Kov. 5 . 10.50
*ACETYLATION
AND
BENZOYLATION OF XLKYLBENZENES
order rate constant was obtained by dividing the corrected first-order rate by the average aromatic concentration followed by multiplication with a thermal factor consisting of the ratio of the density of acetic acid a t 25' to that of the reaction temperature. Isomer Distribution Measurements.-The reaction mixtures utilized for the isomer distributions were identical in all respects to those employed in the kinetic measurements. T o minimize possible complications arising from polymercuration, the aromatic was used in a 10-fold excess over the mercuric acetate present and the reaction was quenched after it had proceeded to approximately 30q0 completion. I n a typical experiment, 100 ml. of 3.00 Aft-butylbenzene in glacial acetic acid and 200 ml. of 0.150 M mercuric acetate in the same solvent were allowed to come t o thermal equilibrium with the bath at the desired temperature and then were quickly mixed. When the reaction had reached approximately 307, completion, calculated from the rate constants, the reaction mixture was quickly cooled and quenched in an equal volume of water. Excess unreacted t-butylbenzene was remcved by aspiration under vacuum a t room temperature and the soluble t-butylphenylmercuric acetates were converted into the insoluble bromide derivatives by addition of a 2- to %fold excess of bromide ion relative to the total mercuric concentration. The bromide derivatives
[CONTRIRUTIOX FROM
THE
5611
were filtered, dried thoroughly under vacuum, and weighed. The yields of these derivatives were nearly quantitative. The isomeric t-butylbenzenemercuric bromides were suspended in carbon disulfide and converted to the isomeric bromo-1-butylbenzenes by careful treatment with bromine. A slight excess of bromine was added, as evidenced by a slight red color in the carbon disulfide solution, and the mixture was then allowed to stand for several hours to ensure complete conversion of the mercuric derivatives. Excess bromine was removed by washing with sodium bisulfite solution followed by water. The carbon disulfide solution was thoroughly dried over calcium hydride and the proportions of the isomeric bromo-t-butylbenzenes were determined by infrared analysis. The conversions to the isomeric bromo-t-butylbenzenes, as based on the infrared analysis, were essentially quantitative. Infrared analyses for the isomeric derivatives were generally carried out on solutions t h a t were about 0.3 M . T h e wave lengths utilized for the m- and p-bromo-t-butylbenzenes were 12.8 and 12.2p, respectively. o-Bromo-t-butylbenzene possesses a characteristic absorption band a t 13.3 p . .ihsorbancies a t each of the above wave lengths were determined by means of the bnse line technique. LAFAYETTE. ISD
RICHARDB. WETHERILL LABORATORY O F P U R D U E UNIVERSITY]
Rate Data and Isomer Distributions in the Acetylation and Benzoylation of Ethyl-, Isopropyl- and i-Butylbenzene. Partial Rate Factors for the Acylation Reaction1 BY HERBERT C. BROWNAND GIANLORENZO ~IARINO~ RECEIVEDMARCH21, 1959 Rate constants have been determined for t h e reaction a t 25" of benzoyl chloride-aluminum chloride in ethylene chloride solution with ethylbenzene, isopropylbenzene and t-butylbenzene, yielding the relative rates: benzene, 1.00; toluene, 117; ethylbenzene, 101; isopropylbenzene, 93; t-butylbenzene, 70. I n these reactions the isomer distributions are: ethyl.45: m- and 93.09; p ; isopropylbenzene, 3.1% 0-, 4.07, m- and 92.9% p - ; t-butylbenzene, 0% o-, p-. I n t h e series of four alkylbenzenes, t h e values of o j decrease from 29.5 for toluene, to 10.9 for ethyl-, to 0 for t-butylbenzene. Similarly, p , exhibits a decrease with increasing branching of the alkyl group: 633, 563, 519 and 398, respectively. On the other hand, mi exhibits a marked increase from toluene t o ethylbenzene, and then increases moderately: 4.9, 10.3, 11.1, 11.4. Under t h e same conditions, the relative rates of acetylation were determined competitively: henzene, 1.00; toluene, 128; ethylbenzene, 129; isopropylbenzene, 128; t-butylbenzene, 114. The isomer distributions were determined t o be: ethylbenzene, 0.3y0 0-,2.7y0 m- and 97.070 p-; isopropylbenzene, OyO.o-, 3.0$&m-, 97.0% p - ; t-butylbenzene, Oyo0-,3.8% m- and 96.2% p-. The lack of significant substitution in t h e ortho position indicates a higher steric factor for t h e acetylation reaction t h a n for t h e corresponding benzoylation reaction. The changes in mi in t h e four alkylbenzenes, 4.8, 10.4, 11.5 and 13.0, respectively, paralleled these quantities in t h e benzoylation reaction. However, the values of pi, 749, 753, i45, 658, are surprisingly constant, in contrast to the consistent decrease in pf with branching of the alkyl group observed in t h e benzoylation and other substitution reactions. It appears t h a t hyperconjugative contributions of the alkyl substituent must be less important in the acetylation reaction than in the related benzoylation reaction.
The Selectivity Relationship3 correlates with excellent precision the available data on the substitution reactions of t ~ l u e n e . ~ I n order t o test the applicability of this treatment to other aromatic derivatives, quantitative data on the rates and isomer distributions are required for a number of representative electrophilic substitution reactions. We have a,dopted mercuration ( k ~ / k6.4),5 ~ nitration ( k ~ ! ' 2S),6 k ~ acylation ( k ~ , l k120)7 ~ ard (1) Directive Effects in Aromatic Substitution. XXXIX. research associate, 1957--1959. on project no. A t ( l l - l ) - 1 7 0 supported by the Atomic Energy Commission. (3) H. C. Brown and K . L. Selson, THIS J O U R N A L , 76, 6292 (1953). (4) L. M. Stock and H. C. Brown, ibid., 81,3323 (1959). ( 5 ) H. C. Brown and C. W. McGary, Jr., ibid., 77, 2306, 2310 (1955). (6) Fortunately, a considerable quantity of satisfactory nitration d a t a i s available through the efforts of C. K . Ingold and his co-workers. For a summary of t h e d a t a and pertinent references, see C . K. Ingold, "Structure and Mechanism i n Organic Chemistry," Cornel1 L'niversity Press, Ithaca, N . Y . , 1953, Chapter VI. ( 7 ) F. R. Jensen. G. LIarino and H. C. Brown, THISJ O U R N A L , 81, 3303 (1959); H. C. Brown a n d G. Marino, ibid., 81, 3308 (1959); H. C. Brown, G. Marino and L. M. Stock, ibid., 81, 3310 (1959). 1 2 ) Post-doctorate
halogenation ( k ~ / 344 k ~ for chlorination, 605 for bromination)* as reactions of low, moderate and high selectivity t o obtain the required data for this test. I n the present study we undertook to obtain the relative rates of reaction of benzoyl chloride and acetyl chloride in ethylene chloride solution, under the influence of aluminum chloride, with ethyl-, isopropyl- and t-butylbenzene. The isomer distributions realized in the acylation reactions were also determined to permit calculation of the partial rate factors. Results Rates of Benzoy1ation.-A kinetic procedure had been developed previously t o determine the second-order rate constants for the benzoylation of benzene and t o l ~ e n e . ~ This procedure was utilized without change to determine the rate con(8) P. W. Robertson, P. B. D. de l a Mare and W. T. G. Johnston,
J. Chem. SOL.,276 (1943); P. B. D. de la Mare and P. W. Robertson. d i d . , 279 (1943); H. C . Brown and L. Ivl. Stock, T H I S J O U R N A L , 79, 1421, 5175 (1957).
HERBERT C. BROWNA N D GIANLORENZO ~\.IARINO
5612
Vol. 81
stants for the benzoylation of ethyl-, isopropyland t-butylbenzene. The results are suiiiiiiarizetl in Table I.
the rate of acetylation of benzene in a typical kinetic s t ~ d y . However, ~ with a half-life of approximately 2 minutes uiidcr the snine conditions, the correspontlirig reaction with toluene could not TABLE I he hnndlctl by the same methods. Accordingly, KATECOSSTANTS FOR THE REACTION OF BENZOYL CHLORIDE- the relative rate of reaction, k , r / k ~was , determined ALUMINUMCHLORIDE WITH THE MOXOALKYLRESZENES IN competitively, using vapor phase chromatography ETHTLEKE DICHLORIDE AT 25.0' t o analyze for the reaction products, acetophenone R a t e constants and the inethylacetophecones.7 Accordingly, the Complex k2 X 108, Re$ Aromatic concn., -11 niole-' set.-' rate same procedure was adopted to determine the reTo1uen e 0.100 0,763 117 activity of ethyl-, isopropyl- and f-butylbenzene ,100 , nn0 101 Ethylbenzene relativr to toluene. The results are summarized Isopropylbenzene ,100 .C,OG 93 in Table 111. l-Butylbenzene a Benzene 1.00.
.lo0
* Ref. 7 .
.4G2
70
Benzoylation Isomer Distributions.-In earlier studies, we utilized infrared spectroscopy to determine the isomer distribution in the benzoylation of toluene.9 However, in this reaction only niiiior amounts of the meta isomer are formed and an accurate analysis of this component required a tedious fractional crystallization procedure to remove much of the more abundant para isomer. Our success in utilizing vapor phase chromatography for the analysis of the product realized in the acetylation of toluene7led us t o explore the possibility of applying this method for the analysis of the isomeric monoalkylbenzophenones formed in the benzoylation reactions. Utilizing a polyadipate substrate at 246-243' i t proved possible t o resolve the reaction products into individual peaks corresponding to the individual isomers formed in the benzoylation reaction. (Only in the case of toluene was the resolution between the m-methyl and p-methylbenzophenone incomplete.) The individual peaks were trapped and the individual isomers werc identified by means of their infrared spectra. I n order to test the procedure, we synthesized pure samples of m- and p-t-butylbenzophenones and prepared synthetic mixtures for analysis. The analytical results agreed closely with the known compositions of the mixtures. Aforeover, the analysis of the methylbenzophenones by this method proved to be in satisfactory agreement with the earlier results from infrared analysis. The isomer distributions are summarized in Table 11.
TABLE I11 RATESFOR THE REACTIONO F A C E T Y L CIILORIDEAI,Uhnsurvr CIILORIDE XVITII THE MONOALKYI.I?ESZESES IN RELATIVE
&FIYLENE Aromatic
~ ~ l C ! ~ I J . O R I IAI' ~ E 25.0'
Tolueneb Ethylbenzene Isoprop3-lbeiizrrie t-Butylbenzene '' Bcnzcne 1.00. * Ref. 7.
Ilelative rate"
128 129 128 114
I n view of the decrease in reactivity with increasing branching of the alkyl group observed in benzoylation and in essentially all other substitution reactions, lo this essential constancy in reactivity was unexpected. IVe tested the reliability of the method by running a competitive benzoylation of toluene ar.d t-butylbenzene. The value obtained, k t - B u p h / k T 0.62, was considered t o be in satisfactory agreement with the value 0.60 established from the rate constants (Table I). IT'e considered the possibility t h a t the addition and diffusion of the acetylation reagent to the mixture of hydrocarbons niight be the cause for these unusual reactivities. However, slow addition of a dilute solution of the acetyl chloride-aluminum chloride complex to a vigorously stirred solution of the two aromatics did not alter the relative reactivity. Finally, we carried out a competitive alkylation of benzene and p-xylene, and of p xylene with t-butylbenzene. From these reactivity ratios. kti-xxy/kB = 23.7, kl-Buph/kfi--Xy 4.69, the relative reactivity is calculated t o be k t - n u p h / k n 111, in excellent agreement with the previous value. Consequently, it appears t h a t these relative TABLE I1 reactivities (Table 111) are real and not the result ISOMERDISTRIRTJTIOXS I N THE REACTIONOF BENZOYL of any difficulty in the experimental procedure. Acetylation Isomer Distributions.-The reaction CHLORIDE-ALUMINUM CHLORIDE WITH THE ~ ~ O X O A L K Y L products from the acetylation of ethyl-, isopropylRENZESES IS ETHYLEKE DICHLORIDE AT 2.l 11 s2 p-t-Butyl Cyclohcxane W:LS u s c d :IS XJlvCtit.
ii
14 50 14.40 14.40 14 32 12 30 12 08 12 05 12
m
10.8 g. [ i f sernicarbazonc, :i yield of 41' i . Alfter several recr?-stalliz3tioIis from ethatiol, 8.0 g . of crj-itals, 111.1). 188189", w;is obtained. Ami. Calcci. for ClJIleSaO: C , W.95; H, 8.13; S, 18.02. Found: C, 66.60; H, 8.28; S,18.(J8. 'rhe semicarbazone \vas treated with 5 -17 l i ~ d r ~ ~ c l i l ~ ~ r i e acid a t 100' for 3 hours, and the i~t-t-but~-lacct~~~~,hcnolle was tlistilled, b . p . 1U2-103" a t 5 niiii., n a o1.5175 ~ Aim,!, C,ilcd. for CI?H160:C, 81.77; 11, 9.14. l'oulld: C, 81.73; H, 8.91. ~ - ~ - ~ u t y l b e t i z o p l i e i i from ~ ~ i i ethe I'rietiel-Crafts reactioii \ V ~ S purified through tlic scinicarbazc~~ic, 111.1,. 1c~i-1390, llizatiori frotii 98c~70petroleum ether ( l ) . p . 6570') atid 2({, etlinnol. aAnuZ. Calcd. for CIsII?1S30: C, 2 . 1 7 ; €1, i . 1 7 ; K, 14.23. Found: C, 73.13; 13, 7.85; S ,11.26. The semicarbazonc was trarlsforn1ctl into tlic ket~ilieb y the usual procedure, b.p. 167' a t 3 rnin., Z ~ O D 1.378(i.24 iii-t-But~lbenzophenone\vas syiithesized froiri it!-broiiirit-butylbenzene and benzoyl chloride tlirough the cudmiuiii intermediate. T h e crude product, 54cl, yield, n'ai purified by recrystallization of the semicarbazone from 95r; petroleum ether-5yi ethanol, m . p . 179-180". A ~ z e l . Calcd. for C l 8 H 2 , S a 0 : C, 73.17; H , 7.17; S , 14.23. Found: C, 72.92; H , 7.23; 9,14.08. 12.3) G F. Hennion and S. F. deC. McLeese, ibzd., 64, 2121 (1042), report b.p 13:3-194O a t 11 mm., t i Z b 1.3199, m.p. of semicarbazone 220-221 e. (24) I R K ~ ~ p I d nI I, N . Parton and J . Vaughan, ibid., 75, 4341 ( l 9 3 ) , report b p. 198' a t 1.; mm., J l ' o l l 13702.
Nov. 5 , 1959
HALOGENATION OF ~BUTYLBENZENES
5615
The semicarbazone was transformed into the ketone, b.p. TABLE IX 156" at 3 mm., ~ z * O D 1.5748. RETESTION TIMESOF ALKYLACETO-aPAND -BESZOAnal. Calcd. for Cl7HI8O: C, 85.66; H , 7.62. Found: PHENONES~,' C, 85.39: H, 7.29. SeparaEach of the ketones was established t o be of high purity by Temp., Retention time, min. tion, min. vapor phase chromatographic examination. Compound "C. 0mp0-m m-P Rates of Benzoy1ation.-The rates of benzoylation were Methylacetophenone 165 8 . 0 10.4 11.7 2 4 1.3 measured by the technique previously utilized for determin175 7 . 1 10.1 1 2 . 1 3 0 2 . 0 ing the rates of benzoylation of benzene and t o l ~ e n e . ~Ethylacetophenone The reactions were run in duplicate. The rate constants Isopropylacetoreported in Table I are averages of these duplicate measurephenone 185 8 . 9 11.2 2.3 ments with a n average deviation no greater than ~ k 2 7 ~ . t-Butylacetophenone 195 8 . 1 11.0 2.9 Relative Rates of Acetylation.-A solution of acetyl chloride-aluminum chloride complex in ethylene dichloride Methylacetophenone 185 4 . 9 6 . 0 6 . 6 1.1 0 . 6 with excess acetyl chloride was added quickly to a vigorously Ethylacetophenone 185 6 . 3 7.9 9.2 1.6 1 3 stirred solution containing a n excess of toluene and the Isopropylacetosecond hydrocarbon. It was established t h a t the temperaphenone 185 8 . 9 11.2 2.3 ture did not increase by more than 0.5'. An inert internal standard, 1,2,4-trichlorobenzene, was added t o facilitate t-Butylacetophenone 185 10.5 13.9 3.4 the determination of the actual concentrations of the prodMethylbenzophenone 247 9 . 4 11.9 13.9 2.5 2 . 0 ucts. Ethylbenzophenone 247 10.9 1 5 . 4 19.4 4.5 4 . 5 After 4 to 6 minutes, the reaction mixture was quenched in a n ice-sodium hydroxide mixture. The organic layer was Isopropylbenzowashed twice with water. The greater portion of the solvent phenone 247 10.9 1 6 . 2 22.0 5 . 8 5.3 was removed by distillation and the product concentrations t-Butylbenzophenone 247 17.0 24.8 7.4 determined by vapor phase chromatographic examination. The extent of reaction ranged from 80 to 90%. The relative a Column: 2 m. RCX polymeric BGA on Celite. Flow rates were determined using the formula rate 125 cc. of helium per min. Flow rate 350 cc. of helium per min . -ka= - logA/Ao kT log T I T O amination. The characteristic absorption bands for ortho The experimental results are summarized in Table VII. disubstituted benzenes (13-13.5 y ) were definitely absent in Isomer Distributions.-The composition of the products all the spectra. was determined utilizing a polyadipate substrate (Rubber T h e comparative examination of the infrared spectra of nzCorporation of America polymeric BG.4). I n the acetylaand p-alkyl-acetophenones exhibits a n interesting shift of tion products of isopropylbenzene and t-butylbenzene only the absorption bands toward shorter wave lengths with intwo major peaks were observed, corresponding to the meta creasing size of the alkyl groups (Table VIII). and para acetylation products. I n the case of ethylbenzene, Retention times of all the alkplacetophenones and benzoa minor peak a t the position anticipated for the ortho isomer phenones are summarized in Table I X . T h e retention times was observed. This peak was estimated from the area to be exhibit a consistent regularity and the separations improve 0.3'%, but no great precision is claimed for this analysis. with increasing size of the alkyl substituent. The major peaks ascribed to the meta and para isomers were isolated and identified as these isomers by infrared ex- LAFAYETTE, IND.
[CONTRIBUTION FROM THE RICHARDB. WETHERILL LABORATORY OF
PURDUE
UXIVERSITY]
Relative Rates and Isomer Distributions in the Halogenation of f-Butylbenzene and Some of Its Derivatives. Partial Rate Factors for Non-catalytic Bromination and Chlorination in Acetic
c.
BY LEONhf. s T O C K 4 f 5.4ND HERBERT BROWK RECEIVEDMARCH21, 1959 The noti-catalytic bromination of t-butylbenzene in 85Yc acetic acid at 25" yields 1.205; 0 - , 1.4ic;h m- and -97.39; p bromo-t-butylbenzene. Under these conditions the relative rate of bromination of benzene t o t-butylbeiizene is 1.00/138. These d a t a provide the partial rate factors 01 4.7, mi 6.1 and pf 806. Greater t h a n 99% 2- and 3-brorno-4-t-butyltoluetie arc formed in the bromination of p-t-butyltoluene. The isomer distribution for the non-catalytic chlorination of t-butylbenzene in 99.9% acetic acid a t 25" is 21.5Yc 0-,2.29yc m- and 76.2% p-chloro-t-butylbenzene. Under these conditions the relative rate of chlorination of benzene t o t-butylbenzene is 1.00/87.7. The partial rate factors for this reaction are of 56.6, mi 6.02 and p f 401. The chlorination of p-di-t-butylbenzene produces 28.6% p-chloro-t-butylbenzene and 71.47; 2 -chloro-1,l-di-tbutylbenzene. The chlorination of p-t-butyltoluene yields 5.0% p-chlorotoluene and 95% 2- and 3-chloro-4-t-butyltoluene. The observed relative rates of chlorination of these p-alkj'l-t-butylbenzenes, after correction for t h e chloro-de-t-butylation side-reaction, are in good agreement with the relative rates calculated on t h e basis of t h e partial rate factors.
That the electrophilic substitution reactions of toluene are correlated by the *proposed simple (1) Directive Effects in Aromatic Substitution. XL. (2) This research supported in part b y the Petroleum Research F u n d administered b y the American Chemical Society. Grateful acknowledgment is hereby made t o the donors of the Petroleum Research Fund. ( 3 ) Based upon a thesis submitted by Leon M. Stock in partial fulfillment for t h e degree Doctor of Philosophy. (4) Monsanto Chemical Co. Fellow, 1957-1958. ( 5 ) Department of Chemistry, University of Chicago.
linear relationship6+ has been adequately confirmed.$ I t has been suppested that the substitution reactions of other monosubstituted benzenes Unfortunately~ the Obey a number Of quantitative experimental observations for compounds other than toluene is quite limited. (6) H. C. Brown and K. L. Nelson, THIS J O U R N A L , 7 5 , 6292 (1953). (7) H. C. Brown and C. W. McGary, ibid., 77, 2300 (1955). (8) H. C. Brown and C. R Smoot, ibid., 78, 6 2 5 5 (1956).
(9) L. M . Stock and H. C. Brown, ibid., 81, 3323 (1959).
