l d R e s e a r c h M e t h o d .
C . F . R . A v i a t i o n M e t h o d ( 1 9 4 0 ) ( 1 - C ) .
A r m y M e t h o d ( A N 9 5 2 5 ) § ( o f f ic ia l f o r m i l i t a r y s e r v i c e u n t i l J a n . 1 9 4 1 ) .
S p e e d .
J a c k e t t e m p e r a t u r e .
S p a r k a d v a n c e .
M i x t u r e t e m p e r a t u r e .
I n t a k e v a l v e . F u e l b l e n d s f o r
c h e c k i n g e n g i n e .
C o m p r e s s i o n r a t i o f o r c h e c k i n g e n g i n e . J I n s t r u m e n t a t i o n f o r
d e t e r m i n i n g k n o c k .
C o n s t a n t , 9 0 0 ± 9 r . p . m . C o n s t a n t , ± 1 ° F .
H e l d b e t w e e n 2 0 9 ° a n d 2 1 5 ° F . A u t o m a t i c , 2 6 0 °
a t 5 : 1 c o m p r e s s i o n r a t i o . 3 0 0 ° F . ± 2 ° F .
S h r o u d e d . 4 9 - 1 % C . P . b e n z
e n e i n A - 6 e q u a l s 6 4 - 2 % C - l l i n A - 6 ( 6 5 O c t . N o . ) .
5 - 3 : 1.
B o u n c i n g - p i n a n d k n o c k m e t e r .
C o n s t a n t , 9 0 0 ± 9 r . p . m . C o n s t a n t , ± 1 ° F .
H e l d b e t w e e n 2 0 9 ° a n d 2 1 5 ° F . A u t o m a t i c , 1 6 °
a t 5 : 1 c o m p r e s s i o n r a t i o . 2 6 0 ° F . ± 2 ° F .
S h r o u d e d . 4 6 - 7 % C . P . b e n z
e n e i n A - 6 e q u a l s 6 3 - 5 % C - l l i n A - 6 ( 6 5 O c t . ( N o . ) .
5 - 5 : 1.
B o u n c i n g - p i n a n d k n o c k m e t e r .
C o n s t a n t , 6 0 0 ± 6 r . p . m . C o n s t a n t , ± 1 ° F .
H e l d b e t w e e n 2 0 9 ° a n d 2 1 5 ° F . F i x e d , 1 3 0 ° f o r
a l l c o m p r e s s i o n r a t i o s .
1 2 5 ° F . ± 2 ° F . f
S h r o u d e d . 4 8 - 4 % C . P . b e n z
e n e i n A - 6 e q u a l s 7 2 - 5 % C - l l i n A - 6 ( 7 0 0 O c t . N o . ) .
5 - 7 5 : 1.
B o u n c i n g - p i n a n d k n o c k m e t e r .
C o n s t a n t , 6 0 0 ± 6 r . p . m .
C o n s t a n t , ± 1 ° F . H e l d b e t w e e n 2 0 9 ° a n d 2 1 5 ° F . A u t o m a t i c , 2 2 - 5 0
a t 5 : 1 c o m p r e s s i o n r a t i o . R o o m t e m p e r a
t u r e , f
P l a i n . 6 5 O c t . N o .
5 - 3 : 1.
B o u n c i n g - p i n a n d k n o c k m e t e r .
C o n s t a n t , 1 2 0 0 ± 1 2 r . p . m . C o n s t a n t a t 3 7 4 °
± 2 ° F . w i t h i n
± 9 ° F .
F i x e d , 3 5 ° f o r a l l c o m p r e s s i o n r a t i o s .
I n t a k e a i r 1 2 5 ° F .
± 5 ° F . M i x t u r e t e m p e r a t u r e 2 2 0 ° F . ± 2 ° F .
P l a i n .
8 5 % S - l i n M - 2 v s . C . P . b e n z e n e t o g i v e s a m e t h e r m a l p l u g r e a d i n g .
T h e r m a l p l u g .
C o n s t a n t , 1 2 0 0 fz
2 0 r . p . m . C o n s t a n t a t 3 3 0 °
± 2 ° F . w i t h i n
± 5 ° F .
F i x e d , 3 0 ° f o r a l l c o m p r e s s i o n r a t i o s .
R o o m t e m p e r a t u r e . |
P l a i n .
8 8 % i s o - o c t a n e i n n - h e p t a n e v s . C . P . b e n z e n e t o g i v e s a m e t h e r m a l p l u g t e m p e r a t u r e .
T h e r m a l p l u g .
* J . S . B o g e n , U n i v e r s a l O i l P r o d u c t s C o m p a n y ,
f T h i s i s t h e i n t a k e a i r t e m p e r a t u r e , n o t m i x i n g t e m p e r a t u r e , j T h i s r a t i o m u s t b e c o r r e c t e d f o r b a r o m e t e r .
§ 2 § - i n . X 4 J - i n . v a r i a b l e c o m p r e s s i o n c y l i n d e r .
122 EGLOFFANDVANARSDELL: OCTANERATINGRELATIONSHIPSOF
HYDROCARBONS, ALCOHOLS, ETH ERS, AND KETONES. 1 2 3
bution, carburetion, air-fuel ratio, humidity, and engine conditions.
Instantaneous pressures of 1200 psi or higher may occur under knocking conditions; temperatures in the cylinder may then range from 2200° to 2500° C. These conditions are highly significant when oxidation reactions are taking place. As a primary concept, octane ratings may be considered as the oxidation index of a given compound under the combustion conditions prevailing in an engine.
In considering the fundamental structural factors affecting the octane rating of organic compounds, there are a number of generalizations which may be extended to cover other compounds than hydrocarbons. A number of rules were proposed by Lovell, Campbell, and Boyd 4 in the study of hydrocarbons and their relation to anti-knock properties. For the pure hydrocarbons it was found necessary to modify somewhat their first and second generalizations, while consideration of data on the oxidized com
pounds showed that these rules were applicable to the oxidized class of motor fuels as well.
Rule 1.—The longer the straight chain of carbon atoms contained in organic molecules, the lower the octane rating, with the exception of the alkenes, where ethene has a lower octane rating than the two following hydrocarbons.
The following examples in Tables II, H I, and TV show the application of Rule 1 to the hydrocarbons of different series.
T a b l e I I .
Straighl-chain Paraffins, Pure Compounds.
C o m p o u n d .
O c t a n e r a t i n g .
C . F . R . R e s e a r c h A . S . T . M . 3 m e t h o d .*
M e t h a n e . . . . 1 1 0 1 0 0
E t h a n e . . . . . 1 0 4 1 0 0
P r o p a n e . . . . 1 0 0 1 0 0
B u t a n e . . . . . 9 2 9 5
P e n t a n e . . . . 61 5 8
H e x a n e . . . . 2 5 3 4
H e p t a n e . . . . 0 0
O c t a n e . . . . . - 1 7 —
N o n a n e . . . . — 4 5 —
D e c a n e . . . . . - 5 3
The octane ratings given by Smittenberg for the 2-alkenes do not follow
the general smoothness of those reported by Lovell, Campbell, and Boyd in
their work, nor do these values fall in line with Rule 1 as well as do the
paraffins. Table III shows octane ratings for the pure olefins,3 and
Table IV shows octane ratings of the blends of pure olefins with a reference
fuel. In general, it may be seen from the data in Table IV, the octane
ratings of the blends follow the same trend as the pure compounds.
124
EGLO FF AND VAN A R SD E L L : OCTANE RA TING R E LA T IO N SH IPS OF T a b l e I I I .Slraight-chain Olefins, Pure Compounds.
C o m p o u n d .
O c t a n e r a t i n g , 3 C . F . R . A . S . T . M .
E t h e n e . 81
P r o p o n e . . . . 8 5
B u t e n e - 2 . . . . 8 3 6
P e n t e n e - 2 . . . . 8 0
H e x e n e - 2 . . . . 7 8
O c t e n e - 2 . . . . 5 5
Calculated values of the octane ratings of olefins from aniline equivalents,4 as well as the direct determination of octane ratings given by Smittenberg, Hoog, Moerbeek, and van der Zijden,3 show the same trend in lowered rating with increase in chain length. The orientation of the double bond towards the centre of a molecule containing the same number of carbon atoms raises the octane rating.4 Table IV shows the octane ratings deter
mined on the pure compounds and on the blends of 1 gram mole of hydro
carbon per litre of 55 octane rating reference fuel.
T a b l e I V .
Olefin Series,10 Pure and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
M .M . R . M . M . M . R . M .
E t h e n e . . . . 1 0 0 8 5 -5
P r o p e n e 1 0 0 1 0 2 0
B u t e n e - 1 8 0 1 1 1 -5
B u t e n e - 2 83
P e n t e n e - 1 9 2 9 8 -5
P e n t e n e - 2 9 8 1 0 7 1 2 5
H e x e n e - 1 8 0 8 5
H e x e n e - 2 8 9 1 0 0
H e x e n e - 3 9 7
H e p t e n e - 1 5 4 5 5
H e p t e n e - 2 7 0
H e p t e n e - 3 8 4 9 5
O c t e n e - 1 3 9 2 5
O c t e n e - 2 5 5
O c t e n e - 3 7 3
O c t e n e - 4 91
N o n e n e - 1 15
Data listed for the acetylene hydrocarbons are so scarce that no
correlations can be made with the m aterial; however, Table V shows the
octane ratings determined for the straight-chain acetylenes, and Table
VI shows a comparison of these acetylenes with straight chains to the
olefins and paraffins of similar structure.
T a b l e V .
HYDROCARBONS, ALCOHOLS, ETHERS, AND KETONES. 1 2 5
Straight-chain Acetylenes, Pure and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g , r e s e a r c h m e t h o d .
P u r e c o m p o u n d . 7 B l e n d e d c o m p o u n d .
A c e t y l e n e 8 0
P e n t y n e - 2 1 0 8 8
H e p t y n e - 1 8 4 7 6 »
H e p t y n e - 3 4 0 - 3 0 7
O c t y n e - 2 6 6 6 2 9
T a b l e V I .
Pure and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g , r e s e a r c h m e t h o d .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
E t h a n e . . . . > 1 0 0 7
E t h e n e . . . . > 1 0 0 7 8 5 -5 9
E t h y n e . . . . 8 0 7
P e n t a n e . . . . 5 8 11 6 0 9
P e n t e n e - 2 9 8 7 1 2 5 9
P e n t y n e - 2 1 0 8 8
H e p t a n e . . . . 0
7
0 9H e p t e n e - 1 5 4 7 5 5 9
H e p t y n e - 1 8 4 7 7 6 9
H e p t e n e - 3 8 4 7 9 5 9
H e p t y n e - 3 4 0 7 - 3 0 7
While the data presented here are not too conclusive, it seems that the octane-rating effect of the acetylene bonding is intermediate between that of the paraffin bond and the olefin bond.
The pure compounds of the cycloparaffin and aromatic series given in Table V II also follow Rule 1 insofar as the data are given; that is, the length of the substituting straight chain lowers the octane rating.
T a b l e V I I .
C o m p o u n d .
O c t a n e r a t i n g , C . F . R . A . S . T . M .
c y c Z o P e n t a n e . . . . 8 3 8
M e t h y lc y c Z o p e n t a n e 8 2 3
c y c Z o H e x a n e . . . . 7 7 3
M e t h y lc y c Z o h e x a n e 71 3
B e n z e n e . . . . . 1 0 8 3
M e t h y l b e n z e n e ( T o l u e n e ) 1 0 4 3
E t h y l b e n z e n e . . . . 9 6 12
The values derived from calculating aniline equivalents in blends for the alicyclic group show the trend even better than those determined for the pure compound. Table VIII shows the octane ratings for the cych- paraffin series.
T a b l e V I I I .
1 2 6 EGLOFF AND VAN ARSDELL : OCTANE RATING RELATIONSHIPS OF
c y c l oParaffin Series,10 Pu re and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g , r e s e a r c h m e t h o d .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
c y c Z o P e n ta n e . 1 0 0 1 2 5
M e t h y lc y c Z o p e n t a n o 8 1 -5 71
E t h y lc y c Z o p e n t a n e . 6 2 5 9
P r o p y lc y c Z o p e n t a n e 16
B u t y lc i/ c Z o p e n t a n e . — 11
P e n t y lc y c Z o p e n t a n e — 1 9
c y c Z o H e x a n e . . . . 8 6
M e t h y lc y c Z o h e x a n e . 7 4
E t h y lc y c Z o h e x a n e . 4 4
P r o p y lc y c Z o h e x a n e . 2 0
B u t y lc y c Z o h e x a n e 3
P e n t y lc y c Z o h e x a n e . — 8
Table IX shows cydo-defines, and here also the effect of chain lengthening is evident.
T a b l e I X .
c y c l o Olefin Series,10 Blended Compounds.
C o m p o u n d . O c t a n e r a t i n g , r e s e a r c h m e t h o d , b l e n d e d c o m p o u n d .
c y c Z o P e n t e n e . . . . 1 4 0
M e t h y lc y c Z o p e n t e n e 1 4 3
E t h y lc y c Z o p e n t e n e 1 0 2
P r o p y lc y c Z o p e n t e n e 9 6
B u t y lc y c Z o p e n t c n o 8 2
P e n t y lc y c Z o p e n t e n e 6 3
c y c Z o H e x e n e . . . . 1 0 2
M e t h y lc y c Z o h e x e n e 1 3 3
E t h y lc y c Z o h e x e n e 1 0 0
B u t y lc y c Z o h e x e n e 6 3
P e n t y lc y c Z o h e x e n e 5 8
The monoalkyl-substituted series of aromatic hydrocarbon blends shows
a deviation from Rule 1 on the longer straight-chain lowering of octane
ratings. Table X shows that up to propylbenzene the octane rating rises
with increasing chain length; beyond the propyl group, however, the normal
lowering with progressive chain lengthening 14 is exhibited. This seems
to be true only for the blends, since Table VII, on pure compounds, shows
the lowering of octane rating with each successive methyl addition to the
alkyl substitution group.
T a b l e X .
HYDROCARBONS, ALCOHOLS, ETHERS, AND KETONES. 1 2 7
Blended Compounds, 13
C o m p o u n d . O c t a n e r a t i n g , r e s e a r c h m e t h o d , b l e n d e d c o m p o u n d .
B e n z e n e . . . . . 1 0 8
M e t h y l b e n z e n e . . . . 1 2 0
E t h y l b e n z e n e . . . . 1 2 8
P r o p y l b e n z e n e . . . . 1 3 7
B u t y l b e n z e n e . . . . 1 1 5
P e n t v l b e n z e n e . . . . 10 1
H e p t y l b e n z e n e . . . . 4 6
Rule 2.—Branched chain aliphatic compounds have higher octane ratings than the normal compounds.
(a) Monomethyl isomers have higher octane rating than the normal compound, and the dimethyl isomers are higher than either the normal or monomethvl isomers, as shown in Table X I.
T a b l e X I .
C o m p o u n d .
O c t a n e r a t i n g , 3 C . F . R . A . S . T . M .
H e x a n e . . . . . 2 5
2 - M e t h y l p e n t a n e . . . . 73
3 - M e t h y l p e n t a n e . . . . 7 5
2 : 2 - D i m e t h y l b u t a n e 9 6
2 : 3 - D i m e t h y l b u t a n e 9 5
(6) As the monomethyl substitution approaches the centre of the mole
cule, the octane rating is increased over that of the other monomethyl substitutions located nearer the end of the chain; for example, 2-methyl- pentane has an octane rating of 73 and 3-methylpentane a rating of 75 in the group of hydrocarbons shown in Table X II. This is further substan
tiated in blending values given for 2-methylpentane, 69; and 3-methyl
pentane, 84.14 Lovell and Campbell state that centralization of the mole
cule is the factor increasing the octane rating; however, this is not true in the case of the dimethyl compounds, where one carbon atom contains both methyl substituents.
T a b l e X I I .
O c t a n e r a t i n g , 3
C o m p o u n d . C . F . R .
A . S . T . M .
2 - M e t h y l p e n t a n e . . . . 7 3
3 - M e t h y l p e n t a n e . . . . 7 5
Branching of the side-ehains in the olefinic series shows the same effect
as the paraffin branching. The presence of the double bond in its various positions is also seen to influence the octane rating of the olefins.
Tables X III and XIV show the same effects in the olefin series as were discussed in the paraffin group, with some exceptions showing, due possibly to the type of linkages, and not the structure. Table X III shows a higher octane rating for peripheral methyl substitutions where the double bonds lie near the centre, in this instance in the 2-position, while for paraffin hydrocarbons the substitution of methyl radical near the centre of the molecule raises the octane rating.
Ta b l e X I I I .
1 2 8 EGLOFF AND VAN ARSDELL I OCTANE RATING RELATIONSHIPS OF
Pure and Blended Compounds.
O c t a n e r a t i n g .
C o m p o u n d . P u r e
B l e n d e d c o m p o u n d .
c o m p o u n d , m o t o r m e t h o d .
M o t o r m e t h o d .
R e s e a r c h m e t h o d .
3 - M e t h y lp e n t e n e - 2 4 - M e t h y lp e n t e n e - 2
1 0 9 9 1 1 5 7
2 : 4 : 4 - T r i m e t h y l p e n t e n e - 2 3 : 4 : 4 - T r i m e t h y l p e n t e n e - 2
8 9 7 8 5 - 6 13
1 3 3 9 7 2 - 5 13
The olefin compounds having double bonds in the one position give octane relations more nearly like the paraffins in this instance, since the methyl substitutions near the centre of the molecules give higher ratings as in the paraffin series. Table XIV shows the 1-alkenes in this relation.
Ta b l e X I V .
Pure and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
M .M . R . M . M . M . R . M .
4 - M e t h y lh e x e n e - 1 8 5 7 8 6 9
5 - M e t h y l h e x a n e - 1 8 2 7 8 3 9
2 - M e t h y lo c t e n e -1 7 4 -8 13 6 9 - 8 13
3 - M e t h y lo c t e n e -1 8 3 -5 13 7 2 -2 13
Table XV, on the acetylenes, shows a lowering of the octane rating
with the orientation of the triple bond towards the centre of the molecule
which is directly opposite the effect noted in the case of the olefines.
H Y D BO CA EB O N S, ALCOHOLS, E T H E E S , AND K E T O N E S.
129
T a b l e X V .
Pure and Blended Compounds.
C o m p o u n d .
O c t a n e r a t i n g , r e s e a r c h m e t h o d .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
H e p t y n e - 1 . . . . 8 4 7 7 6 9
H e p t y n e - 3 . . . . 4 0 7 - 3 0 7
5 - M e t h y l h e x y n e - l . 8 9 9
5 - M e t h y l h e x y n e - 2 . 8 8 s
(c) Dimethyl substitutions exhibit the same tendencies as the mono
methyl substitutions, as long as the two methyls are on adjacent carbon atoms in the molecule. 2 : 3-Dimethylhexane has an octane rating of 76 and 3 : 4-dimethylhexane has a value of 85. Table X V I shows the ratings in tabular form.
T a b l e X V I .
O c t a n e r a t i n g , 3
C o m p o u n d . C . F . R .
A . S . T . M .
2 : 3 - D i m e t h y l h e x a n e 7 6
3 : 4 - D i m e t h y l h e x a n e 8 5
These types of methyl branching introduce tertiary carbon atoms into the molecule, and the effect is one of diminishing the ease of oxidation.16
In explaining the higher octane ratings of monomethyl isomers and the rise in rating accompanying the movement of methyl groups towards the centre of the molecule, the thermal behaviour of normal and isobutane and of normal and isopentane is relevant.
Under comparable conditions (600-700° C.) isobutane dehydrogenates more readily to isobutene than ra-butane to butenes.17 In the case of normal and isopentane, the iso-compound was shown to yield alkenes with the double bond in the inner rather than the terminal position found in the thermal decomposition products of the normal compound.18 Since the final products of a purely thermal reaction are shown to be olefins, the intermediate products formed at high temperatures and pressures could readily be the free radicals derived from the parent compound. Experi
mental evidence has shown that the energy required to dissociate the C-C bond is about 21,000 cal. less than that required for the C-H bond break. Dissociation energies for C-C and C-H bonds are 71,000 and 92,000 cal., respectively. In the case of oxidation reactions at explosion tempera
tures, Sagulin 19 found the heat of activation of the C-H bond to be about 64,000 cal., which accounts not only for the ease of dehydrogenation, but would, in part, explain the much higher octane ratings of the isomeric compounds in relation to the straight chain. Even at explosion tempera
tures it may be postulated that the momentary formation of the free
radicals, isopropyl and isobutyl, reduces the volume of gases simultaneously
released in the engine cylinder. In contrast, there are many more radicals,
and hence greater volumes, in the case of the straight-chain compounds.
A plausible explanation for the higher anti-knock rating of the hydro
carbons with the centred methyl would be a split of the hydrocarbon mole
cule at the tertiary carbon atom, producing an anti-knock effect which would give two, or possibly three, parts of the molecule instituting chain reactions at the same time in contrast to the number of radicals released by the normal compound. The pressure increase set up by the splitting of the molecule gives the knock effect in accordance with the number and kind of hydrocarbon radicals set free. According to Rice’s explanation, the pressure existing in the automotive cylinder would be conducive to the formation of free isopropyl and ¿erf.-butyl radicals.20 As a further observation it may also be stated that ketones 21 might be reasonably expected under those pressure and temperature conditions existing in the automotive cylinder. Both the oxygen and the hydrocarbon are in an activated state, and in the case of the isomeric hydrocarbons the ketone would also have a higher initial combustion temperature, and therefore greater stability, than the peroxides which are postulated by Lewis and von Elbe.22 A further explanation might be given in the justification of the high anti-knock rating for isopropyl ether, which is 99. In this instance, with one part of the molecule having an oxygen attached, the reaction might be shown to proceed as follows :
C—C—0 —C—C — > C—C— + 0 = C —C + H
I I I I
c c c c
Such a compound would oxidize at the tertiary carbon in one part of the ether, whilst the other part of the ether would have formed a ketone.
Rule 3.—A quaternary carbon atom when oriented towards one end of a hydrocarbon chain increases the octane rating. Table XVII shows the effect of orientation of the quaternary carbon.
T a b l e X V I I .
130 EGLOFF AND VAN AESDELL : OCTANE RATING RELATIONSHIPS OF
C o m p o u n d .
O c t a n e r a t i n g , C . F . R . A . S . T . M .
2 : 2 - D i m e t h y l p e n t a n e . 9 3 3
3 : 3 - D i m e t h y l p e n t a n e . 8 4 3
2 : 2 : 3 : 3 - T e t r a m e t h y l h e x a n e 9 7 23 3 : 3 : 4 : 4 - T e t r a m e t h y l h e x a n e 6 5 24
The effect of the quaternary carbon under purely thermal conditions has not been sufficiently studied to give basis to any such hypotheses as were given for the effect of the tertiary carbon. Frey and Hepp 18 found that neopentane decomposed almost quantitatively at 575° C., according to the following reaction :
C
c + c—c=c
c
I f this reaction may be applied as affecting other compounds of this type, the initial reaction may be shown as a splitting of the molecule at the quaternary carbon. 2 : 2-Dimethylpentane will possibly react to give either the olefin, or more probably to give the free radicals, thus releasing only two gases in the engine cylinder. The pressure increase set up by such a yield of products of decomposition would be much less than that released by the isomeric 3 : 3-dimethylpentane, and this would explain, in part, the reduced octane rating for the 3 : 3-dimethyl compound. This assumption is based on Rice’s 20 statement that under heat and pressure conditions heavier radicals such as tert.-butyl and isopropyl may momen
tarily exist. In 3 : 3-dimethylpentane, with the quaternary carbon centred in the molecule, the octane rating is 84, in comparison with 93 for the 2 : 2-dimethyl compound. In view of Rice’s explanation of the more radicals set free the higher the initial pressure, it may be shown that three gaseous products would be formed in the case of the 3 : 3-dimethyl com
pound rather than two as was shown for the 2 : 2-dimethyl compound.
The pressure increase thus set up would probably institute the knocking more readily than in the other compound.
Orientation of compacted dimethyl and trimethyl substitutions towards one end of the molecule raises the octane rating as shown in Table X Y III.
HYDROCARBONS, ALCOHOLS, ETHERS, AND KETONES. 1 31
T a b l e X V I I I .
C o m p o u n d .
O c t a n e r a t i n g , 3 C . F . R . A . S . T . M .
2 : 2 - D i m e t h y l b u t a n e 9 6
2 : 3 - D i m e t h y l b u t a n e 9 5
2 : 2 - D i m e t h y l p e n t a n e . 9 3
2 : 3 - D i m e t h y l p e n t a n e . 8 9
2 : 4 - D i m e t h y l p e n t a n e . 8 2
3 : 3 - D i m e t h y l p e n t a n e . 8 4
2 : 3 - D i m e t h y l h e x a n e 7 6
2 : 5 - D i m e t h y l h e x a n e 5 2
3 : 4 - D i m e t h y l h e x a n e 8 5
2 : 2 : 3 - T r i m e t h y l p e n t a n e 1 0 2
2 : 2 : 4 - T r i m e t h y l p e n t a n e 1 0 0
2 : 3 : 4 - T r i m e t h y l p e n t a n e 9 7
Rule 4.—Methyl additions on cyclic structures of a given number of carbon atoms lower the octane rating, whilst methyl additions on a chain of a given number of carbon atoms raise the octane rating, except in the case of m-butane to 2-methylbutane, where the octane rating is lowered by adding the methyl substituent. Table X IX shows the relationship.
Rule 5.—In hydrocarbons of the cycfohexyl group the effect of the
ortho, meta, and para positions is one of lowering the octane rating
as the substituents are more widely separated. The ortho position
gives the highest octane rating, and the para position gives the lowest.
In the benzene series the opposite effect is noted for the ortho, meta, and para positions. Tables X X and X X I show the effect of position for cyclohexane and benzene hydrocarbons.
1 3 2 E G L O F F AN.D VAN A E tS D E L L : O C T A N E R A T IN G R E L A T IO N S H IP S OF
T a b l e X I X .
C o m p o u n d .
O c t a n e r a t i n g , 3 C . F . R . A . S . T . M .
n - B u t a n e . 9 2
2 - M e t h y l b u t a n e . . . . 8 9
n - P e n t a n e . 61
2 - M e t h y lp e n t a n e . . . . 7 3
3 - M e t h y lp e n t a n e . 7 5
n - H e x a n e . 2 5
2 - M e t h y l h e x a n e . . . . 4 5
ra - H e p t a n e . 0
3 - M e t h y l h e p t a n e . . . . 3 5
c y c Z o P e n t a n e . . . . 8 3
M e t h y lc y c Z o p e n t a n e 8 2
c y c Z o H e x a n e . . . . 77
M e t h y lc y c Z o h e x a n e 71
B e n z e n e . . . . . 1 0 8
M e t h y l b e n z e n e ( T o l u e n e ) 1 0 4
T a b l e X X .
Pure and Blended Compounds. 10
C o m p o u n d .
O c t a n e r a t i n g , r e s e a r c h m e t h o d .
P u r e c o m p o u n d . B l e n d e d c o m p o u n d .
ortho-\: 2 - D im e t h y lc y c Z o h e x a n e 8 6 7 5
m eto -1 : 3 - D im e t h y lc y c Z o h e x a n e 7 7 6 8
p a r a - 1 : 4 - D im e t h y lc y c Z o h e x a n e 7 4 7 5
1 - M e t h y l- 2 - e t h y lc y c Z o h e x a n e . 7 4 5 5
l - M e t h y l- 3 - e t h y lc y c Z ç h e x a n e . 5 8 3 4
l - M e t h y l- 4 - e t h y lc y c Z o h e x a n e . 5 4 2 7
l - M e t h y l- 2 - p r o p y lc y c Z o h e x a n e 4 9 3 7
l - M e t h y l- 3 - p r o p y lc y c Z o h e x a n e 3 9 2 2
1 - M e t h y l- 4 - p r o p y Ic y c Z o h e x a n e 3 4 2 0
1 - M e t h y l - 2 - b u t y lc y c Z o h e x a n e . 3 9 6
l- M e t h y l- 3 - b u t y lc y c Z o h e x a n e . 3 4 — 5
l- M e t h y l- 4 - b u t y lc y c Z o h e x a n e . 2 8 - 5
T a b l e X X I .
HYDROCARBONS, ALCOHOLS, ETHERS, AND KETONES. 1 3 3
Blended Compounds. 10
C o m p o u n d . O c t a n e r a t i n g , r e s e a r c h
m e t h o d , b l e n d e d c o m p o u n d .
ortho- 1 : 2 - D i m e t h y l b e n z e n e 1 2 1
meta- 1 : 3 - D i m e t h y l b e n z e n e 1 4 4
p a r a- 1 : 4 - D i m e t h y l b e n z e n e 1 5 4
l - M e t h y l - 2 - e t h y l b e n z e n e . 1 0 7
l - M e t h y l - 3 - e t h y l b e n z e n e . 1 3 0
l - M e t h y l - 4 - e t h y l b e n z e n e . 1 4 7
1 - M e t h y l - 2 - p r o p y l b e n z e n e 1 1 4
1 - M e t h y l - 3 - p r o p y l b e n z e n e 1 3 0
1 - M e t h y l - 4 - p r o p y l b e n z e n e 1 3 0
l - M e t h y l - 2 - b u t y l b e n z e n e . 1 0 2
l - M e t h y l - 3 - b u t y l b e n z e n e . 1 1 3
l - M e t h y l - 4 - b u t y l b e n z e n e . 1 2 3
l - M e t h y l - 2 - a m y l b e n z e n e . 9 0
l - M e t h y l - 3 - a m y l b e n z e n e . 9 0
l - M e t h y l - 4 - a m y l b e n z e n e . 1 0 0
1 : 3 - D i e t h y l b e n z e n e 1 4 5
1 : 4 - D i e t h y l b e n z e n e 1 5 8
Al c o h o l s, Et h e r s, a n d K e t o n e s.
Oxidation compounds derivable from the hydrocarbons have been considered for some time as products suitable for motor fuels, and in some instances have been widely adopted, although these compounds have not proved entirely satisfactory. Ketones and ethers have been given some attention, but they have not had as wide application as the alcohols.
The following tables and generalizations show the relative octane rating efficiencies of these compounds.
Rule 6.—With the exception of methyl alcohol, the lengthening of the straight chain of carbon atoms in the alcohol molecule lowers the octane rating.
Table X X II shows the octane ratings of the normal alcohols up to pentyl alcohols.
T a b l e X X I I .
Alcohols.S5
C o m p o u n d . O c t a n e r a t i n g , C . F . R . m o t o r m e t h o d , p u r e c o m p o u n d s .
M e t h y l . . . . 9 8
E t h y l . . . . 9 9
n - P r o p y l . . . . 9 0
w - B u t y l . . . . 8 7
n - P e n t y l . . . . 7 8
Up to and inclusive of n-butyl alcohol, the effect of the OH radical seems to he one of lowering the octane rating in comparison with the normal hydrocarbons. Pentyl alcohol shows a higher octane rating than n-pentane; however, at present there are no data to show whether or not this reversal holds for the remainder of the normal alcohols boiling within the gasoline range.
Isomeric alcohols show that chain branching raises the octane rating, which is in direct agreement with Rule 2 shown previously for the hydro
carbons. Table X X III shows the w-alcohols and their isomers and the effect of chain branching on octane number.
1 3 4 E G L O F F AN D V AN A R S D E L L : OCTANE RATING RELATIONSHIPS OF
T a b l e X X I I I .
Alcohols.
C o m p o u n d . O c t a n e r a t i n g , C . F . R . m o t o r m e t h o d . 25
n - P r o p y l . . . . 9 0
i s o P r o p y l . . . . 1 0 4
n - B u t y l . . . . 8 7
i s o B u t y l . . . . 8 8
sec. - B u t y l . . . . 9 2
i e r t . - B u t y l . . . . 1 0 0 +
« - A m y l ( p e n t y l ) . 7 8
tert. - A m y l ( p e n t y l ) 1 0 0 +
The octane ratings of a number of pure ketones have been determined, and, insofar as the data show, the same rules hold for the ketones as for the alcohols. Table X X IV shows the octane ratings of the ketones.
Ta b l e X X I V .
Ketones.
C o m p o u n d . O c t a n e r a t i n g , C . F . R .
m o t o r m e t h o d .
M e t h y l ( a c e t o n e ) 1 0 0
M e t h y l e t h y l 9 9
M e t h y l p e n t y l 8 0
4 - M e t h y l- 3 - p e n t e n - 2 - o n e 91
2 : 6 - D i m e t h y l - 2 : 5 - h e p t a d ie n - 4 - o n e 7 8
The two latter compounds in Table X X IV are not strictly comparable to the preceding ones in the table, but they serve to show somewhat the effect of branching and the effect of the double bond; however, there is no method in this instance of determining the separate effect of either type of structure.
The material presented for the ethers is not on the pure compounds, but was taken from 25 per cent, blends of the pure ethers in aviation gasoline of 74 octane number.26 The figures of over 100 octane number are extra
polated values.
Tables X X V and X X V I show the effect of chain lengthening and chain branching in ethers which is analogous to that of the preceding compounds, with the exception of the ethyl-substituted compound; in each case the octane rating is higher than the methyl compound preceding. There was a slight indication of this phenomenon in the alcohols, but it was not nearly so marked as in the case of the ethers.
HYDROCARBONS, ALCOHOLS, ETHERS, AND KETONES. 1 3 5
T a b l e X X V .
Ether s. 2 6
C o m p o u n d . O c t a n e r a t i n g , C . F . K . m o t o r m e t h o d , b l e n d e d c o m p o u n d .
M e t h y l i e o p r o p y l 7 3
E t h y l i s o p r o p y l . . . . 7 5
M e t h y l - i e r t . - b u t y l 1 1 1
E t h y l - i e r t . - b u t y l 1 1 5
n - P r o p y l - i e r t . - b u t y l 1 0 3
n - B u t y l - i e r t . - b u t y l 81
n - A m y l - t e r t . - b u t y l 6 3
M e t h y l - i e r t . - a m y l 1 0 8
E t h y l - i e r t . - a m y l 1 1 2
T a b l e X X V I .
Effect of Branched Chain in Ethers.
C o m p o u n d . O c t a n e r a t i n g , C . F . R . m o t o r m e t h o d , b l e n d e d c o m p o u n d .
E t h y l - e e c . - b u t y 1 6 3
E t h y l - i e r t . - b u t y 1 1 1 5
n - B u t y l - i e r t . - b u t y l 81
e e c .- B u t y 1 - ie r t .- b u t y l . 1 0 6
From the foregoing tables on the octane rating of alcohols, ketones, and ethers, it is readily seen that these oxygenated compounds follow approxi
mately the same rules on octane rating relationships as the paraffin hydro
carbons.
Th e r m a l R e l a t i o n s h i p s a n d Oc t a n e Ra t i n g s.
The octane ratings and the relationships due to structure presented for the hydrocarbons are not unique, since other types of constants have shown that thermal stabilities, initial combustion temperature, and critical com
pression ratios are also characteristic functions of the structure involved, and are lowered with the increased chain lengths.
Rule 7.—The thermal stability of the pure normal paraffins is
inversely proportional to the length of the carbon chain.27
The following rules are more or less dependent on Rule 6, yet their applicability in engine performance is more readily seen.
jlule g, The initial combustion temperature is lowered as the carbon content of the normal paraffin increases.
Table XXV II shows the lowered initial combustion temperatures with increased carbon chain length.
T a b l e X X V I I .
1 3 6 E G L O F F A N D VAN A R S D E L L I OCTANE RATING RELATIONSHIPS OF
Lowest Temperatures of In itial Combustion in A ir of N orm al Paraffin Hydro
carbons Compared to their Octane Ratings.™
Compound. I n i t i a l c o m b u s t i o n t e m p e r a t u r e , ° C.
O c t a n e r a t i n g .
M o t o r m e t h o d . R e s e a r c h m e t h o d .
M e t h a n e . 615 125 100
E t h a n e 542 125 100
P r o p a n e . 420 * 125 100
B u t a n e 350 * 91 95
P e n t a n e . 295 64 58
H e x a n e 265 59 34
H e p t a n e . 230 * 0 0
O c t a n e 215 —28
N o n a n e 210 - 2 8
D e c a n e 210 -5 3
* T h e s e v a l u e s w e r e i n t e r p o l a t e d f r o m a c u r v e w h i c h w a s p l o t t e d f r o m t h e o th e r v a l u e s s h o w n g iv in g t e m p e r a t u r e s o f i n i t i a l c o m b u s t i o n a g a i n s t t h e n u m b e r o f c a rb o n a t o m s i n t h e n o r m a l p a r a f f in c h a i n s .
Rule 9.—The critical compression ratio of the pure normal alkanes is in inverse order to the carbon chain length, i.e., the highest critical compression ratios are for the lower members of the series.
Tables X X V III and X X IX show the critical compression ratios for the various hydrocarbons as well as the actual operating compression pressures for the test engine and for the automobile engines under approximate test- engine conditions.
Ta b l e X X V I I I .
Paraffins. C r i t i c a l
C o m p r e s s i o n p r e s s u r e , P . S . I .
c o m p r e s s i o n
r a t i o . A c t u a l v a l u e s C . F . R . e n g in e .
A u t o e n g in e u n d e r a p p r o x i m a t e C . F . R .
c o n d i t i o n s .
M e t h a n e E t h a n e . P r o p a n e B u t a n e . P e n t a n e H e x a n e H e p t a n e
15 : 1 14 : 1
12 : 1
6-4:1 3-8 : 1 3-3 : 1
2-8 : 1
349 323 273 130 67
5 5
42
419 388 327
1 5 6 7 8 6 6 5 0
