STUDIES ON OVERCROWDING
IN ORGANIC MOLECULES
B. van de Graaf
le^^/g^
STUDIES ON OVERCROWDING
IN ORGANIC MOLECULES
Proefschrift ter verkrijging van
de graad van doctor in de
technische wetenschappen
aan de Technische Hogeschool Delft,
op gezag van de rector magnificus
prof. ir. L. Huisman,
voor een commissie aangewezen
door het college van dekanen
te verdedigen op
woensdag 14 juni 1978
te 16.00 uur door
Bastiaan van de Graaf
scheikundig ingenieur
geboren te Rotterdam
Delft University Press / 1978
Dit proefschrift is goedgekeurd door de promotor PROF. DR. IR. B. M. WEPSTER
Voor Els
Drawings: J.M. Dijksman
CONTENTS Page
1 INTRODUCTION
1.1 Scope 1
1.2 Synopsis • 2
References 2
2 NITRATION OF 1,2-DI-tert-BUTYLBENZENE
2.1 Introduction 5
2.2 Results and discussion 6
2.3 Experimental part , 10
References and notes - 15
3 BEHAVIOUR OF 1,2-DI-tert-BUTYLBENZENE AND SOME
DERIVATIVES UPON ELECTRON IMPACT
3.1 Introduction 17
3.2 Results and discussion 17
3.3 Experimental part 25
References and notes 25
4 HYDROGENATION OF 1,2-DI-tert-BUTYLBENZENE ON
PLATINUM AND RHODIUM CATALYSTS. SYNTHESIS OF
ais-AND trans-l,2-DI-tert-BUTYLCYCL0HEXANE
4.1 Introduction 29
4.2 Results and discussion 30
4.3 Experimental part 36
References and notes 39
Page
5 CONFORMATIONAL ANALYSIS OF
ais-
AND
trans-l,2-DI--tert-BUTYLCYCLOHEXANE AND SOME HOMOMORPHS
5.1 Introduction 41
5.2 Results and discussion 42
5.3 Conclusions 50
5.4 Experimental part 51
References and notes 53
6 FORCE FIELD CALCULATIONS ON SOME tert-BUTYL
SUBSTITUTED CYCLOHEXANE COMPOUNDS
6.1 Introduction 57
6.2 Calculations 58
6.3 Results 58
6.4 Discussion 58
6.5 Conclusions 70
References and notes 72
SUMMARY AND FINAL REMARKS 75
SAMENVATTING EN SLOTOPMERKINGEN 81
1 INTRODUCTION
1.1 SCOPE
The importance of s t e r i c e f f e c t s in organic chemistry was
recog-nized f i r s t a t the end of the nineteenth c e n t u r y ' . Ever since the
study of such e f f e c t s has been one of the main t o p i c s in p h y s i c a l
--organic chemistry. In our laboratory e s p e c i a l l y the e f f e c t s of
over-crowding both in benzene derivatives2 and in cyclohexane d e r i v a t i v e s ^
have been i n v e s t i a a t e d . The i n v e s t i g a t i o n s presented in t h i s t h e s i s
may be regarded as a sequel to those s t u d i e s . The chemistry of
1,2--di-t<^ri-butylbenzene and d e r i v a t i v e s and the conformational a n a l y s i s
of air,- and t i ' a n s - l , 2 - d i - t e r É - b u t y l c y c l o h e x a n e are two d i s t i n c t t o p i c s
in t h i s t h e s i s . The s t u d i e s of these t o p i c s , however, serve a common
goal: the increase of the knowledge of the e f f e c t s of overcrowding on
the p r o p e r t i e s of organic molecules.
The i n v e s t i g a t i o n s described in t h i s t h e s i s were s t a r t e d a f t e r
s u i t a b l e routes for the synthesis of 1,2-di-ter't-butylbenzene had been
developed'*. I t should be r e c a l l e d , however, t h a t the a t t e n t i o n was
f i r s t focussed upon 1 , 2 - d i - t e r t - b u t y l b e n z e n e as the r e s u l t s of s t u d i e s
of II.C. Brown and coworkers. In a now almost c l a s s i c s e r i e s of papers^
on the analogies between homomorphic compounds these authors predicted
t h a t the then unknown hydrocarbon would be s t r a i n e d by 25 kcal mol"i.
This r e s u l t e d in attempts to synthesize 1 , 2 - d i - t e r t - b u t y l b e n z e n e ,
attempts which eventually were successful'*. The papers of Brown and
coworkers created also a great deal of i n t e r e s t in the physical and
Chemical properties of this highly strained compound. Therefore, when
the investigations on the chemistry of 1,2-di-teri-butylbenzene were
started in this laboratory similar investigations were started or had
been started in other laboratories^. This competition has been a
limiting factor in the scope of the investigations on the chemistry
of l,2-di-io'/'t-butyl benzene presented in this thesis. It has also been
one of the reasons to shift the attention to the conformational
ana-lysis of
cis-
and trans-1,2-di-tr-rt-butylcyclohexane after these
com-pounds had been synthesized^.
1.2 SYNOPSIS
Chapters 2, 3, and 4 of this thesis deal with the chemistry of
1,2-di-tert-butylbenzene and derivatives, chapters 5 and 6 concern
the conformational analysis of
ais-
and
tr'(2ns-l,2-di-iert-butylcyclo-hexane.
Chapter 2 gives the results of the nitration of
1,2-di-te>^t-butyl-benzene^.
Chapter 3 discusses the behaviour of l,2-di-i:ert-butylbenzene and
some derivatives upon electron impact'^.
Chapter 4 gives the results of the hydrogenation of
1,2-di-tert--butylbenzene on Rh and Pt catalysts. It also describes the preparation
of
'-'•;;-
and traws-l ,2-di-ter't-butylcyclohexane^.
Chapter 5 discusses the conformations of
ais-
and
tran,s-l,2-di-t<2r't--butylcyclohexane and some homomorphic compounds on the basis of
spectroscopic data'".
Chapter 6 gives the results of empirical force field calculations
on some tert-butyl substituted cyclohexane compounds. The results are
compared with available experimental data''.
REFERENCES
' For some leading references see:
S.H. linger
and
C. Hansoh,
Prog.
Phys. Org. Chem. j ^ , 91 (1976).
2 See for i n s t a n c e : B.M. Wepster, Progr. in Sterochem. 2, 99 (1958);
H. Kofod, L.E. Sutton, P.E. Verkade and B.M. Wepster, Reel. Trav.
Chim. Pays-Bas 78, 790 (1959); J.M.A. Baas and B.M. Wepster, Reel.
Trav. Chim. Pays-Bas 9^1, 831 (1972); and J.M.A. Baas, J.M. van der
Toom and B.M. Wepster, Reel. Trav. Chim. Pays-Bas 9 3 , 133 (1974).
In these papers references to e a r l i e r s t u d i e s can be found.
^ See for i n s t a n c e : H. van Bekkum, M.A. Hoefnagel, L. de Lavieter,
A. van Veen, P.E. Verkade, A. Wemmers, B.M. Wepster, J.H. Palm,
L. Schafer, H. Dekker, C. Mosselman and G. Somsen, Reel. Trav. Chim.
Pays-Bas 86, 1363 (1967); H. van Bekkwv, B. van de Graaf, G. van
Minnen-Pathuis, J.A. Peters and B.M. Wepster, Reel. Trav. Chim.
Pays-Bas 89, 521 (1970); and J.D. Remijnse, H. van Bekkum and B.M.
Wepster, Reel. Trav. Chim. Pays-Bas 9_3, 93 (1974). In these papers
references to e a r l i e r s t u d i e s can be found.
"* C. Hoogzand and W. Hubel, Angew. Chem. 73, 680 (1961); E.M. Amett
and M.E. strem, Chem. Ind. 1961, 2008; L.R.C. Barclay, C.E.
MilUgan and N.D. Hall, Can. J . Chem. 40, 1664 (1962); and A.W.
Burgstahler and M.0. Ahdel-Rahman, J . Am. Chem. Soc. 8 5 , 173 (1963).
^ H.C. Brown, G.K. Barbaras, H.L. Bemeis, W.H. Bonner, R.B.
Johannesen, M. Grayson and K. LeBoi Nelson, J . Am. Chem. Soc. 75,
1 (1953); H.C. Brown and X. LeRoi Nelson, J . Am. Chem. Soc. 75, 24
(1953); and H.C. Brown, D. Gintis and L. Domash, J . Am. Chem. Soc.
78, 5387 (1956).
^ For a review s e e : E.M. Amett, J.C. Sanda, J.M. Bollinger and M.
Barber, J . Am. Chem. Soc. 89, 5389 (1967).
^ B. van de Graaf, H. van Bekkum and B.M. Wepster, Reel. Trav. Chim.
Pays-Bas 87, 777 (1968).
^ B. van de Graaf and B.M. Wepster, Reel. Trav. Chim. Pays-Bas 85,
619 (1966).
^ B. van de Graaf, Reel. Trav. Chim. Pays-Bas, submitted.
" B. van de Graaf, H. van Bekkum, H. van Koningsveld, A. Sinnema,
A. van Veen, B.M. Wepster and A.M. van Wijk, Reel. Trav. Chim.
Pays-Bas W , 135 (1974).
B. van de Graaf and B.M. Wepster, Tetrahedron Letters 1975, 2943;
B. van de Graaf, J.M.A. Baas and B.M. Wepster, Reel. Trav. Chim.
Pays-Bas, accepted.
2 NITRATION OF l,2-tej>t-BUTYLBENZENE'
2.1 INTRODUCTION
In the l i t e r a t u r e the n i t r a t i o n o f 1 , 2 - d i - t e r t - b u t y l b e n z e n e i s mentioned t w i c e . Olah and Kuhn^ use n i t r o n i u m t e t r a f l u o r o b o r a t e i n tetramethylene sulfone as n i t r a t i n g agent and r e p o r t t h a t , i n a d d i t i o n to d i s p r o p o r t i o n a t i o n , n i t r a t i o n takes place w i t h 4 n i t r o l , 2 d i t e r t --butylbenzene as the only product. They mention no physical constants of t h e i r product and give no proof o f i t s s t r u c t u r e . Burgstahler e t a l . 3 describe the n i t r a t i o n of 1 , 2 - d i - t e r t - b u t y l b e n z e n e w i t h n i t r i c acid i n a c e t i c anhydride. With n i t r i c acid d 1.42 they r e p o r t mono-n i t r a t i o mono-n t o 4 - mono-n i t r o - 1 , 2 - d i - t e r t - b u t y l b e mono-n z e mono-n e , which they describe as a y e l l o w o i l . With n i t r i c acid d 1.50 they f i n d d i n i t r a t i o n t o 4 , 5 -- d i n i t r o -- l , 2 -- d i -- t e r t -- b u t y l b e n z e n e , a c o l o u r l e s s compound w i t h m.p. 149-150 . T h e i r s t r u c t u r e assignments are based on the elemental analyses, the NMR s p e c t r a , and the f a c t t h a t the d i n i t r o d e r i v a t i v e , a f t e r r e d u c t i o n to the diamino d e r i v a t i v e , can be converted i n t o a q u i n o x a l i n e d e r i v a t i v e .
Our i n v e s t i g a t i o n o f the n i t r a t i o n of 1 , 2 - d i - t e r t - b u t y l b e n z e n e , which was already i n an advanced stage when the above-mentioned p u b l i c a t i o n s appeared, y i e l d e d us the f o l l o w i n g , p a r t l y d i f f e r e n t r e s u l t s .
2.2 RESULTS AND DISCUSSION
The r e s u l t s f o r the m o n o n i t r a t i o n o f 1 , 2 - d i - t e j ' t - b u t y l b e n z e n e w i t h fuming n i t r i c acid i n a c e t i c a c i d - a c e t i c anhydride and f o r the f u r t h e r n i t r a t i o n of the mononitro d e r i v a t i v e s w i t h fuming n i t r i c acid i n a c e t i c anhydride are summarized i n the f o l l o w i n g scheme.
Mononitration of 1,2-di-tert-butylbenzene
The main product of the m o n o n i t r a t i o n , 4 n i t r o l , 2 d i t e r ' t b u t y l -benzene, a pale y e l l o w compound w i t h m.p. 38.8-39.2 , could be ob-t a i n e d by c r y s ob-t a l l i z a ob-t i o n o f ob-the crude r e a c ob-t i o n producob-t from meob-thanol. The s t r u c t u r e assigned f o l l o w s from the elemental a n a l y s i s , the NMR spectrum, and the u l t r a v i o l e t spectrum. The NMR spectrum shows two s i n n l e t s at T 8.41 and T 8 . 3 9 , both w i t h r e l a t i v e i n t e n s i t y nine f o r the t e r t - b u t y l p r o t o n s , and a complicated signal f o r the aromatic protons i n an ABK-pattern; AB p a r t a t T 2 . 0 - 2 . 5 and K p a r t at T 1 . 5 - 1 . 6 , The coupling constants J . p ~ 9.0 eps, J.y^ - 2.2 eps, and Jau ~ 0.6 eps p o i n t t o a 1 , 2 , 4 - t r i s u b s t i t u t e d benzene d e r i v a t i v e . The
u l t r a v i o l e t spectrum shows a maximum a t 271.5 nm w i t h an absorption intens'^ty of 11,000, which points to a nitrobenzene d e r i v a t i v e i n which the n i t r o group i s not i n o r t ^ o - p o s i t i o n r e l a t i v e t o a
-butyl group"* .
The second n i t r a t i o n product, 3 - n i t r o - l ,2-di-•,...'-butylbenzene, a l s o
pale yellow and c r y s t a l l i n e , m.p. 87.7-88.2'^, could be i s o l a t e d by the
a p p l i c a t i o n of, inter alia, p r e p a r a t i v e gas chromatography. The s t r u c
-t u r e was assigned on -the basis of -the elemen-tal analysis and -the NMR
spectrum. This spectrum shows two s i n g l e t s a t T 8.48 and T 8.44, both
with r e l a t i v e i n t e n s i t y nine for the t ^ r t - b u t y l protons, and a
com-p l i c a t e d signal for the aromatic com-protons in an ABK-com-pattern; AB com-part a t
T 2.6-3.1 and K part a t T 2 . 3 - 2 . 5 . The coupling constants ^.g - 8 e p s ,
Joy ~ 8 eps, and c/ = 2 eps point to a nonsymmetrical 1 , 2 , 3 t r i s u b
-s t i t u t e d benzene d e r i v a t i v e .
In view of the fact t h a t the above-mentioned r e s u l t s deviate
con-siderably from those reported by Olah and Kuhn^ and by Burgstahler e t
a l . ^ we have also reproduced the procedures of these a u t h o r s . In both
cases the formation of a considerable amount of 3 - n i t r o - l
,2-di-ici".'--butylbenzene was found. The same holds for a n i t r a t i o n with benzoyl
n i t r a t e in a c e t o n i t r i l e . The r a t i o s of the isomers for a l l the
mono-n i t r a t i o mono-n s c a r r i e d out by us are givemono-n imono-n Table I.
Table I Mononitration of 1,2-di-i;e2't-butylbenzene
N i t r a t i o n method
Ratio of isomers
3 - n i t r o 4 - n i t r o
N i t r i c acid in a c e t i c a c i d - a c e t i c anhydride 13% ~ 87%
Benzoyl n i t r a t e in acetoni t r i l e 17% 83°/
N i t r i c a c i d i n a c e t i c a n h y d r i d e ; p r o c e d u r e of
Burgstahler et a] .'^ 15% 85%
Nitronium t e t r a f l u o r o b o r a t e in t e t r a m e t h y l e n e
s u l f o n e ; p r o c e d u r e of Olah and Kuhn^ 15% 85%
A comparison of the above r e s u l t s w i t h the data of Burgstahler e t a l . ' ^ i n d i c a t e s t h a t the 4 - n i t r o - 1 , 2 - d i - t e } ' t - b u t y l b e n z e n e described by them cannot have been pure, but must have contained 3 n i t r o 1 , 2 d i --tü>^t-butylbenzene.
In a l l the n i t r a t i o n procedures the percentage of 3 n i t r o l , 2 d i -- t c j ' t -- b u t y l b e n z e n e i s remarkably h i g h . stook'=> obtained 10.3% of - 2-- n i t r o 2-- l 2-- t e r 2-- t 2-- b u t y l b e n z e n e and 79.4% o f 4 2-- n i t r o 2-- l 2-- t e r / : 2-- b u t y l b e n z e n e upon n i t r a t i o n of t e r t - b u t y l b e n z e n e w i t h n i t r i c a c i d - a c e t i c anhydride In the case of a d d i t i v i t y of s u b s t i t u e n t e f f e c t s only about 6% of 3 n i t r o l , 2 d i t e 7 ' t b u t y l b e n z e n e would have to be formed from 1 , 2 d i -- t e r t -- b u t y l b e n z e n e under s i m i l a r c o n d i t i o n s . The high percentage may be due t o a n o n - p l a n a r i t y of the aromatic r i n g ^ , or even only t o i t s being more f l e x i b l e , i n consequence o f which the t r a n s i t i o n s t a t e f o r n i t r a t i o n on the 3 - p o s i t i o n o f l,2-di-Éer>t-butylbenzene would have less s t e r i c s t r a i n than t h a t f o r the o r t / j o n i t r a t i o n of t e r t b u t y l benzene. In t h i s connection i t i s s t r i k i n g t h a t 3 n i t r o l , 2 d i t e p t butylbenzene absorbs more s t r o n g l y a t 260 nm than does 2 n i t r o l , 4 -- d i -- t s T ' t -- b u t y l b e n z e n e (e being 3050 and 1250 r e s p e c t i v e l y ) , which p o i n t s t o a smaller t w i s t i n g o f the n i t r o group"*.
Further nitration of the mononitro derivatives
The main product i n the n i t r a t i o n o f 4 n i t r o l , 2 d i t e i ' t b u t y l benzene, the p r a c t i c a l l y c o l o u r l e s s 4 , 5 d i n i t r o 1 , 2 d i t e r t b u t y l -benzene, m.p. 150.5-151.5 , could be i s o l a t e d from the r e a c t i o n pro-duct by c r y s t a l l i z a t i o n . The s t r u c t u r e f o l l o w s from the elemental a n a l y s i s , the NMR spectrum, the d i p o l e moment, and the u l t r a v i o l e t spectrum. The NMR spectrum shows a s i n g l e t a t T 8.38 w i t h r e l a t i v e i n t e n s i t y eighteen f o r the t e r t - b u t y l protons and a s i n g l e t a t T 1.87 w i t h r e l a t i v e i n t e n s i t y two f o r the aromatic p r o t o n s . Both the two t e r t - b u t y l groups and the two aromatic protons t h e r e f o r e are equiva-l e n t . The high d i p o equiva-l e moment o f 7.17 D i n d i c a t e s t h a t the two n i t r o groups are ortho r e l a t i v e to each o t h e r . F i n a l l y we can also exclude 2 , 3 - d i n i t r o - l , 4 - d i - t e j > t - b u t y l b e n z e n e as a l t e r n a t i v e s t r u c t u r e i n view
of the u l t r a v i o l e t spectrum. The absorption i n t e n s i t y of the maximum
a t 266 nm (c 7550) is of the r i g h t order of magnitude for an
undisturb-ed ::;.r':P2o-dinitro system'''", whilst for an r r t ^ o - d i n i t r o system between
two te>'t-butyl groups a much lower absorption i n t e n s i t y must be ex
pected'*. Considering the s i m i l a r i t y of the melting point and the
s p e c t r a , t h i s product i s no doubt i d e n t i c a l with the 4 , 5 d i n i t r o 1 , 2
-- d i -- t c r t -- b u t y l b e n z e n e described by Burgstahler e t a l . ^ .
From the NMR spectrum of a crude n i t r a t i o n mixture of 4 n i t r o l , 2
d i t e r t b u t y l b e n z e n e i t appeared t h a t in addition to 4 , 5 d i n i t r o 1 , 2
-- d i -- t e r t -- b u t y l b e n z e n e a second product was p r e s e n t . This second -
pro-d u c t , which coulpro-d also be pro-detectepro-d with t h i n - l a y e r chromatography on
aluminium o x i d e , in view of the NMR spectrum i s i d e n t i c a l with the
3 , 5 - d i n i t r o - l , 2 - d i - t . ? r t - b u t y l benzene described below.
The main product in the n i t r a t i o n of 3 n i t r o l , 2 d i t e r ' b u t y l
-benzene, viz. 3,5-dini tro-1,2-di-,';f;rt-butyl -benzene, a l i g h t yellow
compound with m.p. 104-106.0 , was i s o l a t e d with the aid of
prepa-r a t i v e t h i n - l a y e prepa-r chprepa-romatogprepa-raphy. The s t prepa-r u c t u prepa-r e follows fprepa-rom the
elemental a n a l y s i s , the NMR spectrum, and the u l t r a v i o l e t spectrum.
The NMR spectrum shows two s i n g l e t s a t T 8.43 and T 8.38, both with
r e l a t i v e i n t e n s i t y nine for the tertbuty^ p r o t o n s , and for the a r o
-matic protons two doublets in an AK-pattern; A part a t ~ 1.47 and K
p a r t at T 1.80. The coupling constant J.^^ - 2.5 eps points to two
non-equivalent aromatic protons in w e t a - p o s i t i o n . The absorption
i n t e n s i t y of the maximum a t 266 nm (c 9550) points to the presence
of a non-hindered n i t r o group"*.
In the crude n i t r a t i o n mixture of 3 - n i t r o - l , 2 - d i - r e r ' t - b u t y l b e n z e n e
both NMR spectrometry and t h i n - l a y e r chromatography on aluminium
oxide pointed to the presence of a second product. The NMR spectrum
of this product, which has not yet been i s o l a t e d , shows for the
aromatic protons two doublets in an AB-pattern; A part at T 2.22 and
B part at T 2.42. The coupling constant J.„ - 9 eps points to two
non-equivalent aromatic protons in ortho-position. In combination with
t h e formation of the product from 3 - n i t r o - 1 , 2 - d i - t e r t - b u t y l b e n z e n e
t h i s makes the s t r u c t u r e 3 , 4 - d i n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e h i g h l y probable.
I t i s f u r t h e r to be noted t h a t f o r the d i n i t r a t i o n s i t was found necessary t o f o l l o w the procedure of Burgstahler e t a l . ^ and t o add the mononitro p r o d u c t , dissolved i n a c e t i c a n h y d r i d e , to the n i t r i c a c i d . When the n i t r i c acid and the a c e t i c anhydride were p r e v i o u s l y mixed, no r e a c t i o n was observed.
2.3 EXPERIMENTAL PART
The NMR spectra were obtained from s o l u t i o n s i n carbon t e t r a -c h l o r i d e or d e u t e r i o -c h l o r o f o r m w i t h a Varian A-60 spe-ctrometer, the u l t r a v i o l e t spectra w i t h a Beekman DK-2 spectrophotometer, and the i n f r a r e d s p e c t r a , o f which only a few c h a r a c t e r i s t i c absorptions between 1250 and 850 cm"' are mentioned, w i t h a P e r k i n Elmer 521 spectrophotometer. Dipole moments were determined i n benzene w i t h the a i d o f a Wiss. Techn. Werkstatten d i p o l e meter, while f o r the c a l c u l a t i o n the method of Halverstadt and Kumler^ was used. For the gas chromatography, w i t h a katharometer as d e t e c t o r and hydrogen as c a r r i e r gas, the a n a l y t i c columns used were an Apiezon column,
3.9 m X 4 mm 25% Apiezon N on Chromosorb W 42-60 mesh, and a s i l i c o n e column, 4 m X 4 mm 20% s i l i c o n e o i l on Chromosorb W 42-60 mesh. For the p r e p a r a t i v e s e p a r a t i o n a s i l i c o n e column, 4 m x 12 mm 20% s i l i c o n e o i l on Camag k i e s e l g u h r 60-80 mesh was used.
1,2-di-tert-butylbenzene
1 , 2 - d i - t e r t - b u t y l b e n z e n e was prepared according t o Hoogzand and HübeÜ^ from d i - t ï s r f c - b u t y l a c e t y l e n e " ; y i e l d 33% ( l i t e r a t u r e : 31%'°, 50%'2), m.p. 2 6 . 5 - 2 3 . 0 ° ( l i t e r a t u r e : 2 4 - 2 7 ° ^ , 2 7 . 5 - 2 8 . 5 ° ' ° , 2 7 - 2 8 ° ' ^ ) .
Nitration of 1, 2-di-tert-butylbenzene with fuming nitric acid in
acetic acid-acetic anhydride
To 7.50 g of 1 , 2 - d i - t e r t - b u t y l b e n z e n e d i s s o l v e d i n 37.5 ml o f a c e t i c anhydride and 15 ml o f a c e t i c a c i d , at 0° 5.25 ml of n i t r i c
acid d 1.52 in 22.5 ml of a c e t i c acid was added dropwise with
s t i r r i n g in one hour. The reaction mixture was l e f t to i t s e l f at room
temperature and a f t e r four days was poured out i n t o i c e - w a t e r . The
mixture was e x t r a c t e d with pentane. The pentane e x t r a c t was washed
with 2 .7 potassium hydroxide and then with water, and was dried on
magnesium s u l f a t e . Gas chromatography of t h i s s o l u t i o n a t 180 on the
s i l i c o n e column showed a complete conversion of the l , 2 d i t e " t
butylbenzene and the presence of two products, 13%'"* of 3 n i t r o l , 2
-- d i -- t e r t -- b u t y l b e n z e n e , r e l a t i v e r e t e n t i o n time with respect to -
1,2-- d i 1,2-- t e r t 1,2-- b u t y l b e n z e n e 3 . 0 , and 87% of 4 1,2-- n i t r o 1,2-- 1 , 2 1,2-- d i 1,2-- t e r t 1,2-- b u t y l b e n z e n e ,
r e l a t i v e r e t e n t i o n time 4 . 1 . F i l t r a t i o n and evaporation of the pentane
gave 9.4 g of a yellow o i l , from which, by d i s s o l u t i o n in methanol and
cooling to -30 , 6.45 g of crude 4 - n i t r o - 1 , 2 - d i - t e r ' t - b u t y l b e n z e n e ,
m.p. 32-35 , was obtained. Repeated c r y s t a l l i z a t i o n s from methanol
gave 4.80 g (52%) of 4 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e , which according
to gas chromatography was pure; pale yellow, almost c o l o u r l e s s needles
with m.p. 3 8 . 8 - 3 9 . 2 ° .
Found : C 71.6 ; H 9.0 ; N 5.9
Calc. for Cii,H2iN02 (235.33): C 71.45; H 9.00; N 5.95.
U l t r a v i o l e t spectrum in iso-octane {\ in nm, e ) : 380, 60; 360, 160;
340, 280; 320, 630; 310, 1600; 300, 3450; 290, 6300; 280, 9300; max.
271.5, 11,000; 260, 8450; 250, 4800; min. 236.5, 2350; 230, 3600; 220,
9400; 215, 11,700; 210, 14,200.
Infrared spectrum in CCli,: 1230, 1188, 1125, 1054, and 898 c m ' ' .
Dipole moment: 4.79 D in benzene a t 25°.
The combined mother liquors were evaporated and the residue was
separated with the aid of preparative gas chromatography through a
s i l i c o n e column. Two dark-coloured f r a c t i o n s were obtained, viz. 460
mg of impure 3 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e and 1120 mg of impure
4 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n 2 e n e . The low y i e l d is probably due to
decomposition in the evaporation chamber. Chromatography of the impure
3 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e over neutral aluminium oxide with
petroleum e t h e r 60-80° as eluent gave 430 mg of a l i g h t y e l l o w o i l c o n t a i n i n g 90% o f 3 - n i t r o - 1 , 2 - d i - t s r t - b u t y l b e n z e n e (gas chromatography). D i s s o l u t i o n i n ethanol and c o o l i n g to -30 gave 255 mg o f crude
3-- n i t r o 3-- 1 , 2 3-- d i 3-- t e r t 3-- b u t y l b e n z e n e , m.p. 843--88 . R e c r y s t a l 1 i z a t i o n from ethanol gave 220 mg (2.4%) of product, which according to gas chroma-tography was pure; pale y e l l o w , almost c o l o u r l e s s needles w i t h m.p. 3 7 . 7 - 8 8 . 2 ° . Found : C 71.5 ; H 9.1 ; N 5.8 Calc. f o r CiitH2iN02 ( 2 3 5 . 3 3 ) : C 71.45; H 9 . 0 0 ; N 5.95. U l t r a v i o l e t spectrum i n iso-octane (X i n nm, c ) : 400, 70; 380, 250; 360, 500; 340, 720; 320, 950; 304-296, 1050; 290, 1100; 280, 1550; 270, 2350; 260, 3050; 250, 3950; 240, 6000; 230, 9700; 220, 12,000; 210, 15,100. I n f r a r e d spectrum i n CCl^: 1240, 1215, 1184, 1055, and 898 c m " ' . Dipole moment: 4.11 D i n benzene at 25 .
Nitration of 1,2-di-tert-butylbenzene with benzoyl nitrate in
aceto-ni trile
To a s o l u t i o n of 19 mg of 1 , 2 - d i - t e r t - b u t y l b e n z e n e and 51 mg of s i l v e r n i t r a t e i n 0.5 ml of a c e t o n i t r i l e was added w i t h s t i r r i n g 42 mg of benzoyl c h l o r i d e i n 0.25 ml of a c e t o n i t r i l e a t 0 . A f t e r f o u r days a t room temperature, 2 ml of saturated sodium c h l o r i d e s o l u t i o n was added and the mixture was e x t r a c t e d w i t h e t h e r . The e t h e r e a l e x t r a c t was washed w i t h 2 /'.' potassium hydroxide and then w i t h w a t e r , and was d r i e d on magnesium s u l f a t e . Gas chromatography a t 220 on the Apiezon column showed t h a t , i n a d d i t i o n to non-converted 1 , 2 - d i - t e r t - b u t y l b e n z e n e , 3 - n i t r o - 1 , 2 - d i - t e r t - b u t y l b e n z e n e and 4-- n i t r o 4-- 1 , 2 4-- d i 4-- t e r t 4-- b u t y l b e n z e n e were present i n a r a t i o of 17:93; r e l a t i v e r e t e n t i o n times w i t h respect t o 1 , 2 - d i - t e r t - b u t y l b e n z e n e 3.7 and 5.7 r e s p e c t i v e l y .
Nitration of 1, 2-di-tert-butylbenzene according to the procedure of
Burgstahler e t al. ^
190 mg of 1 , 2 - d i - t e r t - b u t y l b e n z e n e was n i t r a t e d according to the
procedure of Burgstahler et a l . ^ . Analysis of the reaction product
with gas chromatography a t 180 on the s i l i c o n e column showed t h a t ,
in addition to nonconverted 1 , 2 d i t e r t b u t y l b e n z e n e , 3 n i t r o l , 2
-- d i -- t e r t -- b u t y l benzene and 4--ni t r o -- 1 , 2 -- d i -- / ; e r t -- b u t y l benzene were
p r e s e n t in a r a t i o of 15:85.
Nitration of 1,2-di-tert-butylbenzene according to the procedure of
Olah and Kuhn2 , • _
570 mg of 1 , 2 - d i - t e r t - b u t y l b e n z e n e was n i t r a t e d according to the
procedure of Olah and Kuhn^. Analysis of the reaction product with
gas chromatography a t 180 on the s i l i c o n e column showed t h a t , in
a d d i t i o n to l a r g e amounts of non-converted 1 , 2 - d i - t e r t - b u t y l b e n z e n e ,
inter alia 3 n i t r o 1 , 2 d i t e r t b u t y l b e n z e n e and 4 n i t r o 1 , 2 d i t e r t
--butylbenzene were present in a r a t i o of 15:85.
Nitration of 4-nitro-2,2-di-tert-butylbenzene
A solution of 1.00 g of 4 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e in 6 ml
of a c e t i c anhydride was added dropwise with s t i r r i n g a t -10 to 4 ml
of n i t r i c acid d 1.52. The reaction mixture was l e f t to i t s e l f a t
room temperature and a f t e r two hours was poured out into i c e - w a t e r .
The mixture was e x t r a c t e d with pentane. The pentane solution was
washed with a s a t u r a t e d sodium bicarbonate solution and then with
w a t e r , and was dried on magnesium s u l f a t e . After f i l t r a t i o n , the
pentane was evaporated and the r e s i d u e , a yellow o i l , taken up in
e t h a n o l . Cooling to 30 gave 600 mg crude 4 , 5 d i n i t r o l , 2 d i t e r t
butylbenzene; l i g h t yellow l e a f l e t s with m.p. 148150°. R e c r y s t a l
-l i z a t i o n gave 513 mg (42%) of pa-le ye-l-low, p r a c t i c a -l -l y c o -l o u r -l e s s
l e a f l e t s with m.p. 150.5-151.5° {Burgstahler et a l . ^ 149-150°).
Found C 60.2 ; H 7.2 ; N 10.1
Calc. for Ci4H2oN20it (280.33): C 59.98; H 7.19; N 9.99
U l t r a v i o l e t spectrum in iso-octane (A in nm, e): 400, 36; 380, 190;
360, 520; 340, 950; 320, 1450; 310, 1850; 300, 2450; 290, 3400; 280,
5400; 270, 7350; max. 266, 7550; 260, 7200; min. 254, 6700; 250, 7200;
240, 10,600; 230, 12,800; max. 220, 15,000; 215, 14,600.
Infrared spectrum in CCK: 1230, 1211, 1195, 900, and 857 cm"'.
Dipole moment: 7.17 D in benzene a t 25 .
Attempts to n i t r a t e 4 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e with a
pre-viously prepared mixture of n i t r i c acid and a c e t i c anhydride were
unsuccessful, p r a c t i c a l l y no conversion taking place.
When the above n i t r a t i o n was repeated on a smaller s c a l e , s t a r t i n g
from 100 mg of 4 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e , a NMR spectrum of
the crude reaction product showed, in addition t o the 4 , 5 d i n i t r o
-- 1 , 2 -- d i -- t e r t -- b u t y l b e n z e n e , 12% of a second component, which was
i d e n t i f i e d by means of t h i s spectrum as 3 , 5 d i n i t r o l , 2 d i t e r t
--butylbenzene. Again with t h i n - l a y e r chromatography on aluminium
oxide, petroleum e t h e r 60-80 being used as e l u e n t , only two products
were shown to be p r e s e n t . R^ values for 3 , 5 d i n i t r o l , 2 d i t e r t
--butylbenzene and 4 , 5 - d i n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e 0.38 and 0.20
r e s p e c t i v e l y .
Nitration of 3-nitro-l, 2-di-tert-butylbenzene
A s o l u t i o n of 70.5 mg of 3 - n i t r o - l , 2 - d i - t e r t - b u t y l b e n z e n e in 1 ml
of a c e t i c anhydride was added dropwise with s t i r r i n g at -5 to 1 ml
of n i t r i c acid d 1.52. The reaction mixture was l e f t to i t s e l f at
room temperature and poured out a f t e r four hours into i c e - w a t e r . The
mixture was e x t r a c t e d with pentane. After being washed successively
with a s a t u r a t e d sodium bicarbonate solution and with w a t e r , the
pentane e x t r a c t was dried on magnesium s u l f a t e . Thin-layer
chromato-graphy on aluminium oxide, petroleum e t h e r 60-80° being used as
e l u e n t , showed the presence of two products, R, values 0.18 and 0 . 3 8 ,
of which the l a t t e r was c l e a r l y the main product. After f i l t r a t i o n ,
the pentane was evaporated. The NMR spectrum of the residue was
determined, from which a composition of 14% of 3 , 4 d i n i t r o l , 2 d i
-tert-butylbenzene and 86% of 3,5-dinitro-l,2-di-tert-butylbenzene
could be deduced. Preparative thin-layer chromatography (thickness of
layer 1 mm) on aluminium oxide, petroleum ether 60-80 being used as
eluent, gave 37 mg (44%) of the main product
3,5-dinitro-l,2-di-tert--butylbenzene as yellow crystals with m.p. 104-106 . Crystallization
from ethanol gave 26 mg of light yellow needles with m.p. 104.5-106.0 .
Found C 60.1 ; H 7.3 ; N 9.9
Calc. for C14H20N2O4 (280.33): C 59.98; H 7.19; N 9.99
Ultraviolet spectrum in iso-octane (A in nm, E ) : 380, 180; 360, 410;
340, 840; 320, 1550; 300, 2600; 290, 5250; 280, 8150; max. 266, 9550;
250, 8600; min. 236, 7250; 230, 8050; 220, 13,700; max. 215, 15,300.
Infrared spectrum in KBr: 1228, 1176, 1052, 912, and 874 cm"'.
ACKNOWLEDGEMENTS
We thank Mr.
A. van Veen
and Mr.
A. Sinnema
for measuring and
interpreting the NMR spectra, the late Mr.
M. van Leeuwen
for carrying
out the elemental analyses, Mr.
U.J. van Benschop
for measuring the
infrared spectra, and Mr.
M.A. Hoefnagel
for determining the dipole
moments.
REFERENCES AND NOTES
' B. van de Graaf and B.M. Wepster, Reel. Trav. Chim. Pays-Bas 85,
619 (1966).
2
G.A. Olah
and
S.J. Kuhn,
J. Am. Chem. Soc. 86, 1067 (1964).
^ a)
A.W. Burgstahler, M.0. Abdel-Rahman
and
Ping-Lu Chien,
Tetra-hedron Letters 1964, 61; b) A.W. Burgstahler, Ping-Lu Chien
and
M.0. Abdel-Rahman, J. Am. Chem. Soc. 86, 5281 (1964).
"* See
e.g.
:
J. Burgers, M.A. Hoefnagel, P.E. Verkade, H. Visser
and
B.M. Wepster,
Reel. Trav. Chim. Pays-Bas 77, 491 (1958).
5
L.M. Stock,
J. Org. Chem. 26, 4120 (1961).
^ L.R.C. Barclay. C.E. MilUgan
and
N.D. Hall,
Can. J. Chem. 40,
^ G. Kortüm, Z. Physik. Chem. (Leipzig), B 42, 53 (1939).
8 p. Grammatiaakis ,
Buil. Soc. Chim. France (5) 2J., 103 (1954).
5 I.F. Halverstadt
and
W.D. Kumler, J. Am. Chem. Soc. 64, 2988 (1942).
'O C. Hoogzand
and !•/.
Hübel, Angew. Chem. 7_3, 680 (1961).
" G.F. Hennion and T.F. Banigan, Jr., J. Am. Chem. Soc. 68, 1202
(1946).
'2 c. Hoogzand, private communication.
13 A.W. Burgstahler
and M.O.
Abdel-Rahman, J. Am. Chem. Soc. 85, 173
(1963).
''* For the calculation of the percentages from the areas of the peaks
in the chromatograms it has been assumed that the heat conductivity
for the two isomers is the same.
3 BEHAVIOUR OF 1,2-DI-tert-BUTYLBENZENE AND SOME DERIVATIVES UPON
ELECTRON IMPACT'
3.1 INTRODUCTION i
I n several cases2 s t e r i c s t r a i n i n n e u t r a l molecules has been c o r r e l a t e d w i t h i o n i z a t i o n or appearance p o t e n t i a l d a t a . An example f r e q u e n t l y r e f e r r e d t o i s the c o r r e l a t i o n between the d i f f e r e n c e s i n s t e r i c s t r a i n o f the isomeric d i t e r t b u t y l b e n z e n e s and the d i f f e r -ences i n the appearance p o t e n t i a l s of t h e i r [M-15] i o n s ^ ' S . To e x p l a i n t h i s c o r r e l a t i o n i t has been suggested^ t h a t the molecular i o n o f 1 , 2 - d i - t e r t - b u t y l b e n z e n e isomerizes t o t h a t o f 1,3- and/or 1,4-- d i 1,4-- t e r t 1,4-- b u t y l b e n z e n e , c o n v e r t i n g i n t h i s process the s i z a b l e s t e r i c s t r a i n to i n t e r n a l energy. Considering the i n t e r e s t i n the appearance p o t e n t i a l data i t i s s u r p r i s i n g t h a t the mass spectrum i t s e l f o f 1,2-- d i 1,2-- t e r t 1,2-- b u t y l b e n z e n e has never been published although i t has been measured several t i m e s ^ » ^ .I n t h i s paper mass s p e c t r a l data f o r 1 , 2 - d i - t e r t - b u t y l b e n z e n e and some of i t s d e r i v a t i v e s are compared w i t h those of l e s s s t r a i n e d i s o -mers. The f o r m a t i o n and the subsequent fragmentation of the [M-15]
ions of the i s o m e r i c d i - t e r t - b u t y l b e n z e n e s i s given special a t t e n t i o n . The s i g n i f i c a n c e of the d i f f e r e n c e s i n the appearance p o t e n t i a l s of these ions i s d i s c u s s e d .
3.2 RESULTS AND DISCUSSION
Mass spectrasubsti-Table I P a r t i a l mass spectra^ o f some d i - and t r i - t e r t - b u t y l s u b s t i t u t e d benzene d e r i v a t i v e s Benzene derivativ 1,2-di-tBu 1,2,4-tri-tBu 1,3-di-tBu 1,4-di-tBu 1,3,5-tri-tBu 4-N02-l,2-di-tBu 4,5-di-N02-l,2-di es -tBu 5-N02-l,2,4-tri-tBu 5-N02-l,3-di-tBu 3-N02-l,2-di-tBu 3,5-di-N02-l,2-di 2-N02-l,4-di-tBu 4-Br-l,2-di-tBu^ -tBu
Mt
44(5) 22(6) 20(7) 14(6) 14(6) 41(6) 21(5) 22(4) 11(4) 4(1.3) .5(.3) 38(9) 45(8) Relative [M-15]" 78(9) 76(21) 100(37) 100(40) 100(43) 74(11) 69(17) 77(14) 100(40) 3(.9) 1.5(.9) 100(25) 100(17) abundances: % base [M-43]" 6(.7) 1(.3) 5(2) 2(.8) 1(.4) 4(.6) 2(.5) 1(.2) 10(4) --1(.2) 6(1) peak (% r. 39) [M-57]" 70(8) 12(3) 1(.4) 1(.4) .4(.2) 82(12) 40(10) 18(3) --__b __c 1(.2) 35(6) 57" 100(12) 100(27) 35(13) • 23(9) 42(18) 100(15) 100(24) 100(18) 25(10) 100(34) 100(59) 37(9) 42(7)Obtained w i t h a CH-4 spectrometer o p e r a t i n g a t 70 eV. Compounds were introduced by d i r e c t i n s e r t i o n probe. . , ,
[ M - 5 6 ] • r e l a t i v e abundance 5%. '' [M-56|- r e l a t i v e abundance .5%.
tuted benzene derivatives are given in Table I. The mass spectra of
the isomeric di-tert-butylbenzenes are also given in full in Figures
1-3.
The spectra of 1,3- and l,4-di-tert-butylben2ene, apart from a
characteristic difference in intensity for the ion at m/e 160, are
rather similar in appearance. They are dominated by the peak at m/e
175 corresponding to the a-cleavage ion [M-CH3] ; in both spectra this
ion carries about 40% of the total ion current. The appearance of
the mass spectrum of 1,2-di-tert-butylbenzene is more complex. The
base peak is shifted to m/e 57 and at the high end of the mass
spec-trum there are several intense fragment ion peaks besides the peak at
m/e 175. Of these ion at m/e 133 ([M-Ci+Hg] ) is of special interest
because its intensity suggests considerable aryl-alkyl cleavage; a
fragmentation which normally is not important with polyalkylbenzenes^.
We will discuss the formation of the [M-C^Hg] ions from the molecular
ion of 1,2-di-tert-butylbenzene below.
The data in Table I show that derivatives of
1,2-di-tert-butylben-zene are, just as 1,2-di-tert-butylben1,2-di-tert-butylben-zene itself, easily
differen-tiated from positional isomers. The most characteristic features in
the mass spectra of the 1,2-di-tert-butylbenzene derivatives are the
base peaks at m/e 57 and the relatively abundant [M-Ci,Hg] or
[M-Ci^Hg]- ions (Table I, notes b and c ) . In view of the steric strain
of these compounds it is remarkable that their molecular ions are,
with the exception of those of 3-nitro- and
3,5-dinitro-l,2-di-tert--butylbenzene, as abundant as those of the less strained compounds
in Table I^. In the case of 3-nitro- and
3,5-dinitro-l,2-di-tert--butylbenzene the relatively low abundance of the molecular ions as
well as the formation of [M-Ci+Hg] • ions instead of [M-C^Hg]" ions may
be explained by an additional ortho-effect.
Metastable characteristics
In Tables II and III metastable characteristics are given from DADI
(or MIKE) spectra of the molecular ions and the [M-CH3]" ions of the
isomeric di-tert-butylbenzenes. These data show that both the
mole-> 1 O) CD C71 t/) C\J NJ 0 )
^
4 J U O l C L U~i%.
TO
s ^ 1 • 1 — -a en on ro Ho+ •
r» -r e l a t i v e a b u n d a n c e r e l a t i v e a b u n d a n c e O) O) > 1^)
tl) s-•4-> U 0 ) Q . l / l ?H ttl 4^i 1• ,
—
-a in-4
o \ ^ E C5 CVJ r e l a t i v e a b u n d a n c ec u l a r ions and the [M-CH3] ions decomposing w i t h i n the metastable
Table I I Metastable c h a r a c t e r i s t i c s of the molecular ions of the isomeric d i - t e r t - b u t y l b e n z e n e s
Benzene d e r i v a t i v e : 1 , 2 - d i - t B u 1 , 3 - d i - t B u Metastable t r a n s i t i o n % T(meV) % T(meV)
1 , 4 - d i - t B u % T(meV)
190" > 175" 100 81 100 19 100 21
i g o t - 133" 3.3 — < 0.1 — < 0.1
Obtained from DADI scans on a Varian 311 spectrometer. Percentages give r e l a t i v e abundances w i t h respect t o the most intense metastable t r a n s i t i o n . Releases of k i n e t i c energy (T) were c a l c u l a t e d from peak widths a t h a l f h e i g h t , values given are c o r r e c t e d f o r the width o f the main beam.
Table I I I Metastable c h a r a c t e r i s t i c s of the [M-CH3J ions of the isomeric d i - t e r t - b u t y l b e n z e n e s Benzene d e r i v a t i v e : Metastable t r a n s i t i o n 1 , 2 - d i - t B u % T(meV) 1 , 3 - d i - t B u % T(meV) 1 , 4 - d i - t B u % T(meV)
1 7 5 " ^
175" .
175" ->
175" ->
1 7 5 " ^
1 7 5 " .
1 7 5 " .
160t
147"
133"
119"
105"
91"
57"
< 0.1
3.1
100
2.4
3.6
0.7
< 0.2
-163
19
22
17
-2.4
69
10
30
1.4
1.1
100
-69
32
27
-26
100
24
8.2
13
0.8
0.4
30
27
122
77
33
-32
See note a. Table I I ,
time-window have not rearranged to common s t r u c t u r e s . This i s contrary t o a suggestion made by Loudon and Mazengo^. They pointed o u t t h a t a rearrangement of the molecular ion of 1 , 2 - d i - t e r t - b u t y l b e n z e n e can e x p l a i n the e n e r g e t i c s of the formation of the [M-CH3] i o n s . We w i l l discuss the s i g n i f i c a n c e of these e n e r g e t i c s below.
The data i n Tables I I and I I I show t h a t the c h a r a c t e r i s t i c [M-Ci+HgJ ions are formed from the molecular ion o f 1 , 2 - d i - t e r t - b u t y l b e n z e n e by two d i f f e r e n t pathways: ( i ) d i r e c t cleavage o f the a r y l - a l k y l bond i n the molecular i o n ; ( i i ) via the [M-CH3I ion by l o s s of C3Hg. The f i r s t pathway is r a t h e r unusual f o r ( p o l y ) a l k y l b e n z e n e s , i t s a b i l i t y to compete w i t h the more usual a-cleavage r e a c t i o n can be r a t i o n a l i s e d by the almost t o t a l r e l i e f of the s t e r i c s t r a i n when the a r y l - a l k y l bond i s c l e a v e d , as compared w i t h - as shown below - a p a r t i a l r e l i e f of s t e r i c s t r a i n i n the a-cleavage r e a c t i o n . However, i f the assumption i s made t h a t the frequency f a c t o r s f o r the cleavage of the a r y l - a l k y l bond and of the b e n z y l i c bond are o f the same order of magnitude^ i t f o l l o w s t h a t the data i n Tables I I and I I I i n d i c a t e that the f i r s t pathway given above f o r the formation o f the [M-C^Hg] ions i s the minor one. Formation of Ci+Hg ions (m/e 57) by cleavage of the a r y l -- a l k y l bond e v i d e n t l y also occurs w i t h almost t o t a l r e l i e f o f s t e r i c s t r a i n . No metastable was found f o r t h i s r e a c t i o n i n the DADI spectrum of the molecular ion of 1 , 2 - d i - t e r t - b u t y l b e n z e n e but a very weak meta-s t a b l e wameta-s found i n a high voltage meta-scan. The r e a c t i o n might compete more e f f e c t i v e l y w i t h the a-cleavage r e a c t i o n a t higher molecular ion i n t e r n a l e n e r g i e s . I t should be n o t e d , however, t h a t the abundance
(% Ï. 39) of the Ci+Hg ions i s r a t h e r s i m i l a r f o r the isomeric d i t e r t --butylbenzenes. On the whole, t h e r e f o r e , i t seems t h a t the charac-t e r i s charac-t i c feacharac-tures of charac-the mass speccharac-trum of 1 , 2 - d i - charac-t e r charac-t - b u charac-t y l benzene are l a r g e l y a r e s u l t o f secondary fragmentations o f the [M-CH3] ions
For 1 , 2 - d i - t e r t - b u t y l b e n z e n e the a c t i v a t i o n energy f o r the formation of the [M-CH3J ions i s r e l a t i v e l y s m a l l . The r e l a t i v e l y l a r g e release of k i n e t i c energy accompanying t h i s f o r m a t i o n , t h e r e f o r e , must be a t t r i b u t e d to a s i z a b l e reverse a c t i v a t i o n e n e r g y ' 0 . This i s i n l i n e w i t h an unrearranged molecular ion and a r e l a x a t i o n process going on 22
during the formation of the [M-CH3J''" ions to minimize the steric strain
in these.
Loss of a methyl radical from an even-electron ion usually is not
a low energy process. Yet in case of the [M-CH3J ions from
1,4-di--tert-butylbenzene this reaction shows the most intense metastable
and differentiates the mass spectra of 1,3- and
1,4-di-tert-butylben-zene. This can be explained by resonance stabilization of the product
ions at m/e 160 (ion structure |) for 1,4-di-tert-butylbenzene. Such
an explanation is consistent with the conclusions reached above
regarding the structures of the molecular ions and the [M-CH3] ions
a; m/e 160
of the isomeric di-tert-butylbenzenes.
Energetics of the formation of the [M-C//3J ions
It remains to show that the conclusions about the structures of the
[M-CH3J ions of the di-tert-butylbenzenes are consistent with the
energetics of their formation. The appearance potentials of the ions
formed in their ground states at the threshold is given by: • .
A([M-CH3]") = AH^([M-CH3]") + A H ^ ( C H 3 ) - AH^(M) + e" + e° - ë^ (1)
i n which e i s the excess a c t i v a t i o n energy ( k i n e t i c s h i f t ) , e° the reverse a c t i v a t i o n energy and e, the average heat content of the molecule before i o n i z a t i o n . Under i d e n t i c a l c o n d i t i o n s ê. w i l l be prac-t i c a l l y equal f o r isomers. T h e r e f o r e , s u b s prac-t r a c prac-t i n g prac-the prac-two eqns. (1) f o r 1,2- and 1 , 4 - d i - t e r t - b u t y l b e n z e n e y i e l d s :A([M-CH3]") - A([M-CH3]" = A H . ( [M-CH3]") - A H . ( [M-CHj]") - A H . ( M ) +
p . 0 . 1 ' n T ' n I n
A H . ( M ) + (e + e°) - ( e * + e°)
in which
o
and
p
are suffixes denoting 1,2- and
1,4-di-tert-butyl-benzene, respectively. Experimentally^ the difference between the heats
of formation of the isomers (22.3±0.5 kcal mol"') is equal but
oppo-site to the difference between the appearance potentials of their
[M-CH3]" ions (24.4±3.1 kcal mol"'). Thus:
A(rM-CH3]") - A([M-CH3]") == A H . ( M ) - A H . ( M ) (3)
'
•' p o
T o ' P
and in combination with eqn. (2):
AHf([M-CH3]p - AH^([M-CH3]p
~-
(e* + e " ) ^ - (e* + e ° ) ^ (4)
The near equality of expression (3) has received considerable
attention because it seems to be a specific example of an equation
proposed and used by
Jalonen
and
Pihlaja^'^
to correlate strain in
neutral molecules with appearance potential data. They derived their
equation for the case that the fragment ions have similar heats of
formation, assuming that terms as in the righthand side of expression
(4) can be neglected".
Loudon
and
Mazengo^
have pointed out, and it
will be shown that they were right, that in case of the
di-tert-butyl-benzenes it is unlikely that the [M-CH3] ions will have similar
heats of formation if no rearrangements have taken place. Accepting in
fact the equation proposed by
Jalonen
and
Pihlaja'^'^
they, therefore,
suggested a rearrangement of the molecular ion of
1,2-di-tert-butyl-benzene to that of 1,3-di-tert-butyl1,2-di-tert-butyl-benzene and/or that of
1,4-di-tert--butylbenzene. In that case both sides of expression (4) would be
equal to zero and expression (3) would be an exact equation. The
meta-stable characteristics given in this paper, however, show that both the
molecular ions and the [M-CH3] ions have not isomerized to common
structures. Therefore, the near equality of expression (3) is just
fortuitous and an evaluation of expression (4) seems in order.
Since the ionization potentials of the di-tert-butylbenzenes are
quite similar^ and as the molecular ions do not isomerize it follows
that the activation energies for the formation of the [M-CH3] ions
differ considerably, (A( [M-CH3])"-IP) being 47 and 68 kcal mol"' for
1,2- and 1 , 4 - d i - t e r t - b u t y l b e n z e n e , r e s p e c t i v e l y . This i m p l i e s a con-s i d e r a b l e d i f f e r e n c e i n the k i n e t i c con-s h i f t con-s . Thicon-s d i f f e r e n c e i con-s estimated as 11 kcal m o l " ' by using an approximate rate equation
14
w i t h i d e n t i c a l frequency f a c t o r s o f 2.4*10 and one t h i r d o f the o s c i l l a t o r s a c t i v e ' ^ " ' ' * . Taking i n t o a c c o u n t ' ^ t h a t there i s a s i z a b l e reverse a c t i v a t i o n energy f o r the loss o f CHj from the molecular i o n of 1 , 2 - d i - t e r t - b u t y l b e n z e n e one f i n d s from expression (4) t h a t the
[M-CH3J ions from l , 2 - d i - t e 2 ' £ - b u t y l b e n z e n e are s t r a i n e d by 9.7 kcal m o l " ' compared w i t h those from 1 , 4 - d i - t e r t - b u t y l b e n z e n e . This r e s u l t i s c o n s i s t e n t w i t h the conclusions about the [M-CH3] ion s t r u c t u r e s .
3.3 EXPERIMENTAL PART
Mass s p e c t r a l data were obtained using CH-4 and Varian 111 and 311 spectrometers; see notes a and d o f Table I and note a o f Table I I .
Most compounds used were p r e v i o u s l y r e p o r t e d ' ^ ' ' ' ^ a n a l y t i c a l
samples. N i t r a t i o n o f 1 , 2 , 4 - t r i - t e r t - b u t y l b e n z e n e ' ^ i n the usual way'^ w i t h n i t r i c a c i d i n a c e t i c a c i d a c e t i c anhydride gave 5 n i t r o l , 2 , 4 t r i t e r t b u t y l b e n z e n e ; y i e l d 30% w i t h m.p. 9292.6° a f t e r c r y s t a l -l i z a t i o n from e t h a n o -l . Bromination o f 1 , 2 - d i - t e r t - b u t y -l b e n z e n e w i t h bromine i n a c e t i c acid gave 27% 4 - b r o m o - l , 2 - d i - t e r t - b u t y l b e n z e n e besides 2 - b r o m o - t e r t - b u t y l b e n z e n e ' ^ . Using t h i s m i x t u r e mass spectra were obtained by GC-MS (note d , Table I ) .
REFERENCES AND NOTES
' S. van de Graaf, Reel. Trav. Chim. Pays-Bas, s u b m i t t e d .
2 J. Jalonen and K. Pihlaja, Org. Mass Spectrom. 7, 1203 (1973). 3 E.M. Amett, J.C. Sanda, J.M. Bollinger and M. Barber, J . Am. Chem.
Soc. 89, 5389 (1967).
•* J. Jalonen and K. Pihlaja, Org. Mass Spectrom. 6, 1293 (1972). ^ A.G. Loudon and R.Z. Mazengo, Org. Mass Spectrom. 8, 179 (1974). 6 L.R.C. Barclay, C.E. MilUgan and N.D. Hall, Can. J . Chem. 40,
1664 (1962); E.M. Amett and M.E. Strem, Chem. Ind. 1961, 2008. -^ H.M. Grubb and S. Meyerson, in F.W. McLafferty ( E d . ) , Mass
ö This observation can be accounted for if the P(E)-curves have a
minimum between 2 and 3 eV for all compounds (see:
F.W. McLafferty,
T. Wachs, C. Lifshitz, G. Innorta and P. Irving, J. Am. Chem. Soc.
92, 6867 (1970)).
9 The slightly tighter transition state one might expect for the
a-cleavage reaction being compensated for by the statistical factor
of three in favor of this reaction.
'0 R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable
Ions, Elsevier, Amsterdam, 1973, p. 104.
" This assumption is not correct unless the molecular ions of the
isomers examined isomerize to common structures. See the following
discussion.
12 s.A. Benezra and M.M. Bursey, Org. Mass Spectrom. 7, 241 (1973).
13 The kinetic shift of 36 kcal mol"' estimated this way for
1,4-di--tert-butylbenzene may be compared with the 22 kcal mol"' which can
be calculated from the kinetic energy released in the reaction
(Table II) by using
Franklin's
empirical relationship
(T = e /(0.44*S);
M.A. Haney
and
L.J. Franklin,
J. Chem. Phys. 4 8 ,
4093 (1968)) and by assuming that the reverse activation energy
is negligible.
1"* The same procedure gives for the loss of CHÓ from the molecular ion
13
of ethylbenzene (frequency factor = 4*10 ) a kinetic shift of 16
kcal mol"', which is satisfactory close to the value of 17 kcal
mol"' that can be calculated from A([M-CH3]")-IP (I.
Howe
and
D.H.
Williams,
J. Am. Chem. Soc. ^ , 7137 (1969)) and tabulated values
of heats of formation
{F.H. Field
and
L.J. Franklin,
Electron
Impact Phenomena, Academic Press, New York, 2nd ed. (1970)).
1^ Assuming that the reverse activation energy for
1,4-di-tert-butyl-benzene is negligible (see note 13) and that that for
1,2-di-tert--butylbenzene is released completely as kinetic energy. Because of
the latter assumption the value of 9.7 kcal mol"' for the difference
between the heats of formation of the [M-CH3] ions can be regarded
as an upper limit.
^ B. van de Graaf and B.M. Wepster, Reel. Trav. Chim. Pays-Bas 8 5 ,
619 (1966).
'^ A.J. Hoefnagel, J.H.A.J. Nunnink, A. van Veen, P.E. Verkade and
B.M. Wepster, Reel. Trav. Chim. P a y s - B a s ^ , 386 (1969); J.M.A. Baas,
H. van Bekkum, M.A. Hoefnagel and B.M. Wepster, Reel. Trav. Chim.
Pays-Bas 88, 1110 (1969); and J.M.A. Baas and B.M. Wepster, Reel.
Trav. Chim. Pays-Bas 9 1 , 1002 (1972).
8 U. Krüerke, C. Hoogzand and W. Hübel, Chem. Ber. 94, 2817 (1961).
^ In c o n t r a s t to l i t e r a t u r e data [A.W. Burgstahler, Ping-Lu Chien and
M.0. Abdel-Rahman, J . Am. Chem. Soc. 86, 5281 (1964)) considerable
bromode-tert-butylation was observed in repeated experiments.
4 HYDROGENATION OF 1,2-DI-tert-BUTYLBENZENE ON PLATINUM AND RHODIUM
CATALYSTS. SYNTHESIS OF
cis-
AND trans-1,2-DI-tert-BUTYLCYCLOHEXANEl
4.1 INTRODUCTION
It is known that a considerable steric strain is present in
1,2-di--tert-butylbenzene^. In view of this it may be expected that in
reactions in which the ring loses its aromatic character
di-tert--butylbenzene may show a behaviour different from less strained
1,2--dialkylbenzenes. In this article we describe the hydrogenation of
1,2-di-tert-butylbenzene on rhodium and platinum catalysts. Apart
from in the course and the kinetics of the hydrogenation, we were
also interested in the reaction products,
cis-
and
trans-1,2-di-tert--butylcyclohexane; an investigation into the properties of these
stereochemically interesting compounds forms a sequel to other work
in our laboratory^.
The catalytic hydrogenation of 1,2-di-tert-butylbenzene has not
been investigated before. An indication that the aromatic ring in
1,2--di-tert-butylbenzene and derivatives is readily hydrogenated was
found by
Burgstahler
et 31."*'^ during the preparation of
1,2-di-tert--butylbenzene from l,2-bis(l,l-dimethyl-2-butylthioethyl)benzene by
desulfurization with Raney nickel. Further
Burgstahler et
al."*
ob-tained a completely saturated product by hydrogenation of
2,3-di-tert--butylcyclohexene and 2,3-di-tert-butyl-1,4-cyclohexadiene, both
obtained by reduction of 1,2-di-tert-butylbenzene with lithium in
liquid ammonia. Considering our results, this product, which was not
investigated further by them, must have been a mixture of
ais-
and
trans-1,2-di-tert-butylcyclohexane.
4.2 RESULTS AND DISCUSSION
The course of the hydrogenations
The hydrogenation of 1,2-di-tert-butylbenzene has been investigated
with the following catalysts: rhodium on carbon, platinum on carbon,
and reduced platinum oxide. In these hydrogenations, in addition to
the two isomeric cyclohexanes, an intermediate product appeared to
be formed, which according to elementary analysis, refractive index,
and NMR spectrum was 2,3-di-tert-butylcyclohexene already described
by
Burgstahler
et al."*. In Table I the experimental data of the
hydro-genations of 1,2-di-tert-butylbenzene are listed, in Table II data of •
hydrogenations of the intermediate product
2,3-di-tert-butylcyclo-hexene.
Table I Hydrogenation of 1,2-di-tert-butylbenzene
Exp. Amount Products in % ^^^^ „^ Half-life time
no.
Catalyst ,- . Solvent . , n, b u ^ J.
•^ of cat.
azs trans
al kene substrate
1 Rh(5% on C) 200 mg EtOH 71.1 28.9 30.1 3 h. 4'
2 Rh(5% on C) 200 mg AcOH 56.5 43.5 45.2 23'
3 Pt(5% on C) 200 mg EtOH 91.5 8.5 5.7 22 h.
4 Pt(5% on C) 200 mg AcOH 90.0 10.0 7.5 23'
5 Pt02 25 mg AcOH° 84.0 16.0 11.5 1 h. 12'
Hydrogenations were carried out at 25 and atmospheric hydrogen
pressure. In all experiments 500 mg of 1,2-di-tert-butylbenzene was
used in 50 ml of solvent.
2,3-di-tert-butylcyelohexene.
Table II Hydrogenation of 2,3-di-terÉ-butylcyclohexene
Catalyst
Rh (5% on C)
Pt (5% on C)
Solvent
AcOH
AcOH
Products
ais4.0
3.5
in
%
trans96.0
96.5
^ Hydrogenations were carried out at 25° and atmospheric hydrogen
pressure.
The course of the hydrogenation of 1,2-di-tert-butylbenzene has
been represented in a graph for two characteristic experiments,
experiment 2 in Fig. 1 and experiment 5 in Fig. 2^.
•
%
hydrogenation
Fig. 1 Hydrogenation of 1,2-di-tert-butylbenzene over rhodium on
car-bon (see Table I, experiment 2 ) . • , 1,2-di-tert-butylbenzene; A , 2,3
-di-tert-butylcyclohexene; O , ets-l,2-di-tert-butylcyclohexane; A s
trans-1,2-di-tert-butylcyclohexane.
From Fig. 1, hydrogenation with rhodium on carbon as catalyst, it
appears that one of the isomeric 1,2-di-tert-butylcyclohexanes is
formed substantially directly from the aromatic, whilst the other, so
100 80 ^ 60 +-> o "O o 40 s-CL 20-50 60 70 80 90
• % hydrogenation
Fig. 2 Hydrogenation of 1,2-di-tert-butylbenzene over reduced platinum
oxide (see Table I, experiment 5). • , 1,2-di-tert-butyl benzene; A ,
2,3-di-tert-butyleyclohexene; O , et's-1,2-di-tert-butylcyclohexane;
• , trans-1,2-di-tert-butylcyclohexane.
far as can be ascertained, is formed exclusively
via
the intermediate
product 2,3-di-tert-butyleyclohexene (see Scheme 1 ) .
Scheme 1
trans
On the basis of the idea that hydrogenation of an aromatic in one
residence on the catalyst surface takes place completely, or in any
case substantially, through al l-cjis-addition of hydrogen'''^ the
cis--configuration has been assigned to the isomer which is formed directly
from the aromatic. This assignment of configuration is confirmed by
the NMR and IR spectra of the isomers. A discussion of these spectra
w i l l be given i n a f o l l o w i n g p u b l i c a t i o n ' 3 .
I n the hydrogenations w i t h rhodium on carbon a great s e l e c t i v i t y i s to be found as regards the hydrogenation of the aromatic as compared w i t h t h a t of 2 , 3 - d i - t e r t - b u t y l c y c l o h e x e n e . The maximum q u a n t i t y o f 2 , 3 - d i - t e r t - b u t y l e y e l o h e x e n e obtained i s equal t o the t o t a l q u a n t i t y t h a t has been desorbed. In the l i t e r a t u r e only a s i n g l e example approx-i m a t approx-i n g t h approx-i s s e l e c t approx-i v approx-i t y has been g approx-i v e n approx-i ° . Wapprox-ith the platapprox-inum c a t a l y s t s the s e l e c t i v i t y i n t h i s respect i s s m a l l e r . Thus i n experiment 5 ( F i g . 2) we f i n d a maximum of 11.5% of 2 , 3 - d i - t e r t - b u t y l c y c l o h e x e n e , w h i l e from the u l t i m a t e q u a n t i t y of trans i t can be i n f e r r e d t h a t a t o t a l q u a n t i t y of about 17% has been desorbed. In t h i s ease again the hydro-genation indeed probably proceeds e n t i r e l y as i n d i c a t e d i n Scheme 1 , In our opinion the great s e l e c t i v i t y observed when using rhodium on carbon as c a t a l y s t must be a t t r i b u t e d , apart from t o a g r e a t d i f f e r e n c e i n hydrogenation r a t e , t o a p r e f e r e n t i a l a d s o r p t i o n of the aromatic on the metal s u r f a c e .
Another type of s e l e c t i v i t y i s found to a pronounced degree i n the hydrogenation o f 2 , 3 - d i - t e r t - b u t y l c y c l o h e x e n e : both w i t h platinum and w i t h rhodium c a t a l y s t s t r a n s - 1 , 2 - d i - t e r t - b u t y l c y c l o h e x a n e i s formed i n l a r g e excess (see Table I I ) . Such a predominance of the traws-isomer i s a t variance w i t h what may be expected on the basis of a mechanism f o r the hydrogenation o f cyclohexene d e r i v a t i v e s advanced by Sauvage et a l . i ' . These i n v e s t i g a t o r s found t h a t i n a c e t i c acid w i t h reduced p l a t i n u m oxide as c a t a l y s t more t r a n s i s o m e r i s formed from 1 , 4 d i a l -kylcyelohexenes according as the s u b s t i t u e n t i n p o s i t i o n 4 i s b u l k i e r ; thus 4 - t e r t - b u t y l - l - m e t h y l c y c l o h e x e n e gives 63% of t r a n s - i s o m e r as a g a i n s t 1,4-dimethylcyclohexene 43%. In order t o account f o r t h i s r a t h e r small e f f e c t , they base themselves on the geometry of the c y c l o -hexene molecule i n the adsorbed s t a t e ' ^ . They assume t h a t i n the adsorbed s t a t e the molecule i s i n a boat conformation i n which the bonds with the metal surface are i n exo p o s i t i o n r e l a t i v e to the r i n g (see F i g . 3a). There are two p o s s i b i l i t i e s w i t h such a geometry: the s u b s t i t u e n t i n p o s i t i o n 4 may be i n exo or endo p o s i t i o n . Sauvage et 33
a l . " take the view that according as this substituent is bulkier there
will be greater preference for the
exo
position, so that hydrogenation
will produce the irans-isomer in excess.
a b c F i g . 3 Possible geometries o f the t r a n s i t i o n s t a t e of hydrogenation.
A p p l i c a t i o n o f t h i s mechanism t o the hydrogenation of 2 , 3 d i t e r t -- b u t y l c y c l o h e x c n e leads t o the p r e d i c t i o n t h a t the ei.s--isomer would be formed p r e f e r e n t i a l l y , f o r i n the geometry l e a d i n g to the trans--isomer the s u b s t i t u e n t i n p o s i t i o n 3 would occupy a f l a g p o l e p o s i t i o n , which i s e n e r g e t i c a l l y very unfavourable.
For cyclohexenes w i t h a bulky s u b s t i t u e n t , such as a t e r t - b u t y l group, i n p o s i t i o n 1 we propose a d i f f e r e n t geometry f o r the t r a n s i t i o n s t a t e t o the half-hydrogenated s t a t e , viz. a boat conformation in which the bonds w i t h the metal surface are i n endo p o s i t i o n r e l a t i v e to the r i n g . I t i s t r u e t h a t the p o s i t i o n of the r i n g r e l a t i v e t o the metal surface i n t h i s case i s not so f a v o u r a b l e , but t h i s is compensated f o r a large s u b s t i t u e n t i n p o s i t i o n 1 by the f a c t t h a t the exo p o s i t i o n which t h i s s u b s t i t u e n t now occupies i s more favourable (see Figs 3b, 3c).
I f we accept t h i s p i c t u r e , we f i n d f o r 2 , 3 - d i - t e r t - b u t y l c y c l o h e x e n e t h a t i n the t r a n s i t i o n s t a t e leading t o the ais-isomer ( F i g , 3b) the t e r t - b u t y l group i n p o s i t i o n 3 occupies an e n e r g e t i c a l l y unfavourable f l a g p o l e p o s i t i o n , w h i l s t i n the t r a n s i t i o n s t a t e leading t o the trans-- isomer ( F i g . 3c) the t e r t trans-- b u t y l group i n p o s i t i o n 3 occupies a bowtrans-- bow-s p r i t p o bow-s i t i o n .
We would add t h a t the r e s u l t s of hydrogenations of other c y c l o -34
hexenes w i t h a 1 - t e r t - b u t y l s u b s t i t u e n t are also i n c o n f o r m i t y w i t h t h i s mechanism. Thus hydrogenation over platinum and rhodium c a t a l y s t s o f 1 , 4 d i t e r t b u t y l c y c l o h e x e n e gives mainly c i s 1 , 4 d i t e r t b u t y l -cyclohexane, since the t e r t - b u t y l croup i n p o s i t i o n 4 i n the t r a n s i t i o n s t a t e p r e f e r s the exo p o s i t i o n over the endo p o s i t i o n , w h i l s t 2 t e r t b u t y l 3 c a r b o x y c y c l o h e x e n e mainly gives t 2 ^ a n s 2 t e r t -- b u t y l eyelohexanecarboxylic a c i d ' ^ .
F i n a l l y two solvent e f f e c t s may be pointed t o . The f i r s t concerns the i n f l u e n c e o f the s o l v e n t on the r a t e of h y d r o g e n a t i o n , s i m i l a r t o the e f f e c t s which have been observed by other i n v e s t i g a t o r s ' ' * . In a d d i t i o n , when rhodium on carbon i s used as c a t a l y s t , the q u a n t i t y o f 2 , 3 - d i - t e r t - b u t y l e y c l o h e x e n e t h a t i s desorbed appears to be dependent upon the s o l v e n t (see Table I ) . No reasonable e x p l a n a t i o n o f these phenomena can be given a t the moment. .
The kinetics of the hydrogenations
The hydrogenation r a t e of 1 , 2 - d i - t e r t - b u t y l b e n z e n e has been com-pared w i t h t h a t o f benzene and t h a t o f some 1,2-dialkylbenzenes w i t h rhodium on carbon as c a t a l y s t . The hydrogenations are a l l zero order i n s u b s t r a t e . The r e s u l t s are l i s t e d i n Table I I I .
Table I I I K i n e t i c s of hydrogenation^
