Oct. 5, 1954
PARTIAL RATEFACTORS IN CHLORINATION OF BENZENE, TOLUENE AND XYLENES 5175 [CONTRIBUTION FROM THE
RICHARD B.
WETHERILL
LABORATORY O F PURDUE UNIVERSITY]
Rates of Chlorination of Benzene, Toluene and the Xylenes. Partial Rate Factors for the Chlorination R e a ~ t i o n ' ~ ~ ~ ~ BY HERBERT C. BROWNAND LEONM. STOCK RECEIVED APRIL 18, 1957 The isomer distribution in the non-catalytic chlorination of toluene in acetic acid a t 25" has been determined to be 59.78% ortho, 0.48% meta and 39.74% para. The relative rate of chlorination of benzene and toluene under these conditions is 1.00/344. These data establish the partial rate factors for the non-catalytic chlorination of toluene to be of = 617, mr = 4.95 and pi = 820. Excellent agreement is observed between the observed rates of chlorination of the isomeric xylenes and the rates calculated using these partial rate factors. Chlorination of m-xylene yields 23,0y0 2- and 77.0% 4-chloro-1,3dimethylbenzene. The small discrepancy with the calculated values, 27.3% 2- and 72.7y0 4-, is attributed to the "double ortho effect" accompanying substitution in the 2- position.
I n examining the partial rate factors for the halogenation of toluene, a large discrepancy was noted between two possible sets of partial rate factors for the chlorination r e a ~ t i o n . ~One set had been developed by Condons and was based upon the isomer distribution observed in the catalyzed chlorinatioq6 the relative rates of the uncatalyzed chlorination of benzene and t o l ~ e n eand , ~ an assumed value of mf: of = 600, mf = 5.0 and pf = 870. The second set was obtained by solution of the simultaneous equations for the relative rates of chlorination of benzene, toluene, 0- and p-xylene7: of = 402, mf = 8.2 and p f = 1280. Excellent agreement had been realized previously in both the bromination4 and the mercuration reactions* between partial rate factors based primarily upon the observed isomer distribution in toluene and those based upon relative rates. Consequently, a detailed study of the chlorination reaction appeared desirable in order to resolve the discrepancy. Results Bromination of aromatics in acetic acid solution exhibits complicated kineticsg and the reactivities are generally established by determining the time required for 10 or 20Y0 reaction under identical conditions. Fortunately, the chlorination reaction does not exhibit these kinetic difficulties. Thus it has been observed that the chlorination of phenolic ethers and anilides1° and the chlorination of a number of aromatic hydrocarbon^^^^^^^^ exhibit clean second-order kinetics. l 3 Accordingly, the relative reactivities were established by measuring the second-order rate con(1) Directive Effects in Aromatic Substitution.
XVIII.
(2) Supported b y t h e Petroleum Research F u n d of t h e American Chemical Society. (3) Based upon a thesis submitted b y Leon M. Stock in partial fulfillment of t h e requirements for t h e degree of Doctor of Philosophy. J O U R N A L , 79, 1 4 2 1 (1957). (4) H. C . Brown a n d L. M . Stock, THIS (5) F. E. Condon, ibid., 70, 1963 (1948). (6) E. Wertyporoch, A n n . , 493, 753 (1932). (7) P. B. D. de l a M a r e and P . TV. Robertson, J. Chem. Soc., 279 (1943). (8) H. C. Brown a n d C . W. M c C a r y , Jr., THISJOURNAL, 77, 2310
(1955). (9) See ref. 4 for pertinent references. (10) A. E . Bradfield a n d B. Jones, J . Chem. Soc., 1006 (1928), a n d subsequent papers in t h i s series. (11) K. Lauer a n d R. Oda, Ber., 69, 1061 (1936). See criticism of this s t u d y b y d e la Mare a n d Robertson, ref. 7. (12) M . J. S. Dewar a n d T. Mole, J . Chem. Soc., 342 (1957). (13) It has been reported t h a t hydrogen chloride, generated in t h e reaction, results in a small autocatalysis (ref. 7). However, this phe. nomenon was not observed in t h e present study.
stants for the reaction of chlorine with the aromatics in 99.87 mole acetic acid. Light was carefully excluded to avoid reaction of the chlorine with the side chains. A number of check experiments indicated that such side-chain chlorination was less than 1-2Y0 and could be neglected. Typical results are summarized in Table I. TABLE I
SECOND-ORDER RATE CONSTANTSFOR
THE REACTION OF CHLORINE WITH BENZEXE AND METHYLBENZENES IS 99.87% ACETIC ACID AT 25.0 O
Compound
Benzene
Concentration, M [A r H I [Ch]
R a t e constant
kz X 104, 1. mole-lsec.-l 0.0147 f 0.0003
0.1990 0.0893 ,0158 i ,0005 ,0885 ,1990 5.27 It .04 ,0313 Toluene ,1908 5.34 f .I7 .1990 ,0342 32.0 i .5 ,1895 o-Xylene ,0252 33.1 f .4 ,2000 ,0398 32.5 i .2 ,1776 ,0211 32.0 f .I p-Xylene .I905 ,0281 i 80 m-Xylene ,0132 .0132 2870 i 100 ,0130 ,0120 2990 2840 i 110 .00619" .0167 2820 i 40 .00613* ,0166 =t100 .0197" .0197 2530 a Solutioii contained benzene, 0.0646 M . Benzene, 0.0993 M . Benzene, 0.1974 M.
m-Xylene was very reactive and it was necessary to reduce both the chlorine concentration and the m-xylene concentration in order to reduce the rate of the reaction sufficiently to permit measurement. In the case of o-xylene it was demonstrated that a similar decrease in the chlorine concentration did not alter the rate constant significantly (Table I). It has been noted that the halogenation reaction is quite sensitive t o changes in the polarity of the reaction m e d i ~ m .To ~ ~test ~ ~the ~ possible effect of the decrease in hydrocarbon concentration on the rate constant, the chlorination of m-xylene was examined in the presence of added benzene in several concentrations (Table I ) . With the possible exception of the highest concentration, the variations in the observed second-order rate constants resulting from this procedure are no greater than the experimental uncertainty. It was therefore concluded that in the concentration ranges utilized in the present experiments the effect of (14) R . M. Keefer, A. Ottenberg and L. J. Andrews, THIS JOURNAL, 78, 255 (1956).
HERBERT C. BROWN AND LEONM. STOCK
5176
changes in the hydrocarbon concentration could be neglected. All previous studies of the isomer distribution in the chlorination of toluene have employed electrophilic catalysts and have been carried out generally in the absence of a solvent. Thus Wahl and co-workers observed 58% 0- and 42% p-chlorotoluene with metallic iron as catalyst15 while Wertyporoch noted 58% 0- and 42% p- using antimony pentachloride and 55% 0- and 45% p- with iodine as the catalyst.G The presence of "traces" of the meta isomer was reported in an early chlorination of toluene in the presence of molybdenum pentachloride. l6 For purposes of the present investigation it was necessary to determine the isomer distribution under the identical conditions utilized for the rate measurements. Moreover, it was especially important to determine accurately the meta isomer formed in the reaction. Accordingly, the chlorination of toluene was carried out in the absence of catalyst by the slow addition of chlorine in acetic acid to a large excess of toluene in the same solvent. The product was isolated and analyzed by infrared. The results are summarized in Table 11.
VOl. 79
and the well established value for pf. This procedure indicates that the product should contain 0.48 mole yo m-chlorotoluene. I n view of the excellent agreement between these different approaches, i t would appear that considerable confidence may be placed in the experimental value of 0.48 mole yo. It was of interest to examine also the isomer distribution in the chlorination of m-xylene. The hydrocarbon was chlorinated using procedures identical with those adopted for toluene. The Droduct was isolated. distilled and analvzed bv inkrared spectroscopy. ' The results are iummaiized in Table 111. TABLE I11 ISOMERDISTRIBUTION IN THE KOS-CATALYTIC CEILORIXATION OF WZ-XYLENE IN ACETICACID AT 25" Run
Procedure"
Product, mole % 2-Chloro 4-Chloro
1 B 22.0 B 24.1 2 Mean 23.0 i 1 . 0 See Experimental Part.
78.0 76.9 77.0 =t1 . 0
Recovery
%
97.5 88.8
Discussion
TABLE I1 For convenience in following the discussion, the ISOMER DISTRIBUTION IN THE KOS-CATALYTIC CHLOR~NA-average values of the second-order rate constants TION OF TOLUENE IN ACETICACID AT 25" are summarized in Table IV, together with the Run
Procedure"
ortho
Product, mole
mefa
%
para
1 A 60.32b 39. 67b A 60.58 0.45 38.98 2 3 R 59.77 .51 39.72 4 13 59.30 .52 40.18 5 13 59.21 .44 40 35 Mean 59.78 rt 0.42 0 . 4 8 i 0.04 30.74 f 0.42 a See Experimental Part. These values were normalizcd with 0.048% meta to obtaiu the mean values.
Based upon our experience with analysis of synthetic mixtures, we believe that the values for the ortho and para isomers are probably accurate within the limits of uncertainty indicated in Table 11. However, the limits of uncertainty for the ineta isomer in the analysis of these synthetic mixtures appeared to be *0.10. Consequently, the excellent agreement in the analytical results for this component may be somewhat fortuitous. Fortunately, the meta partial rate factor and the mole per cent. meta isomer formed in the chlorination reaction may be estimated from independent data. First, solution of the set of simultaneous equations for the rates of chlorination of the three xylene leads to a value for mf. This value indicated that the chlorination of toluene should result in the formation of 0.50 mole yo m-chlorotoluene. Second, using the value of of from the rate and percentage ortho isomer in the chlorination of toluene, and the rate data for p-xylene, it is possible to calculate mf. This value predicts a concentration of O . W j & for the meta isomer. Finally, it is possible to utilize the Selectivity Relationship'? log pf = 1.310 log ( p i / m r ) (15) A. Wahl. G. h-orman a n d G. Vermeylen, Bull. soc. c h i m . , 141
31, 570 (1922). (16) B. Aronheim a n d G. Dietrich, Bcr., 8. 1401 (1876); J. S . Reese, Chein. Revs., 14, 55 (1934). (17) H. C. Brown and C. R. Smoot, THISJ O U R N A L , 7 8 , 8255 (1956).
available literature values. The second-order rate constant for the chlorination of benzene, 14.8 X lo-' (24') l.mole-lsec.-', obtained by de la Mare and Robertson,? is in serious disagreement with that reported by Lauer and (30') 1.mole-' sec.-l. HowOda," 4.2 X ever, the present value, 15.4 X lo-? l.mole-Isec.-' supports the lower value of the former workers.1a The relative rate data for toluene and p-xylene are in excellent agreement with the earlier values of de la Mare and Robertson.' However, our value for o-xylene is less than half of the earlier value. Some confidence may be placed in the present value on the basis that i t leads to a consistent set of partial rate factors, whereas the higher value results in serious discrepancie~.'~ The relative rate of chlorination of m-xylene has not been reported previously. The reaction is cxceedingly fast. Even utilizing very low concentrations of the reactants results in a relatively fast reaction with a half-life of approximately 3 minutes. As a result, the uncertainty in this value map be somewhat greater than in the case of the other, less reactive hydrocarbons. Utilizing the relative rates of Chlorination of benzene and toluene and the observed isomer distribution in toluene, we can calculate the partial rate factors. These are listed in Table V. The (18) Dewar a n d Mole (ref. 12) have reported a value of 0 2z 1 X I O 7 1. mole-: sec.-l f o r this reaction, However, they utilized solutions relatively concentrated in t h e hydrocarbon, 0.6 M , and t h e lower rate constant m a y result from t h e decrease in t h e polarity of t h e reaction medium (ref. I O , 11). (19) I n preliminary experiments utilizing o-xylene of 99.5 mole % purity we observed a n initial rapid depletion in t h e chlorine concentration. I t was suspected t h a t t h e phenomenon was due t o t h e highly reactive m-xylene present a s a n impurity. Utilization of a sample of o-xylene of 99.99 mole yo purity avoided this difficulty. It is possible t h a t t h e high value for 0-xylene observed by d e la M a r e a n d Robertson is due t o this cause.
Oct. 5, 1957
PARTIAL RATEFACTORS IN CHLORINATION OF BENZENE, TOLUENE AND XYLENES 5177
TABLE IV OBSERVED A N D CALCULATED RELATIVE RATESOF CHLORINATION OF BENZENE AND AT
Compound
Rate constantsa k, X 104, .mole-l set.-'
THE
METHYLBENZENES IN ACETICACID
25' Relative rates, k / k o
7
Obsd.
Calcd.
Lit.6
Benzene 0.0154 1.00 1.oo 1.00 Toluene 5.30 344 350 344 o-Xylene 32.3 2.10 x 108 4 . 6 x 103 2.37 x 103 2850 1 . 8 5 X lo6 m-Xylene 2.32 X lo5 +Xylene 32.0 2.08 x 103 2 . 2 x 103 2.04 x 103 Hernimellitene 8.37 x 105 Pseudocumene 7.34 x 106 Mesitylene 1.56 X 108 Prehnitene 4.13 X lo6 Isodurene 5.15 X 108 Durene 3.11 X 106 Pentamethylbenzene 1.27 x 109 a Mean values based on data in Table I together with additional determinations. Ref. 7. Benzene and toluene were compared in 80% acetic acid and the remaining compounds in 99% acetic acid by measurement of the 10 and 20% reaction time at 24". Based on of 617, mr 4.95, pr 820.
agreement with Condon's estimated values6 (of = GOO, mf = 5.0, pf = 870) is remarkably close. A set of partial rate factors can also be calculated from the observed rates of chlorination of benzene, toluene, 0- and p-xylene. In a similar manner it is possible to derive a set of partial rate factors from the relative reactivity data for the three xylenes. Finally, the partial rate factors can be based upon 9, and the Selectivity Relationships for toluene. The results of these different approaches to the partial rate factors are summarized in Table V. TABLE V PARTIAL RATEFACTORS FOR THE CHLORINATION OF TOLUENE IN ACETICACID AT 25' Source
Isomer distribution and k T / k g Relative rates: k g , k T , k,-xr, ko- XY Relative rates: k ~ k ,p - x y , ko-xy,
km-xy
Selectivity relationship Log (#f/w). Ref. 17.
Partial rate factors Selectivity 01 mr P I factor, S r
617 4 . 9 5 820
2.219
681 4 . 5 8 694
2.180
604 5.17 615 529 4.87 820
2.076 2.224
culated rates for the mercurations and the brominationof all of the methylben~enes.~ The measure of the agreement is indicated by Figs. 1 and 2. On this basis, considerable confidence may be placed in the calculated values for the chlorination of the higher methylbenzenes.20
2.4
rr-
j
1.8
n 0
v
0 Y
2 1.2 -
Considering the possible effects of experimental uncertainties in these different approaches to the partial rate factors, the agreement in the values may be considered excellent. Since the values based upon the isomer distribution in toluene and the toluene/benzene relative rate involve a direct calculation without any additional assumptions, we shall adopt this set for the chlorination reaction. The Hammett reaction constant, p, may then be calculated t o be -9.80, Utilizing these partial rate factors, of 617, mf 4.95 and pf 820, i t is possible to calculate the rates of chlorination of all of the methylbenzenes (Table IV). I n the case of three xylenes the observed chlorination rates agree with those calculated from the partial rate factors with an average deviation of 10%. Unfortunately, the rates of chlorination of the higher methylbenzenes are far too fast to measure by the present procedure and experimental values for their rates could not be obtained for comparison. However, earlier studies have revealed good agreement between the observed and cal-
0.6
0 0
0. 6
I.2
1.8
2.4
log k / k o (calc.). Fig. 1.-Observed
and calculated relative rates of mercuration of the methylbenzenes.
The partial rate factors also permit a calculation of the isomer distributions to be expected in the chlorination reaction. The values are summarized in Table VI. Experimental determination of the isomer distribution in the chlorination of m-xylene indicated the formation of 23.070 2-chloro-1,3-dimethy1benzene and 77.070 4-chloro-1,3-dimethyIbenzene. (20) The zinc chloride-catalyzed bromination and the iodine chloride iodination of the methylbenzenes have also been examined: L. J. Andrews and R. M. Keefer, THISJOURNAL, 78, 4549, 5623 (1956). However, in these cases the agreement between the observed and calculated relative rates is somewhat less satisfactory.
HERBERT C. BROWNAND LEON11. STOCK
5178
12
IO
/
/
A
d rn
e
1,3,5-
1,2,3,4-
n
1,2,4,5.
0
1,2,3-
1
0 x
1,2,4-
6
\
x
0
-0
4
2
0 0
2
6
4
l o g k/k,(calc
e
12
IO
),
Fig. 2 --Observed and calculated relative rate\ tion of the methylbenzenes.
I if
broiiiiiia-
This is in reasonable agreement with the calculated values: 27.3y0 2- and 72.770 4. The somewhat lower value observed experimentally for the 2- isomer may be due to a slightly retarded rate of substitution in the 2-position where the chlorine atom must substitute ortho t o two methyl groups. It was pointed out previously that retarded substitution rates may be anticipated for such "doublcovtho" positions.812?n TABLE VI DISTRIBUTION IS THE CHLORISATIOU METHYLBCXZENES IS ACETICACID AT 25'
CaLCULATED ISOMER OF THE
Mole
Compound
2-
Toluene o-Xylene m-Xylene Hemimellitene Pseudocumene
59.7
'0
monochloro isomer in prod i c t s 345-
0 5 42.9
27 3 42.8
39 8 57 1 72 7 997 ,56 9
The isomeric chloro-nz-xylenes were prepared from the corresponding xylidines in concentrated hydrochloric acitl solutions.** However, this technique did not result in increased yields over those realized with the Sandmeyrr procedure for these compou1ids.~3 The 2-chloro-l,3-dimcthylbenzene was obtained in Soy0 yield, while the 4-chloro-l,3dimethylbenzene was obtained in 40y0 yield. These conipounds were purified by two steam distillations from aqiicou5 sodium hydroxide, extraction with ether, followed by fraetionation. The following properties and purities ( i i i i n o l c yo) were observed: 2-chloro-1,3-dimethylbenzene,b.11. 183.9' at 750 a m . , f.p. -34.96", %%OD 1.Ti268, 99.6%,; 4chloro-1,3-dimethylbenzene,b.p. 185.1" a t 740 mm., f.1). -32.49', %'OD 1.5235, 99.1%. Carbon disulfide and acetic acid were purified by proccdures previously d e ~ c r i b e d . ~Chlorine (Matheson, 99.5 was used directly, after drying by passage through sulfu acid. The acetic acid used in the kinetic experiments was purified by distillation followed by a low temperature sublimation, f.p. 16.585", 99.87 mole Yo purity. The acetic acid in the isomer distribution studies was used aftcr fractionation, but without sublimation, f.p. 16.25", 99.3 mole yo purity. Kinetic Measurements.-It was established that the rate of the dark reaction of chlorine with acetic acid was slow a t 'ioo, kl = 7.0 X set.-', and negligible a t 25' compared to the rate of reaction with the aromatics. Consequently, no correction for this side reaction was necessary. All rate measurements mere performed in black flasks with a long narrow neck in an enclosed constant temperaturebath a t 25.00 =t 0.03". Experiments were initiated by mixing thoroughly acetic acid solutions of known concentrations of aromatic and chlorine. An aliquot was withdrawn to establish zero time, thereby minimizing the effects of initial losses due to the volatility of the chlorine and the effect of trace impurities in the aromatic. Analyses for unreacted chlorine were made iodimetrically. The kinetic experiments were carried out to 75-80y0 completion, except in the case of benzene which had proceeded to 10% reactior only in five days. (In the latter case, a small correctinti, 0.001 M eli, established in blank experiments was applied t o conipensate for small losses in chlorine during the removal of aliquots over the extended reaction time.) The specific second-order rate constants wcre calcu1:itctl for each point taken, 5 to 7 for each kinetic study. Tllc second-order rate constants are reported in Table IV. Rate data for a typical determination are reported in Table V I I.
TABLE TTII
6-
Rxm
DATA FOR THE
REACTION OF CIILORINE W I T H TOLTJ-
87yoACETIC .%CID 0.002 0 3
Time, sec.
0 .3
The partial rate data define a point which fits precisely on the Selectivity Graph for the substitution reactions of toluene.21 It may be concluded that the chlorination reaction clearly follows all other electrophilic substitution reactions previously examined in obeying the Selectivity Relationship.
Experimental Part Materials .-Samples of all of the aromatic hydrocarbons were available from previous studies488 with purities of 99.570 or better. The o-, nz- and p-chlorotoluenes utilized as infrared standards were commercial materials, purified by fractionation. p-Chlorotoluene was further purified by fractional crystallization. The mole yo purities mere established by analysis of the cooling curves. The products utilized exhibited the following physical properties and purities (in mole yo): o-chlorotoluene, b.p. 158' a t 747 mm., nza: 1.5258, f.p. -3
