Proc. Natl. Acad. Sci. USAVol. 74, No. 10, pp. 4121-4125, October 1977Chemistry

Electrophilic mercuration and thallation of benzene and substitutedbenzenes in trifluoroacetic acid solution*

(electrophilic substitution/selectivity)

GEORGE A. OLAH, IWAO HASHIMOTOt, AND HENRY C. LINttInstitute of Hydrocarbon Chemistry, Department of Chemistry, University of Southern California, Los Angeles, California 90007; and the Department ofChemistry, Case Western Reserve University, Cleveland, Ohio 44101

Contributed by George A. Olah, July 18, 1977

ABSTRACT The mercuration and thallation of benzene andsubstituted benzenes was studied with mercuric and thallictrifluoroacetate, respectively, in trifluoroacetic acid. With theshortest reaction time (1 sec) at 00, the relative rate of mercu-ration of toluene compared to that of benzene was 17.5, withthe isomer distribution in toluene of: ortho, 17.4%; meta, 5.9%;and para, 76.7%. The isomer distribution in toluene varied withthe reaction time, significantly more at 25° than at 00. Thecompetitive thallation of benzene and toluene with thallic tri-fluoroacetate in trifluoroacetic acid at 150 showed the relativerate, toluene/benzene, to be 33, with the isomer distribution intoluene of: ortho, 9.5%; meta, 5.5%; and para, 85.0%. Withincreasingly higher reaction temperatures in both mercurationand thallation reactions of aromatics, isomerization (both in-tramolecular and intermolecular) within the relevant ortho- andpara-metallated intermediate ions and/or of the isomers be-comes more important. Competitive rates and isomer distri-butions of mercuration and thallation of benzene and substi-tuted benzenes were also determined. The predominant parasubstitution in both mercuration and thallation of methylben-zenes reflects, besides some steric factors, the strong stabilizingeffect of para methyl groups on the arenium ion intermediates.Under predominantly kinetically controlled conditions, noanomalous increase in the amount of meta substitution wasobserved.

The mercuration of aromatic hydrocarbons with mercurictrifluoroacetate in trifluoroacetic acid at 250 has been studied,predominantly by Brown and coworkers (1-3). The rate ratioof toluene to benzene (kT/kB) was found to be 9.89 and theisomer distribution in toluene was: ortho, 12.2%; meta, 8.6%;and para, 79.2%. From these results, as well as from their pre-ceding studies in acetic acid solution with perchloric acid cat-alyst giving kT/kB = 3.6-7, and 12-16% meta isomer, it wasconcluded that the relatively high meta isomer ratio observedwas due to the high electrophilic reactivity of the mercuratingagent causing the observed low selectivity substitutions. Theseresults were correlated by the so-called Brown selectivity re-lationship (for a review, see ref. 4).

It should be pointed out that, in their original paper onmercuration of toluene and benzene, Brown and Nelson, (5)extending the claimed quantitative selectivity relationshipgoverning isomer distributions in aromatic substitution, com-mented on the preceding work of Klapproth and Westheimer(6) who found that, in glacial acetic acid with perchloric acidas catalyst, toluene was mercurated by mercuric acetate at 250to give 6% of the meta isomer. Brown and Nelson claimed tohave observed 12% meta isomer under similar conditions andstated "we are, however, unable to account for the differencesbetween the values of the meta isomer." In the former case (6),a precise radiochemical technique was used in establishingorientation, whereas infrared analysis was utilized in the latter.Comparison of the experimental sections of Brown and Nelson'sand Klapproth and Westheimer's papers, however, indicatesthat the claim of reinvestigation "under similar conditions" isincorrect. Klapproth and Westheimer carried out their exper-

4121

iments at 250 and quenched the mixtures after 5 min, whereasBrown and Nelson carried out the reaction for 6.5 hr. In sub-sequent work, in view of the importance of both the mechanisticand practical implications of the orientation-rate correlation,Brown and McGary (7), carried out a more detailed study ofthe mercuration reaction. They concluded: "A redeterminationof the isomer distributions and relative rates indicates excellentagreement with the linear relationship of orientation and rel-ative rate." They recognized, however, that the isomer distri-butions change with time but still claimed that the correctedmeta isomer distribution in the mercuration of toluene is 9.5+ 0.5% and remarked, "there remains a relatively minor dis-agreement with the values reported by Klapproth and West-heimer (6% meta) for which we are unable to account."

In preceding work (8) we have suggested that, regardless ofthe substrate selectivity in electrophilic aromatic substitutions(which generally can be most conveniently expressed by kT/kBrate ratios), the regioselectivity (positional) of the reactionsremains high, with usually predominant ortho-para substitu-tion, the amount of meta isomer being 2-5%.We considered it possible that the reported relatively high

meta isomer ratio observed in preceding studies of mercurationof toluene (as well as related alkylbenzenes) was still the resultof ongoing faster isomerization and/or disproportionation ofthe ortho and para isomers, compared to the more stable metaisomer (7) or related rearrangements in the a-complex inter-mediates of the reactions. These would fully explain the dis-agreements with Klapproth and Westheimer's work. Conse-quently, it appeared desirable to reinvestigate in detail themercuration of benzene and substituted benzenes under pre-dominantly kinetically controlled conditions, minimizing thepossibility of isomerization and disproportionation affectingthe results.The thallation of benzene and substituted benzenes with

thallium trifluoroacetate in trifluoroacetic acid, the mechanismof which was considered to be similar to that of related mer-curations (9-11), was also studied under similar conditions.

MATERIALS AND METHODSMaterials. Benzene, toluene, and alkylbenzenes were of

highest available purity. Trifluoroacetic acid (Cationics, Inc.)was obtained in sufficient purity to be used without furtherpurification (boiling point 730). Mercuric and thallium triflu-oroacetate were commercial materials (Aldrich Chemical Co.,Inc.).

Abbreviation: kT/kB, ratio of reaction rates for toluene and ben-zene.* Paper no. 40 in the series "Aromatic Substitution." Paper no. 39 wasOlah, G. A., Pelizza, F. Kobayashi, S. & Olah, J. A. (1976) J. Am.Chem. Soc. 98,296.

t Visiting Scientist from the Wakayama Technical College, Japan.* Senior Research Associate (1974-1975).

Proc. Natl. Acad. Sci. USA 74 (1977)

r - - -- 1 Table 1. Gas/liquid chromatographic parameters

E ~G

Quench

FIG. 1. Flow-quenching apparatus for the mercuration ofpolymethylbenzenes. Two 50-ml syringes (A and B) are driven by asyringe pump (C; Sage Instruments, model 351). The separate solu-tions came together via two flexible Teflon tubes (D and E, insidediameter 1.4 mm; length, 60 cm), mix at F, react during passage G, andare quenched at the end of G.

Competitive Mercuration of Benzene and SubstitutedBenzenes. Mercuric trifluoroacetate (25 ml of 0.2 M solutionin trifluoroacetic acid) was added rapidly to 25 ml of 1 M so-lutions of the aromatics in the same solvent with vigorous stir-ring, both precooled and kept at 00. After specific time inter-vals, the reaction mixture was quenched with aqueous sodiumbromide, maintaining a bromide ion to mercuric ion ratio of3:1. The precipitated arylmercuric bromides were filtered andthoroughly dried under reduced pressure. Then, the mixtureof arylmercuric bromides was slowly treated with bromine inchloroform at 00 until a permanent red color developed. Stir-ring was continued for 5 hr at 0° and then for an additional hourat room temperature. Conversion of arylmercuric bromides tothe corresponding bromoarenes under these conditions wasfound to be quantitative for all practical purposes (the slowerconversion of the parent phenylmercuric bromide withoutraising the temperature to ambient is not always quantitative).The reaction mixture was washed with sodium bisulfite solutionand water and then dried over sodium sulfate. Bromoareneswere subsequently analyzed by gas/liquid chromatography.

Competitive Mercuration of Polymethylbenzenes. Therates of mercuration of polymethylbenzenes are so fast that itis difficult to carry out the reactions by the usual methods.Therefore, a modified flow-quench apparatus (12) was usedto allow short reaction times (of the order of 1 sec) (Fig. 1).Quenching was with aqueous sodium bromide surrounded byice. The quenched products were worked up, brominated withbromine in chloroform, and then analyzed by gas/liquidchromatography.

Competitive Thallation of Benzene and SubstitutedBenzenes. The thallations were carried out with exclusion oflight, in a manner similar to that for the mercurations. Thereaction mixture was quenched in aqueous sodium chloride,and the solution was evaporated under reduced pressure untila white solid remained. The bromination of arylthalliumchlorides was performed with bromine in carbon tetrachlorideaccording to the procedure described by Wright (13). Productbromoarenes were analyzed by gas/liquid chromatography asin the case of mercuration.

Analytical Procedure. All products were analyzed by gas/liquid chromatography, with a Perkin-Elmer model 900 gaschromatograph equipped with a hydrogen flame ionizationdetector and either open tubular capillary columns or a packedcolumn. Peak areas were calculated with the Infortronics modelCRS-100 electronic printing integrator.

Characteristic retention times of bromo compounds alongwith type of column and conditions are listed in Table 1. Au-thentic samples of pure substituted bromobenzene isomers wereavailable in our laboratory from previous work. A 12-ft 4,4'-azoxyanisole-packed column was utilized at 1300 to achievecomplete separation of bromotoluene isomers (14).

Bromo compoundBromobenzeneo-Bromotoluenem-Bromotoluenep -Bromotolueneo-Bromoethylbenzenem -Bromoethylbenzenep-Bromoethylbenzeneo -Bromoisovronvlbenzene

p -Bromoisopropylbenzeneo-Bromo-tert- butylbenzenem -Bromo-tert- butylbenzenep -Bromo-tert-butylbenzene4-Bromo-o-xylene4-Bromo-m-xylene2-Bromo-p-xyleneBromoesitylene4-Bromo-1,2,3-trimethylben-

zene5-Bromo-1,2,4-trimethyl-benzene

5-Bromo-1,2,3,4-tetramethyl-benzene

4-Bromo-1,2,3,5-tetramethyl-benzene

BromodureneBromopentamethylbenzeneo-Bromofluorobenzenem-Bromofluorobenzenep-Bromofluorobenzeneo-Bromochlorobenzenem -Bromochlorobenzenep-Bromochlorobenzeneo -Dibromobenzenem -Dibromobenzenep -Dibromobenzeneo-Bromoanisolem-Bromoanisolep-Bromoanisole

Column*DDDDDDDDDDDDDCCCC

C

C

Conditionst ofretention

Column Time, minI 5.9I 10.0I 11.0I 12.3I 10.3I 12.0I 14.2I 11.7I 13.8I 18.3I 16.2I 19.3I 22.3I 2.9I 2.7I 3.0III 3.2

I

I

5.1

4.8

C II 4.5

C II 4.5C III 6.6C III 17.8B IV 12.2B IV 9.8B IV 10.5B V 15.1B V 13.0B V 13.2B V 25.8B V 21.9B V 22.2A VI 10.9A VI 9.6A VI 10.1

* A, stainless steel open tubular column, 150 ft X 0.01 inch, wall coatedwith butanediol succinate; B, stainless steel open tubular col-umn, 150 ft X 0.01 inch, wall coated with m-bis(m-phenoxyphen-oxy)benzene modified with 20% Apiezon L grease; C, stainless steelpacked column, 12 ft X 1/8 inch, with solid support 80-100-meshChromosorb W and liquid phase 1.75% butanediol succinate; D,stainless steel packed column, 12 ft X 1/8 inch, with solid support80-100-mesh Chromosorb W and liquid phase 15.0% 4,4'-di-methoxyazoxybenzene.

t Column conditions [column temperature, °C (carrier gas, helium,pressure (psi) or flow rate (ml/min)]: I, 130 (20 ml/min); II, 140° (20ml/min); III, 145° (20 ml/min); IV, 950 (20 psi); V, 1300 (20 psi); VI,1500 (20 psi).

The apparent decrease of kT/kB with time is accompaniedby an increase in the amount of the ortho and meta isomers atthe corresponding expense of the para isomer. The possibleisomerization (transmercuration) in the involved reaction in-termediates, although not necessarily of the arylmercury tri-fluoroacetate products, was not fully recognized previouslywhen limited product isomerization was only considered. Weare forced to conclude that the higher proportions of the meta(and ortho) isomer reported previously may be a result of theisomerization (disproportionation) of the kinetically favoredinitially formed para and ortho arenium ions, compared withthe thermodynamically more stable meta substituted ions.

4122 Chemistry: Olah et al.

Proc. Nati. Acad. Sci. USA 74 (1977) 4123

Table 2. Variation of kT/kB for mercuration of benzene andtoluene and of isomer distribution in toluene with reaction

time at room temperature (22°)*

Time, Isomer distribution, %min kT/kB Ortho Meta Para

0.15 sec 15 21.3 6.9 71.80.91 sec 15 23.7 7.6 68.71 14 23.3 7.9 68.82 13 26.6 8.7 64.73 11 30.0 9.1 60.96 10 30.2 9.7 60.115 10 32.5 10.1 57.4160 8 37.4 12.6 50.0

* Concentrations: total [ArH], 1.00 M; [Hg(OCOCF3)2], 0.10 M.

RESULTS AND DISCUSSIONMercuration. We first restudied the competitive mercuration

of toluene and benzene with mercuric trifluoroacetate in tri-fluoroacetic acid at ambient room temperature (220). [The ratesof mercuration of monoalkyl- and polymethylbenzenes withmercuric acetate in acetic acid have been determined (15).] Theproducts were precipitated as aryl mercuric bromides, filteredoff, and then convertedby bromination into bromobenzene andthe corresponding bromotoluenes, which were analyzed bygas/liquid chromatography.

Hg(OCOCF)

+ Hg(OCOCF3)2 riiiaq.NaBr

HgBr Br

Br,

min CHC1.

Because the mercuration of benzene and alkylbenzene intrifluoroacetic acid is fast, it was considered necessary to usea flow-quenching apparatus (12) to allow proper mixing of thereagents before reaction.The relative rates and isomer distributions in the competitive

mercuration of toluene and benzene at 220, summarized inTable 2, show the variation of the data with reaction time, aswell as the difficulties one can encounter due to the lack of ki-netic control when intermolecular and/or intramolecularisomerizations affect the data.

CH3 CH3 CH3

__ ~~~~~~~~Hetc.

HAHgOCCF3 X+ HgOCCF,HgOCCF3 0

0~~~~The ready mercury shifts in arenemercurenium ions was

previously demonstrated in our studies under stable long-livedion conditions.Because of the changing isomer distributions encountered

in the mercuration of benzene or toluene at 220, we next studiedthe reaction at 0° (the lowest practical temperature attainable)to slow down the isomerization processes.The amount of meta isomer in the mercuration of toluene

was 5.9% with the shortest reaction time (1 sec) studied at 00,compared to the lowest proportion of meta isomer, 6.9% ob-

Table 3. Competitive mercuration of monosubstituted benzeneswith mercuric trifluoroacetate in trifluoroacetic acid at 00

with 2-sec reaction time*

Isomer distribution, %

ArH kArH/kB Ortho Meta Para

Benzene 1Toluene 18 20.6 6.4 73.0Ethylbenzene 15 2.3 3.5 94.2Isopropylbenzene 14 0.6 5.4 93.9tert-Butylbenzene 10 0 8.4 91.6Fluorobenzene 0.24 27.6 0.2 72.2Chlorobenzene 0.03 27.3 0.1 72.6Bromobenzene 0.2 18.7 1.7 79.6Anisole 860 18.8 0.2 81.1

* Concentrations: total [ArH], 1.0 M; [Hg(OCOCF3)2], 0.1 M.

served at 220 (kT/kB = 18; 17.4% ortho, 5.9% meta, and 76.7%para). There was no significant variation of kT/kB and only asmall variation of the isomer distribution (particularly of theortho/para ratio) with reaction times of up to 60 min, whenkT/kB = 18 with an isomer distribution of 25.6% ortho, 7.7%meta, and 67.2% para.

Consequently, in further studies the mercuration of mono-substituted benzenes with mercuric trifluoroacetate in triflu-oroacetic acid was carried out at 00 in order to minimize thepossible effect of isomerization (disproportionation) on theisomer distributions and kT/kB, and the competitive experi-ments were performed with the shortest feasible reaction times(2 sec). The data obtained are summarized in Table 3.

In the mercuration of monoalkylbenzenes, a decreasing orderof the relative reactivities (Me > Et > i-Pr > t-Bu) was ob-served, accompanied by an increase of meta substitution anda decrease of ortho/para isomer ratio.The mercuration of polymethylbenzenes was also studied

by the competitive method with reaction times of 2 sec at 00.The data were compared with the relative stabilities ofthe corresponding ir- and a-complexes in order to show acomparable trend indicative of the rate-determining step (16).The relative mercuration rates and corresponding ir- anda-basicities of benzene and methylbenzenes are shown in Table4.The rate of mercuration of polymethylbenzene was so fast

that competitive mercuration of polymethylbenzenes andbenzene could not be directly evaluated. Therefore, the com-petitive mercurations of 1,2-, 1,3-, and 1,4-dimethylbenzeneswere measured against toluene, those of 1,2,3-, 1,2,4-, and1,3,5-trimethylbenzenes against 1,3-dimethylbenzene, andthose of 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-tetramethylbenzenes andpentamethylbenzenes against 1,3,5-trimethylbenzene. Fromthese data and the kT/kB value, the relative reactivity of eachpolymethylbenzene with respect to that of benzene was ob-tained. It is significant to note the low reactivity of 1,4-di-methylbenzene and particularly of 1,2,4,5-tetramethylbenzene.An unsubstituted ring position para to a methyl group seemsto be necessary in order to achieve high substrate reactivity.The correlation coefficient between the logarithms of the

relative mercuration rates and the logarithms of the corre-sponding ir- and c-basicities of methylbenzenes is 0.82 forir-complexes and 0.89 for a-complexes.

In an attempt at more quantitative treatment of the resultson monosubstituted benzenes, both the Hammett-Brown c++plinear relationship and the improved multiparameter Yu-kawa-Tsuno relationship (17), taking into consideration bothconjugative and inductive effects of substituents, were used.The Hammett-Brown treatment gave a p value of -6.31 and

Chemistry: Olah et al.

Proc. Natl. Acad. Sci. USA 74 (1977)

Table 4. Relative rates of mercuration of benzene andpolymethylbenzenes at 00 with reaction time of 2 sec and

comparison with r and a basicities*

Relative RelativeRelative 7r-complext a-complex$

mercuration stability stabilityArH rates* with HCl with HF-BF3

Benzene 1 1.0 1Toluene 18 1.5 7.9 X 1021,2-Dimethylbenzene 335 1.8 7.9 X 1031,3-Dimethylbenzene 362 2,0 1061,4-Dimethylbenzene 41 1.6 3.2 X 1031,2,3-Trimethylben-

zene 2 X 104 2.4 2 X 1061,2,4-Trimethylben-

zene 1 X 103 2.2 2 X 1061,3,5-Trimethylben-

zene 5.7 X 104 2.6 6.3 X 1081,2,3,4-Tetramethyl-benzene 2.3 X 105 2.6 2 X 107

1,2,3,5-Tetramethyl-benzene 1.35 X 105 2.7 2 X 109

1,2,4,5-Tetramethyl-benzene 67 2.8 107

Pentamethylbenzene 4.4 X 105 2 X 109* Concentrations: total [ArH], 1.0 M; [Hg(OCOCF3)2], 0.1 M.t Andrews, L. J. & Keefer, R. M. (1964) Molecular Complexes inOrganic Chemistry (Holden-Day, San Francisco, CA), and refer-ences quoted therein.Mackor, E. L., Hofstra, A. & van der Waals, J. H. (1958) Trans.Faraday Soc. 54, 66, 187, and references given in ref. 7, p. 241.

a correlation coefficient of 0.95. The Yukawa-Tsuno treatmentgave an n-value of 0.69, p = -5.82, and a correlation coefficientof 0.98. These results suggest that the transition state of highestenergy in mercuration of methylbenzenes is predominantly ofa-complex nature. It also should be noted that a large primarykinetic hydrogen isotope effect was observed in aromaticmercuration (kH/kD = 6.0 + 0.1 at 250) (18). Consequently,proton elimination can at least be partially rate determiningand a relatively slow process gives rise to a significant isotopeeffect. Intramolecular rearrangements in a relatively long-liveda-intermediate ion prior to deprotonation are thus quite fea-sible.

Thallation. The competitive thallation of benzene and tol-uene with thallium trifluoroacetate was studied in trifluo-roacetic acid in a manner similar to the mercurations describedabove. Thallated aromatics were isolated by precipitation withaqueous sodium chloride, brominated in carbon tetrachlorideto form the corresponding bromoarenes, and then analyzed assuch by gas/liquid chromatography.

Tl(OCOCF3)2

+ Tl(OCOCF3X3 [In order to establish whether the studied thallations indeedinvolved real competition of the substrates, variation of theconcentration of toluene and benzene in the competitive thal-lation experiments was carried out at 150. Varying the tolu-ene/benzene concentration ratio from 1:1 to 1:8 caused noapparent change in the kT/kB ratio, 35. The isomer distributionalso remained constant (9% ortho, 5% meta, and 86% para).To determine whether diarylthallium complex formation

(9) had any influence on the data, the molar ratio of toluene tothallium trifluoroacetate was also varied from 1:1 to 10:1 and

Table 5. Variation of relative rate and isomer distribution withtemperature in thallation of benzene and toluene

in trifluoroacetic acid*

Temp., Time, Isomer distribution, %°C min kT/kB Ortho Meta Para

10 2 34 8.8 5.1 86.115 3 35 9.5 5.5 85.015 60 34 9.0 5.7 85.6

33 9.1 5.9 85.025 3 33 9.7 5.8 84.625 60 28 9.3 6.2 84.650 3 21 10.6 7.5 81.950 60 19 12.6 7.8 79.773 30 18 12.2 8.3 79.473 60 14 15.0 10.7 74.373 180 8 19.6 17.0 63.473 360 4 25.8 27.2 47.073 1440 2 28.6 46.1 25.4

* Concentrations: total [ArH], 1.00 M; [Tl(OCOCF3)3], 0.10 M.

the reaction was carried out up to approximately 60% com-pletion. There was no significant variation in the isomer dis-tributions observed, and thus there is no evidence for diar-ylthallium compounds.No significant variation of the relative rate and of isomer

distributions of thallation with time was observed at 15°.Therefore, under the experimental conditions used, isomer-ization and disproportionation of the isomeric reaction productsin the thallation of toluene may be considered insignificant.

In order to study further the influence of reaction tempera-ture on the isomer distribution, the competitive thallation oftoluene and benzene was also carried out at higher tempera-tures. The relative rates and isomer distributions are summa-rized in Table 5.

At higher reaction temperatures, specifically at 730 (the boil-ing point of trifluoroacetic acid), a marked decrease in kT/kBwas observed and the ortho and meta isomers increased at theexpense of the kinetically favored para isomer. These resultsmean that aromatic thallation under the more severe reactionconditions is an increasingly reversible electrophilic aromaticsubstitution, accompanied by rapid isomerization and trans-thallation.

Because no isomerization (transthallation) was apparent inthe thallation of toluene below 15', the thallation of substitutedbenzenes was also carried out at 100 with reaction times of 2min. The relative reactivities and the isomer distributions aresummarized in Table 6.The results in Table 5 show that the relative reactivities of

the thallation of halobenzenes are in the order F > Cl > Br,TlCl2 Br

Br, (~aq. NaCIG in aq. KBr + CC14

accompanied by a decrease of the ortho substitution in theabove order. The thallation of monoalkylbenzenes under thesame experimental conditions gave the order of reactivities: Me< Et < i-Pr < t-Bu. This order is the reverse of that of themercuration of the same monoalkylbenzenes. However, theortho isomer obtained in the thallation of ethylbenzene was only0.6% and in the cases of isopropylbenzene and tert-butylben-zene, no ortho isomer was formed. These results indicate thatthe thallation reaction evidently has larger steric requirementsand also seems to be in accord with Arnett and Abboud's (19)

4124 Chemistry: Olah et al.

Proc. Nati. Acad. Sci. USA 74 (1977) 4125

Table 6. Competitive thallation of monosubstituted benzeneswith thallium trifluoroacetate in trifluoroacetic acid at 100

with 2-min reaction time*

Isomer distribution, %

Ar kAr/kB Ortho Meta Para

Benzene 1Toluene 34 8.8 5.1 86.1Ethylbenzene 70 0.6 2.2 97.2Isopropylbenzene 90 0 2.1 97.9tert-Butylbenzene 200 0 0.5 99.5Fluorobenzene 0.14 4.5 0.1 95.3Chlorobenzene 0.03 1.8 0.9 97.4Bromobenzene 0.02 1.6 1.8 96.7Anisole 270 8.0 0.1 91.9Anisole 1900t 14.0 0.6 85.5

* Concentrations: total [ArH], 1.0 M; [Tl(OCOCF3)3], 0.1 M.t Reaction temperature, -200.

recent conclusion that the Baker-Nathan effect is substantiallyof steric origin.The thallation of polymethylbenzenes with thallium triflu-

oroacetate was also carried out in trifluoroacetic acid at 00 with2-min reaction time (Table 7). The correlation coefficientsbetween the logarithms of the relative rates of the thallation ofpolymethylbenzenes and the logarithms of either the 7r- or

a-basicities of the corresponding polymethylbenzenes are 0.97for ir-complexes and 0.95 for a-complexes. The Hammett-Brown y+p equation gave a p value of -6.92 and a correlationcoefficient of 0.96, whereas the Yukawa-Tsuno treatment gave

,y = 0.44 with p = -8.34, and a correlation coefficient of 0.98.The similar values of both -r- and a-correlation coefficientsin thallations per se would not allow differentiation of the na-ture of the transition state of highest energy in the thallationof polymethylbenzenes. Primary kinetic hydrogen isotope ef-fects in the thallation of both heavy benzene and toluene (kH/kD= 3.7 for benzene and 3.6 for toluene) (20), however, seem tosupport the a-complex nature of the transition state of highestenergy.

Competitive thallation of polymethylbenzenes and benzenecould not be directly evaluated because the rates of thallationsof polymethylbenzenes were too fast compared with that ofbenzene. Therefore, competitive thallation of 1,2-, 1,3-, and1,4-dimethylbenzenes with toluene, of 1,2,3-, 1,2,4-, and1,3,5-trimethylbenzenes with 1,3-dimethylbenzene, and of1,2,3,4-, 1,2,3,5-, and 1,2,4,5-tetramethylbenzenes and pen-tamethylbenzenes with 1,3,5-trimethylbenzene was deter-mined. From these results and the kT/kB the relative reactivitiesof polymethylbenzenes with respect to benzene were estab-lished.The leveling off of the value of the relative reactivities in

thallation can be explained by the fact that the activation en-ergy of the reactions in trifluoroacetic acid must be relativelylower than that of mercuration or other electrophilic aromaticsubstitions of higher selectivity (7).The predominant para substitution observed in the mercu-

ration and thallation of methylbenzenes reflects, besides somesteric factors, also the strong stabilizing effect of para-methylgroups on the arenium ion intermediates. Isomer distributionswere somewhat affected by intramolecular rearrangementswithin the reaction intermediates or by the reversibility of thesystems, but meta substitution, under predominant kineticcontrol, stayed low (i.e., 5-6% or less), in good agreement withKlapproth and Westheimer's (6) report of 6% in the mercura-tion of toluene. In contrast, the higher meta values reported byBrown and McGary (21) are, in our view, due to thermody-

Table 7. Relative rates of thallation of benzene andpolymethylbenzenes at 100 with 2-min reaction time*

ArH kApjkB

Benzene 1Toluene 341,2-Dimethylbenzene 1451,3-Dimethylbenzene 15001,4-Dimethylbenzene 331,2,3-Trimethylbenzene 72001,2,4-Trimethylbenzene 15501,3,5-Trimethylbenzene 77501,2,3,4-Tetramethylbenzene 63001,2,3,5-Tetramethylbenzene 63501,2,4,5-Tetramethylbenzene 2300Pentamethylbenzene 7500

* Concentrations: total [ArH], 1.0 M; [Tl(OCOCF3)3], 0.1 M.

namically controlled intramolecular or intermolecular processesaffecting primarily the ortho/para metallated derivatives andnot to enhanced meta substitution. The discrepancy thus canbe resolved by pointing out that in the latter case the necessityto differentiate kinetic from thermodynamically controlledprocesses, when considering substrate and regioselectivities inaromatic substitution, was not fully taken into account.

Support of our work by the National Science Foundation is gratefullyacknowledged.

1. Brown, H. C. & Wirkkala, R. A., (1966) J. Am. Chem. Soc. 88,1447-1452.

2. Brown, H. C., & Wirkkala, R. A. (1966) J. Am. Chem. Soc. 88,1453-1456.

3. Brown, H. C. & Wirkkala, R. A. (1966) J. Am. Chem. Soc. 88,1456-1458.

4. Stock, L. M. & Brown, H. C. (1963) in Advances in PhysicalOrganic Chemistry, Vol. I, ed. Gold, V. (Academic Press Inc.,New York), Vol. I, pp 35-154.

5. Brown, H. C. & Nelson, K. L. (1953) J. Am. Chem. Soc. 75,6292-6299.

6. Klapproth, W. J. & Westheimer, F. H. (1950) J. Am. Chem. Soc.72,4461-4465.

7. Brown, H. C. & McGary, C. W., Jr. (1955) J. Am. Chem. Soc. 77,2300-2306.

8. Olah, G. A. (1971) Acc. Chem. Res. 4,240-248.9. Henry, P. M. (1970) J. Org. Chem. 35,3083-3086.

10. McKillop, A., Hunt, J. D., Zelesko, M. J., Fowler, J. S., Taylor, E.C., McGillivray, G. & Kienzle, F. (1971) J. Am. Chem. Soc. 93,4841-4844.

11. McKillop, A., Swann, B. P. & Taylor, E. C. (1970) TetrahedronLett., 5281-5284.

12. Olah, G. A. & Lin, H. C. (1974) J. Am. Chem. Soc. 96, 549-553.

13. Maliyandi, M., Sawatzky, H. & Wright, G. F. (1961) Can. J.Chem. 39, 1827-1835.

14. Dewar, M. J. S. & Schroeder, J. P. (1964) J. Am. Chem. Soc. 86,5235-5239.

15. Brown, H. C. & McGary, C. W., Jr. (1955) J. Am. Chem. Soc. 77,2310-2312.

16. Condon, F. E. (1952) J. Am. Chem. Soc. 74,2528-2529.17. Yukawa, Y., Tsuno, Y. & Sawada, M. (1966) Bull Chem. Soc. Jpn.

39,2274-2286.18. Kresge, A. J. & Brennan, J. F. (1963) Proc. Chem. Soc. London

215.19. Arnett, E. M. & Abboud, J. L. M. (1975) J. Am. Chem. Soc. 97,

3864-3865.20. Brody, J. M. & Moore, R. A. (1970) Chem. Ind. (London),

803.21. Brown, H. C. & McGary, C. W., Jr. (1955) J. Am. Chem. Soc. 77,

2300-2306.

Chemistry: Olah et al.