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Interconversion of some pentylbenzenes by aluminum chloride
Tetrahedron Lettera, Ro.6,~~3-8, 1959.Pergamon &es@ LM. Printed in Great Britain.
I~~CO~SrOR
ENES BY ALBEIEDX CELORIDE
OF SOME Pm
Royaton Y. Roberts and Yuri W. Ran Depertmentof Chemlstry,University of Texas, Austin, Texas (Reoeived5 May 1959)
AS a continuationof our study of alkylbenzenesre have exaatined tha behavior of several pentylbenzenesin the presence of aluminum chloride. Recently Nenitzezouand -oo-vorkers published an account of experi1 mente quite similar to oure. Their observationof the conversionof eec-but&benzene into isobutylbenzenewas in agreementwith our ozn2 but, ozing to the qualitativenature of their analysis, they vere unaware of the reversibilityof this rearrangement.These vorkere also reported the conversionof &pentylbenzene into 2-methyl-j-phenylbutane, and of the latter into neopentylbenzeneby more extended treatmentwith aluminum ohloride.We now wish to report the reeults of a quantitative study of the reaction of these three pentylbenzeneedth aluminum chloride and to explain the relationshipbetween them in term8 of a mechanism vhioh oorrelatesthis system zith the rearrangementsof propyl- and
1
C. D, Nenitsescu,I. Necsoiu, A. &at% and Y. Zalman, Chem. Be,
n
g,
10
(1959).
' B, Y. Roberts, Y. W. Han C. H, S&mid and D. A. Davis, J. Amer, Chem. Sot, ii, 640 (1959).
5
6
Interconversion
butylbenaenes Analysis
of some pentylbensenes
. of the pentylbensenee
was made by means of infrared The pentylbenaene fraction higher-boiling
after
spectrometry
material by fractional
aluminum chloride
reoovered
6
separated from lower- and
(loO”,
1 hr) with water-activated
used with -set- and isobutylbenaene,*
were neopentylbensene
(30$) and s-pentylbenaene
0
and vapor chromatography.
distillation.
treated
in the proportion
The pentylbensenes
treatment with aluminum chloride
was previously
&-Pentylbenzene was firet
3-phe~lbu~e
by aluminum chloride
I2
(b) 2-Methyl-3-pknylbutane:
I6
(5%) .3
24
30
-3
P-methyl-
After milder treatment
36
a=,-pentylbenzene benzene ~-Z-methyl
(65$),
42
’
.-“&&yl-phenylbulone
3 These three pentylbenaenes made up 9% of the distillation fraations the other $ consisted of the craoking products sand/or isobutylbenzene (these two were not separated on the vapor chromatogram) end isopropylbenrene.
Interconversionof eome pentylbenzeneeby aluminum chloride (in benzene solution at ca. t30°,0.5 hr) however, the major admponentof the pentylbenzeneewas 2-methyl-3-phenylbutane (66) in accordancewith Nenitzesaulafinding, accompaniedby neopentylbenzene(2%) and i-pentylbenzene (1%). Further experimentsin which &pentylbensene was isomerized by heating with catalyst for various periods of time are auoniwizedin Fig. 1.
At first, 2-methyl+phenylbutane was produced rapidly,
neopentylbenzenelee8 rapidly, but the latter isomer was elorly converted into the former as i-pentylbenzenedisappeared. When P-methyl3-phenylbutanewas the etarting material, essentiallythe same mixture of isomer8 was produced after 24 hr.
When neopentylbenzenewas treated
similarly for 24 hr it wae virtually unchanged;
leee then 2$ of P-methyl-
3-phenylbutaneand about 246of cracking products were found, No evidence of the productionof &y
other pentylbenzenesin any of the experiments
was obtained by either infrared epectrometryor vapor chromatography, A meohaniem which ia consistentwith these data is outlined below:
7% CH3-y-CH2-+ CHs
FH3 CH,-
C-
RH t
CHz-CH3
CH,
k(
CH,-L-CHs RH Re ti
c--t
hH3
j4 Il.2
II
FH3 CH,-+CH-CH, Y” I
C”, IX-
4 H-yH-CH3 59
mo
8
Interconversionof Bornepentylbenzenesby aluminum chloride
This scheme is similar to those proposed by Nenitaescu,'but incorporates the suggestionof Schmerlingand co-workers4of anchimericassistancein the abstractionof RIO from i-pentylbenzene.It is directly analogous to thoae we have recently proposed for the rearrangementsof propyl- and
5 In the present system, the intermediateion set- and isobutylbensenes. III is favored over II energeticallynot only by the hyperconjugative effect, but also by the steric effect, since the i-butyl group causes sev6 ere steric inhibitionof resonance in IIa. Hence 2-methyl-j-phenylbutane is produced more rapidly than neopentylbenoeneby the action of aluminum chloride on i-pentylbenzene. Although neopentylbenzeneis produced more slowly, the reaction is irreversible. This must be due to the difficultyof abstractingHz@ from the side-chainof neopentylbenzene. The nine primary hydrogensare Y to the ring, there are no &hydrogens, and the two a-hydrogensare shieldedby a &-butyl group. One is led to draw the interestingdeduction that there is a significantsterio requirement for hydride abstraction,by either aluminum chloride-watercomplex or by a carboniumion. It is probable that this factor is involved in the lack of rearrangementof n-butylbenzene,?isosmylbenzenel and perhaps some other alkylbensenes.*
4 Who first described the rearrangementof t-pentylbenseneto P-methylj-phenylbutane;L. Schmerling,R. W. Welch;and J. P, Luvisi, J Amer Chem Sot 14, 2636 (1957)iL. Schmerlingand J. P. West, Ibiw 1917).
5 R. M. Roberts and J. E. Douglass, Chem. k Ind, In press (1959). 6 C. Baddeley,J. Chadwickand H. T. Taylor, J. Chem. SOD, 2405 (1954); T. C. Van Hoek, P. E. Verkade and B. II.Wepster, Rec. Trav. Chimti 11, 559 (1958). 7 R. M. Roberts, S. G. Brandenbergerand S. G. Psnayides,J. Amer, Chem. Sot, 22, 2507 (1958). 8 We are grateful to the National Science Foundationfor a grant which supported this research.
I~~CO~SrOR
ENES BY ALBEIEDX CELORIDE
OF SOME Pm
Royaton Y. Roberts and Yuri W. Ran Depertmentof Chemlstry,University of Texas, Austin, Texas (Reoeived5 May 1959)
AS a continuationof our study of alkylbenzenesre have exaatined tha behavior of several pentylbenzenesin the presence of aluminum chloride. Recently Nenitzezouand -oo-vorkers published an account of experi1 mente quite similar to oure. Their observationof the conversionof eec-but&benzene into isobutylbenzenewas in agreementwith our ozn2 but, ozing to the qualitativenature of their analysis, they vere unaware of the reversibilityof this rearrangement.These vorkere also reported the conversionof &pentylbenzene into 2-methyl-j-phenylbutane, and of the latter into neopentylbenzeneby more extended treatmentwith aluminum ohloride.We now wish to report the reeults of a quantitative study of the reaction of these three pentylbenzeneedth aluminum chloride and to explain the relationshipbetween them in term8 of a mechanism vhioh oorrelatesthis system zith the rearrangementsof propyl- and
1
C. D, Nenitsescu,I. Necsoiu, A. &at% and Y. Zalman, Chem. Be,
n
g,
10
(1959).
' B, Y. Roberts, Y. W. Han C. H, S&mid and D. A. Davis, J. Amer, Chem. Sot, ii, 640 (1959).
5
6
Interconversion
butylbenaenes Analysis
of some pentylbensenes
. of the pentylbensenee
was made by means of infrared The pentylbenaene fraction higher-boiling
after
spectrometry
material by fractional
aluminum chloride
reoovered
6
separated from lower- and
(loO”,
1 hr) with water-activated
used with -set- and isobutylbenaene,*
were neopentylbensene
(30$) and s-pentylbenaene
0
and vapor chromatography.
distillation.
treated
in the proportion
The pentylbensenes
treatment with aluminum chloride
was previously
&-Pentylbenzene was firet
3-phe~lbu~e
by aluminum chloride
I2
(b) 2-Methyl-3-pknylbutane:
I6
(5%) .3
24
30
-3
P-methyl-
After milder treatment
36
a=,-pentylbenzene benzene ~-Z-methyl
(65$),
42
’
.-“&&yl-phenylbulone
3 These three pentylbenaenes made up 9% of the distillation fraations the other $ consisted of the craoking products sand/or isobutylbenzene (these two were not separated on the vapor chromatogram) end isopropylbenrene.
Interconversionof eome pentylbenzeneeby aluminum chloride (in benzene solution at ca. t30°,0.5 hr) however, the major admponentof the pentylbenzeneewas 2-methyl-3-phenylbutane (66) in accordancewith Nenitzesaulafinding, accompaniedby neopentylbenzene(2%) and i-pentylbenzene (1%). Further experimentsin which &pentylbensene was isomerized by heating with catalyst for various periods of time are auoniwizedin Fig. 1.
At first, 2-methyl+phenylbutane was produced rapidly,
neopentylbenzenelee8 rapidly, but the latter isomer was elorly converted into the former as i-pentylbenzenedisappeared. When P-methyl3-phenylbutanewas the etarting material, essentiallythe same mixture of isomer8 was produced after 24 hr.
When neopentylbenzenewas treated
similarly for 24 hr it wae virtually unchanged;
leee then 2$ of P-methyl-
3-phenylbutaneand about 246of cracking products were found, No evidence of the productionof &y
other pentylbenzenesin any of the experiments
was obtained by either infrared epectrometryor vapor chromatography, A meohaniem which ia consistentwith these data is outlined below:
7% CH3-y-CH2-+ CHs
FH3 CH,-
C-
RH t
CHz-CH3
CH,
k(
CH,-L-CHs RH Re ti
c--t
hH3
j4 Il.2
II
FH3 CH,-+CH-CH, Y” I
C”, IX-
4 H-yH-CH3 59
mo
8
Interconversionof Bornepentylbenzenesby aluminum chloride
This scheme is similar to those proposed by Nenitaescu,'but incorporates the suggestionof Schmerlingand co-workers4of anchimericassistancein the abstractionof RIO from i-pentylbenzene.It is directly analogous to thoae we have recently proposed for the rearrangementsof propyl- and
5 In the present system, the intermediateion set- and isobutylbensenes. III is favored over II energeticallynot only by the hyperconjugative effect, but also by the steric effect, since the i-butyl group causes sev6 ere steric inhibitionof resonance in IIa. Hence 2-methyl-j-phenylbutane is produced more rapidly than neopentylbenoeneby the action of aluminum chloride on i-pentylbenzene. Although neopentylbenzeneis produced more slowly, the reaction is irreversible. This must be due to the difficultyof abstractingHz@ from the side-chainof neopentylbenzene. The nine primary hydrogensare Y to the ring, there are no &hydrogens, and the two a-hydrogensare shieldedby a &-butyl group. One is led to draw the interestingdeduction that there is a significantsterio requirement for hydride abstraction,by either aluminum chloride-watercomplex or by a carboniumion. It is probable that this factor is involved in the lack of rearrangementof n-butylbenzene,?isosmylbenzenel and perhaps some other alkylbensenes.*
4 Who first described the rearrangementof t-pentylbenseneto P-methylj-phenylbutane;L. Schmerling,R. W. Welch;and J. P, Luvisi, J Amer Chem Sot 14, 2636 (1957)iL. Schmerlingand J. P. West, Ibiw 1917).
5 R. M. Roberts and J. E. Douglass, Chem. k Ind, In press (1959). 6 C. Baddeley,J. Chadwickand H. T. Taylor, J. Chem. SOD, 2405 (1954); T. C. Van Hoek, P. E. Verkade and B. II.Wepster, Rec. Trav. Chimti 11, 559 (1958). 7 R. M. Roberts, S. G. Brandenbergerand S. G. Psnayides,J. Amer, Chem. Sot, 22, 2507 (1958). 8 We are grateful to the National Science Foundationfor a grant which supported this research.