FREE-RADICAL REACTIONS WITH AROMATIC ETHERS PART I. BENZOYL PEROXIDE WITH ANISOLE'

BRIAN M. LYNCH? AND RALPH B. MOO RE^ Chemistry Department, Afemorial University of Nezvfoundland, St . Johtt's, Newfoz~ndland

Received January 32, 1962

ABSTRACT

A re-examination of the products of thermal decomposition of benzoyl peroxide in anisole has resolved anomalous findings by previous worlm-s. The expected phenylation of the sub- strate is accompanied by appreciable benzoyloxylation, and the distribution of isomers in the phenylation and benzoyloxylatio~~ p r o d ~ ~ c t s has been examined; the theoretical inlplications of the results are discussed.

INTRODUCTION

Phenylation of aromatic substrates by benzoyl peroxide, discovered by Gelissen and Hermans (I) in 1925, was recognized as a free-radical process by Hey and Waters ( 2 ) , and continues to receive much attention (for general reviews, see Augood and Williams (3), Dermer and Edmison (4), Walliilg (5), and Williams (6, 7)). In a series of solne 20 papers, Hey and Williams and their co-worlrers (8, and previous papers cited therein) have applied Ingold's conlpetitive method (Ingold and Shaw (9)) to evaluate the relative reactivities of many aromatic co~npounds towards the aryl radicals liberated by decom- posing benzoyl peroxides.

Recently, it has been pointed out (5, 10-12) that there is considerable uncertainty attached to the application of the competitive neth hod in such reactions, since many side reactions occur, and their importailce varies markedly for different aromatic substrates. Further reservations have been expressed since the phenylation process does not occur by any one nlechallis~n; differing processes may be dominant a t different concentrations (Eliel et al. (13)). I t is apparent that selection of the relative yields of substituted biphenyls as a measure of reactivity towards free-radical attack must be regarded with considerable reserve; a more acceptable procedure involves consideration of the distribution of products among the various possible reaction paths (cf. Lynch and Pausacker (11, 12)).

In the present paper, we have used this approach in a study of the various reaction products obtained when benzoyl peroxide deco~nposes in hot anisole, and we have resolved suggestions (14; 6, p. 65) that anisole is in some way anonlalous in its behavior. A previously published study of this reaction system (15) neglects the occurrence of benzoyloxylation of the substrate.

EXPERIMENTAL

Analyses are by the Schwarzliopf i\iIicroa~lalytical Laboratory, Woodside, N.Y., and the Australian Microanalytical Service a t the Chemistry Department, University of Melbourne. Infrared spectra were measured using a Unicam SPlOO prisl11-grating spectrollleter, and ultraviolet spectra were recorded with a Beclcman DIC2A spectrophoto~neter.

'Presented i n part at the 44th A n n z ~ a l Conference of the Chenzical Institz~te of Canada, ilIontrea1, Quebec, A z ~ g z ~ s t 2-5, 1961.

=To wJzom reqz~ests for reprints shozdd be addressed, at the CIze~tzistry Department, S t . Francis Xaeicr U?ziz~ersity, ilntigonislz, ~Vova Scot ia.

JA?nericaiz Chemical Society - Petrolez~m Researclz F Z L ~ Undergraduate Scholar, 1961-1962.

Canadian Journal of Chemistry. Volume 40 (1962)

1461

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1462 CANADI:\N JOURNAL O F CHEMISTRY. VOL. 40. 1962

il!faterials Anisole (Matheson, Coleman, and Bell) was redistilled before use, b.p. 154", noz6 1.5151; the various

samples contained from 0.02-0.0870 phenol. Benzoyl peroxide was purified (minimum purity, 98y0) by reprecipitation from ice-cold chloroforln by methanol. Reference samples of the isomeric methoxybiphenyls, methosyphenols, and ~nethoxyphenyl benzoates, and of phenol and phenyl benzoate, were obtained by purification of co~nrnercially available samples or by established common syntheses; the physical constants of these samples agreed with those recorded in the literature.

General Procedz~re Benzoyl peroxide and anisole were heated together under varying conditions (see Table I) . After com-

pletion of reaction, the various products were separated and isolated as follows: the carboz dioxide evolved mas estimated by passage through a gas-absorption train. Free benzoic acid was isolated by extraction with saturated sodium hydrogen carbonate solution, followed by acidification. Excess of substrate was removed under reduced pressure, and the residue was heated under rellux with 5% potassium hydroxide in ethanol (16 hours). The ethanol was removed under reduced pressure, and the residue was acidified to pH 2 and extracted with ether. 'l'he phenolic frartion and an additional quantity of benzoic acid (be~zzoic acid frof~z hydrolysis) were obtained fro111 the extract by standard methods, and the nzethoxybiplre~zyls were isolated, after evaporation of the ether, by careful distillation under reduced pressure, or by steam distillation. Any involatile residue was weighed and reserved. This procedure is based on that employed by Lynch and Pausac1;er (11).

The Plze~zolic Fractions: Distribzrtio~z of Isonzers in Benzoyloxylation Element analysis, and infrared and ultraviolet spectra, indicate that the phenolic fractions isolated

from the various experiments consist largely of methosyphenols, with little phenol being present. The following infrared absorption bands were noted for the three methoxyphenols, and for a typical phenolic fraction (from expt. 2): guaiacol (capillary film) (cm-I, intensity): 745 vs, 835 m, 920 w, 1005 w, 1022 s, 1040 m, 1112 s, 1220 m, 1265 s, 1380 111, 1440 m, 1480 rn, 1510 vs, 1610 vs, 1620 m, 2840 m, 2960 ~ n , 3010 m, 3430 s, 3500 s ; n7-methoxyphenol (capillary film) : 690 s, 770 s, 840 m , 925 m , '396 w, 1045 s, 1080 w, 1160 vs, 1200 vs, 1290 s, 1445 m, 1470 m, 1500 s, 1600 vs, 1615 vs, 2835 111,2950 111,3000 m, 3400 s ; p-methosyphenol (KC1 disl;): 733 s, 825 vs, 1033 vs, 1105 s, 1151 w, 1170 w, 1182 m, 1240 s, 1280 s, 1302 m, 1379 s, 1455 s, 1510 vs, 1608 s, 2830 m, 2950 s, 3015 w, 3035 w, 3400 s ; phenolic fraction from expt. 2 (capillary film): 690 vw, 735-760 s, 820 s, 840 s, 920 w, 1030 s, 1040 s, 1110 s, 1155 s, 1180 m, 1225 s, 1270 s, 1380 111, 1460 m, 1480 111, 1510 vs, 1600 vs, 1620 s, 2840 m, 2955 s, 3010 w, 3430 s, 3500 s. Virtually all of these bands can be assigned without diffic~~lty, following the lines of previous work by 1Catritzl;y et al. (16, and previous papers cited therein). I t is evident that guaiacol and j-methosyphenol are major constituents of the phenolic fraction examined (the infrared spectra of the phenolic fractions were all closely similar), but the dilferences in the spectra of the three isomers are not sufficient for accurate analysis of the ratios of isomers.

Similarly, although the ultraviolet spectra of the mixtures of phenols from expts. 1-3 and 6-8 corresponded well to that of guaiacol, the differences between the spectra of the various isomers are snlall (in water con- taining 1% ;;;methanol by volume, guaiacol has A,,, 216 and 275 m p , log E 3.76 and 3.33; 717-methoxyphenol has A,,, 217, 273, and 27'3 mp, log E 3.83,3.28, and 3.23; p-methoxyphenol has A,,, 283 and 28'3 mp, log E 3.83 and 3.41; phenol has A,, 211,270, and 276 mp, log E 3.75,3.18, and 3.08; and the various phenolic fractions showed A,,, 216 and 275 m p , log E 3.9 and 3.32). The infrared spectra of the three methoxyphenyl benzoates were recorded, and it was found that the meta and para isomers had characteristic absorption bands in the aromatic C-H deformation region. The following absorption bands were observed (bands characteristic of an isomer are italicized): guaiacyl benzoate (ICCI disl;) (cm-I, intensity): 710 vs, 760 s, 810 w, 1025 s, 1040 m, 1065 vs, 1080 m, 1110 s, 1155 w, 1170 s, 1200 s, 1260 vs, 1310 w, 1450 s, 1585 w, 1600 s, 1735 vs, 2835 m, 2940 s, 2975 vs, 3000 s, 3065 s ; nz-methoxyphenyl benzoate (KC1 disk): 690 s, 710 vs, 744 w, 782 s, 870 s, 950s, 996 w, 1023 s, 1063 vs, 1081 vs, 1142 vs, 1160 m, 1175 m, 1190 s, 1270 vs, 1318 s, 1450 s, 1490 s, 1589 vs, 1605 vs, 1736 vs, 2825 In, 2930 s, 2950 s, 3000 s, 3065 111; j-methosyphenyl benzoate (KC1 disk): 710 vs, 765 111, 815 s, 870 rn, 1030 m, 1040 m, 1070 s, 1075 s, 1115 w, 1185 m, 1205 s, 1255 s, 1280 s, 1320 w, 1440 m , 1520 vs, 1605 m , 1735 vs, 2835 s, 2'335 s, 2960 s, 2905 vs, 3065 m. Samples of mixed methoxyphenyl benzoates, prepared by benzoylation of the phenolic fractions from expts. 1 and 2, showed the following bands (ICCI disl;): 710 vs, 760 111, 815 w, 870 w, 1024 s, 1040 m, 1065 vs, 1077 s, 1113 rn, 1170 s, 1200 s, 1265 vs, 1315 w, 1450 s, 1520 s, 1600 s, 1735 vs, 2830 s, 2940 s, 2970 s, 2998 s, 3065 m. The absence of absorption a t 690, 744, 782, 930, and 996 cm-I in the samples of mixed benzoates indicated that very little nz-methoxyphenyl benzoate was present; comparison with synthetic ~uixtures containing the other isomers showed that as little as 3% of the meta isomer could be detected readily by the above absorption bands. Comparison of the intensities of the 710 cm-I and 815 cm-I bands in the mixed benzoates with those in a series of synthetic mixtures (the 710 cm-I band is common to all three isomers; the 815 cm-I band is char- acteristic of the para isomer) indicated ortho:para ratios of from 20-25:1, suggesting that very little $-methoxyphenyl benzoate was present. However, a referee has drawn attention to bands in the infrared spectra of the phenolic mixtures (at 1155 cm-I, and a t 1180 and 820 cm-I) indicating the presence of signi- ficant alllounts of both m-and p-methoxyphenols; it thus appears possible that some fractionation of isomers occurred during esterification.

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LYKCI-I AND MOORE: FREE-RADICAL REACTIONS

as G' G' - as, A 33% . .C" . I coo .omd+do

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1464 CANADIrIN JOURNAL OF CHEMISTRY. VOL. 40, 1062

Consequently, further examination of the various phenolic mixtures was made by gas-liq~iid chromatog- raphy using a Beckman GC-2A gas chromatograph with a "Carbowax 4000 DioleateV-on-firebricli column No. 70007, operated using helium a t an inlet pressure of 30 p.s.i. (outlet pressure, atmospheric), a t a column temperature of 160" C. Trial experiments established that the three ~nethosyphenols were satisfactorily resolved under these conditions, (the order of elution being ortho, para, meta), and that the ratio of peak areas was accurately proportional to the quantities of the isomers. The following isomer percentages were deduced: mixture from expt. 1: ortho, 65; para, 35%; from expt. 2: ortho, 70; para, 30$$; from espt. 3: ortho, 82; para, 18yo. No nt-methoxyphenol was detected, confirming in part the results deduced from the infrared spectra of the mixed methoxyphenyl benzoates. The results of the trial experin~ents indicate that the individual experimental error is f 2%.

The Methoxybipltenyls: Isomer Ratios in Pl~e?tylatio7t The mixtures of methoxybiphenyls isolated from expts. 1 and 2 were analyzed by gas-liquid chroma-

tography and by differential ultraviolet spectrophotometry. In the gas-liquid chromatography, a Beckman GC-2A gas chro~natograph was used, wit11 the standard

silicone-on-firebrick column No. 74346; helium a t an inlet pressure of 30 p.s.i. (outlet pressure, at~nospheric) was the carrier gas, and the column temperature was 190' C. Under these conditions, the three isomers were satisfactorily resolved (the order of elution being ortho, meta, para), and experiments with synthetic mixtures showed that the ratio of peak areas was proportional to the isomer ratios. The results obtained were: mixture from expt. 1: ortho, 76.2; meta, 10.5; para, 13.37;; misture from expt. 2: ortho, 77.4; meta, 10.7; para, 11.9%. Results obtained with the synthetic mixtures indicate that the esperi~nental error is f2%.

The ultraviolet absorption spectra of the various methoxybiphenyls differed s~~fficiently for analysis of the mixtures by a standard method (cf. Lynch and Pausaclcer (17), Cadogan, Hey, and Williams (18)). Calibration spectra in 95Yo ethanol were recorded in the wavelength range 220-360 nip, n i th the following results: o-methoxybiphenyl showed A,,, 246 and 285 mp, log e 4.09 and 3.70; m-methosybiphenyl showed A,, 250 and 280 (shoulder) mp, log 6 4.18 and 3.62; p-methoxybiphenyl showed Anlax 261 mp, log 6 4.28. The proportions of ortho, meta, and para isomers were found to be 80, 10, and 109; for the mistures from expts. 1 and 2 (the two mixtures gave identical ultraviolet spectra). The experimental error is probably f 5 % .

The infrared spectra of the various methosybiphenyl mixtures were consistent with the isomer ratios as determined above, and no bands other than those due to the three isonlers were present. The following absorption bands were noted for the methoxybiphenyls: ( a ) o-methosybiphenyl (capillary film) (cnl-1, intensity): 700 vs, 732 s, 755 vs, 915 111, 937 w, 998 w, 1012 s, 1031 s, 1060 s, 1078 m, 1126 s, 11G6 111, 1182s, 1241 vs, 1264 vs, 1300 m, 1433 vs, 1465 111, 1485 vs, 1508 s, 1585 s, 1600 s, 2830 m, 2035 s, 2055 s, 3000 m, 3025 m, 3060 s ; 711-methosybiphenyl (capillary film): 600 vs, 755 vs, 790 111, 860 m, 908 \v, 1020 m, 1038 111, 1053 m, 1077 w, 1180 s, 1215 vs, 1270 sh, 1298 s, 1420 s, 1480 vs, 1570 s, l G O O vs, 2832 m, 2035 s, 2950 s, 3000 m, 3030 In, 3060 s ; p-methoxyl~iphenyl (KC1 disk): 688 m, 760 vs, 832 vs, 000 \v, 1001 IT, 1015 m, 1038 s, 1042 m, 1120 m, 1186 n1, 1202 s, 1253 vs, 1271 In, 1.200 s, 1441 m, 1450 In, 1-166 n1, 1-100 vs, 1527 m, 1582 w, 1609 vs, 2835 m, 2035 s, 2960 s, 3000 In, 3030 nl, 3070 111.

Infrared Spectra of I7tvolatile Residues These were recorded for the products f r o ~ n the various experiments, using tlie potassium chloride disli

technique for sample preparation; all spectra were closely similar. i\ typical example (a sample prepared from the involatile residue from espt. 7) showed the following strong barlds: 700, 755, 830, 1035, 1080, 1120, 1180, 1230, 1250, 1300, 1410, 1480, 1500, 1590, 1600, 2830, 2930-2960, 3000, 3030, 3060 cm-I.

DISCUSSION

In i~lost experiments, the suins of the yields of carbon dioxide, free benzoic acid, and bellzoic acid from hydrolysis account alnlost quantitatively for the reacted benzoyl peroxide, and the yields of other products are quite reproducible. Althougl~ some losses are inevitable in the working-up processes, i t appears that reasonably quailtitative estimates of the various reaction products have been made.

111 the following discussion, we interpret the anonlalous behavior reported by previous worlcers, and attempt to show that anisole actually behaves in a typical manner in its reactions with the radicals froin decomposing benzoyl peroxide.

The " Anomaloz~s" Behavior of Anisole The distribution of products in the present work differs fro111 tha t reported previously

(Suehiro (15)). Under conditions sinlilar to those of expts. 4 and 5, Suehiro obtained the following yields of products (mole/mole peroxide): carbon dioxide, 0.51; free benzoic

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acid, 1.22; rnethoxybiplle1l)7ls, 0.36. Thus, the contributioil of the stoicl~iometi-ic expression (A) to the total reaction is greater for our experiments.

Results similar to Suehiro's, i l l that yields of benzoic acid well in excess of 1 mole/mole peroxide were obtained, have also been reported by AA~igoocl (1-1) ancl Rondestvedt and Blanchard (19). Since the stoichioinetric equations (.A), summarizing phen)-lation, and (B), sumnlarizing benzoylox>lation by benzo)-1 peroxide, predict a mnxim~~n7 yield of free benzoic acid of 1 mole/mole peroxide, processes other than (A) and (B) must account for the abnorniall~. high jrields of free benzoic acid.

Xugood and \TTilliams (3) ancl \TTilliams (G, p. G5) suggest that the high yields of benzoic acid noted in their reactions result from 11) clrogen abstraction fro111 ailisole b~ a benzoate radical, ~~ie ld ing the phenox\ methg-1 radical and benzoic acid (process (C')):

this radical is assumed to be incorporated in the high-boiling residues. If this suggestion is correct, i t might be expected that I,2-diphenoxyethane (formed bl- dimerization) and/or phenoxymetliylai~isoles (by nuclear substitution in anisole) would occur in the "methoxybipl~enyl" fractions.": Ho~vever, there were no indications of the presence of these substances in the methosybiphenyls (see Experimeiltal); furthermore, the infrared spectra of the involatile residues from the various experiments, and their met1lox)~l content (cf. 'Table I ) , indicate that these residues are probably of the quaterphenyl or tetl-ahydroquaterpl~e~~yl type to be espectecl from climerization ol the aclduct between a pheilyl radical and auisole (cf. L>,nch and Pausacl:er (1 I ) , Pausaclrer (21 ), DeTai- and 1,ong (22)). Thus there is little supporting evidence for suggestions involving the extensive participation of process (C) in the total reaction scheme.

The key to an explanation of the anomalous yields of lree benzoic acid obtained previously is simple; it is provided by the fact that although in the present worl: there is still an excess of lree benzoic acid over the isolated nuclear substitution products, it is f7.r less mar1:ed than reported previously. This implies that the anisole samples used ill the various sets of expel-iments differ in some wa).. Since me noted in the course of the present worl: that our anisole samples contained traces of free phenol (which I-eacts extremely readily with benzoyl peroxide, yicldiiig benzoic acid and other products (cf. refs. 23-25), it seems evident that the much higher yields oi free benzoic acid obtained by previous worlrers are a result of the presence of higher proportions of free phe~lol as a contaminant in their ai~isole samples, and are not a consecluence of any anomalous behavior of the anisole itself. Confirmation is afforded by the results of expts. 9 and 10, ~vllere deliberate addition of phenol to reaction mixtures marl:edly increased the yields of free benzoic acid.

I t is also feasible that hg7drolysis of the peroxide by small amounts of water present in the solveilt might lead to increased yields of free benzoic acid (cf. the behavior of ioclosobenzene dibenzoate and of lead tetrabenzoate (17, 26, 27); however, added \\rater had no appreciable effect (expt. 11).

"I'fordte a w l L e l ~ s c h v c i (ZO) Irn.~"e obtniized 1 ,2 -d ip I ze l rosye t l~u~1~ i l l ln7il yield b y .iiiadicitiolz of n?iisolr 7oilh i/ltr.cr~iolet l ight , n?zd Menbest, 12eid, atid Slii l i lrg (2O(a)) hur!e isolated pl~eilosyii~etli .~lallisnles froila t l ~ e tl~eiiiral dccon~positio?l of f-Dzltyl pe~o.vide in. n~l isolc .

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1466 CANADlAN JOURNAL OF CI-IEMlSTRY. VOL. 40, 1982

I t is possible that another process may contribute to some of the excess of free benzoic acid noted in the present series of experiments, although there is no direct evidence in its favor: demethylation of anisole to phenol could occur by a route analogous to that commonly postulated for the deallcylation of tertiary amines ( 5 , 28), i.e.

I n unpublished experiments, we have shown that this reaction path is significant when substituted benzoyl peroxides react with anisole, since free phenol is generated from the substrate in sufficient amount for ready isolation from coinpleted reaction mixtures. Cowley, Norman, and Waters (29) have fouild that inetl~yl radicals effect deal1;ylation of anisole, and I<harasch and Huang (30) have shown that many phenol ethers can be deallcylated by free radicals, thus lending some support to this suggestion.

Nliclear Szibstitzition Prod~~cts

(a) Metlzoxyphenyl Benzoates When benzenoid aronlatic coinpounds react with bellzoyl peroxide, benzoyloxylation

(process (13) above) generally accompanies phenylation (process (A)) as a side reaction of varying importance.

Thus, benzoyloxylatioil llas been observed when beilzoyl peroxide decoinposes in beilzene (31), chlorobenzene (12, 31), nitrobenzene (12, 17), biphenyl (27, 31), p-dichloro- benzene (27), 1,2,3-trimethoxybenzene (32), pyridine (33), q~iinoline (33), and p-di- methoxybenzene (34). With the monosubstituted compounds, the extent of benzoyl- oxylation is small, and in some instances, the products (generally determined after hydrolysis to the corresponding substituted phenols) could have been forined by nucleo- philic substitution of excess substrate during the hydrolysis step (e.g., nitrobenzene is converted into o- and p-nitrophenols by treatnlent with allcali in the presence of air (35, 36)).

However, in our present work we find that a t 80° (expts. 6 and 7), the yields of methoxyphenyl benzoates are approximately one-half those of inethoxybiphenyls; a t higher temperatures, the extent of benzoyloxylation decreases, since the benzoyloxy radicals decarboxylate more rapidly.

These results iinply that anisole is more reactive towards decoinposing beilzoyl peroxide than an17 monosubstituted benzene examined previously (since in a inore reactive solvent, the benzoyloxy radical is more lilcely to react with the substrate before decarboxylating), conflicting with findings in previous studies employing the competitive method (3, 19, 37). Further discussion of this point is given below.

Acyloxylation of anisole under conditions favoring free-radical attack has been reported previously: with lead tetraacetate in acetic acid a t 80°, 9-methoxyphenyl acetate is a nlajor product (38), while electrolysis of aqueous acetic acid in the presence of anisole gives inixed methoxyphenyl acetates (ortho: para ratio, 7 : 3) (39, 40).

In the present worlc, the most reliable values for the isomer distribution range from 65-82% 0-methoxyphenol and froin 18-35% p-methoxyphenol; no m-methoxyphenol could be detected. The large scatter in the isonler distributions for the various experiments (far beyond errors of analysis) probably reflects the occurrence of some preferential oxidation of the p-methoxyphenol during the hydrolysis of the methoxyphenyl benzoates

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LYNCH .-ISD MOORE: FREE-R.\DICZ\L REACTIOXS 14G7

(which is effected under conditioils lrilowil to oxidize phenols having electron-donating substituents (cf. ref. 40(a))). The apparent selectivity nlay thus result froill the inore rapid oxidation of @-methoxyphenol (for recent evidence indicating that 9-methoxy- pheilol is oxidized more rapidly than o-methoxyphenol, see McClure and Williams (40(b))).

The occurrence of extensive ortho benzoyloxylation is in contrast to the results noted for thermal acetoxylatioil by Cavill and Solomon (38) ; it is suggested that the mechanism of ortho benzoyloxylation of ailisole is analogous to that proposed for the benzoyl- oxylation of phenols by benzoyl peroxide (Walling and Hodgdoil (41)). Thus, the second stage of the suggested deallrylatioil reaction (p. 1466) is replaced by coupliilg between the anisole ion-radical and a benzoate radical, followed by a cyclic rearrangement, i.e.

Walling and Hodgdoil found that the reaction rates of phenols were increased by electron- donating substituents, and we find (34) that @-dimethoxybenzene is benzoylox~~lated rapidly and almost quailtitatively by benzoyl peroxide a t 80' (apparent first-order rate constants for 0.125 Al peroxide are (105k, sec-I): p-dimethoxybenzeile, 58.8; anisole, 8.3; benzene, 3.8), thus lending some slender support to the suggested mechanism.

A re-examination of the acetoxylation of anisole by lead tetraacetate in acetic acid, using infrared spectroscopy in the examination of products (42), further confirmed Cavill and Solomon's finding (38) of preferred para substitutioil (we could detect no o-methoxy- phenyl acetate). I t is possible that the differences in orientatioil between benzoyloxylatioi~ and acetoxylation reflect the occurrence of @olar acetoxylatioil by lead tetraacetate (cf. Mosher and Kehr (43)).

Free phenol, present in our anisole samples, could lead to the occurrence of catechol in the pheilolic fractioils (arising froin the above ortho benzoyloxylation, followed by hydrolysis), thus lesselling the significance of the isomer ratios deduced above. This possibility was checked by extracting any pheilolic material from completed reaction mixtures with aqueous sodium hydroxide, and brominating the extracted illaterial; only

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1468 CANADIAN JOURN.+\L O F C1-IEMISTl<IT. VOL. -10, 1962

very small quantities (0.02 g from a reaction under conditions similar to expt. 6) were isolated. Therefore, this poteiltial source of error is not significa~lt (see also Experimental, p. 1462.

(b) .illetlzoxy biplzenyls The isomer ratios for the free-radical phenylation of anisole a t 154' (present worli)

and 100' (Suehiro (15)) and for inethylation a t 140' (Cowley et al. (29)) are assenlbled in Table 11.

TABLE I1 Percentages of isomers in thc phcnylation ancl the mcthylation of anisole

-

Ortho Mcta Para

Phenylation &At 15-1": analysis by gas-licluid chromatography 76.8 10.6 12.6

anal1,sis by ~~l t raviole t spectrophoton~etry 80 10 10 At 100": analvsis by infrared s ~ e c t r o s c o ~ v 67 18 15

il1efl;ylation *At 140": analysis by gas-liquicl chromatography 7-1 15 11

and infrared spectroscopy

Changes in orientation with temperature probably do not account for the differences between our results and Suehiro's, since isoiuer ratios in free-radical substitutions are not greatly temperature dependent. In our experience, infrared spectroscopy did not prove very suitable for the ailalysis of the isomer ratio in the phei~ylation of anisole, since considerable overlapping occurred among the various characteristic bands in the C-H out-of-plane deformation region (see Experimental) ; me believe that the figures based on gas-liquid cl~romatograpl~y are more reliable. Holvever, the general pattern of the results is similar, and the agreement is reasonable OII an absolute basis, although ally revision of isomer distributiolls will have a large effect on partial rate factors.

Comparison of our results for phenylation of anisole with those for methylation reveals little difference in the behavior of the two radicals; in particular, no sigliificallt differences in the proportions of ortho isomer are noted, although it has been suggested (6, 29) that there may be slightly less steric hiildrailce to methylation than to phenylation. Since recent morli (Weingarten (-14)) indicates that the phenyl radical approaches almost perpendicularly to the plane of the aromatic ring in the substrate, steric effects will norn~ally be insignificant. The appreciably lower meta:para ratio noted for phenylation probably reflects the greater electrophilicity of the phenyl radical.

High ort11o:para ratios are typical of free-radical phenylatioils (for a summary, see Williams (7)), yet still await satisfactory explanation. Although preferred ortho-para orie~ltatioll is to be expected fro111 resollallce theory if a n-complex (the iiWl~eland inter- mediate" Ia-Ib-Ic) is regarded as the transition state (since a n-complex mill be stabilized by co~~j~lga t ion or hyperconjugatioll by all substituellts ortho or para to the site of attack), it is doubtful whether the formation of a n-complex can be regarded as rate controlling in phenylations.

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LYSCII ASD MOORE: ITRICE-R:\DICXL REACTIONS 14G9

The lacli of selectivity noted in the phenylation and the methylation oi naphthalene (12, 45-47) indicates that in the transition states for these reactions, the aromatic carbon being attaclced is still partly conjugated ~i~it l l the rest of the sj.stem and the attacliing radical is weakly bonded. Since the free-radical p h e ~ l ~ latioiis of all aromatic substrates are certainlj. very fast reactions wit11 very Io\v activation energies, showing low selectivity betweell the various possible positions of attacli, and very little contrast in relative reaction rates (xvhich, although derived using the somewhat suspect "competitive" methocl, possess empirical significance), a transitioll state involving a long "n-lilte" bond between radical and substrate (i.e., (11)) is probably appropriate for all phenylations (Walling ( 5 , pp. 312, 434) has suggested that analogous "n-complexes" may be involved in other reactions involving the intel-action of radicals with aromatic molecules). If so, hoxvever, the esplanation of preferred ortho-para orielltatio~l noted above loses much of its force, a l t l~ougl~ the relative stabilities of the various "n-complexes" may parallel those of the \VI~elancl intermediates.

General Comnze?zts on the Sign$ca7zce of Partial Rate Factors and Relatsve Reactivities towards Phetzyl-Radical .Ittack

Recent downward revisions of the scale of relative reactivities of aromatic compounds toxvards phenyl-radical attack (8), as evaluated by application of the "competitive" nletllod, indicate that calculatio~ls of partial rate factors may not be meaningful for these reactions. Partial rate factors which are appreciably less than unity are noted for the meta positions of nitrobenzene (0.86)' toluene (0.71), ethylbenzene (0.76), and isopropyl- benzene (0.61). Further, ii a relative reactivity of cd. 1.6 is assigned to anisole (cf. refs. 3, 6, 8, 14), then the calculated partial rate factor for a ~ n e t a position is ca. 0.5. I t is difficult to suggest a mechanism for deactivation of the meta positions relative to a benzene position.

Olah and his co-worlters, in a study of another reaction (48) presenting a similar problem of low selectivity betxveen different substrates, together with non-statistical isomer distribution (i.e., nitration of alltylbenzenes by nitronium tetrafluoroborate), suggest that partial rate factors are meaningless under such conditions, since the primary competition between substrates involves their molecular n-electron systems as entities, and not the individual positions of these molecules.

Unfortunately, such an interpretatioll accentuates the difficulty in suggesting a satis- factory esplanation of the marlted preference for ortho substitution found with phenyl- ations. Waters' naive but remarltably fruitful suggestion that free radicals are essentially electrophilic (40-31) may be relevant, together with Rondestvedt and Blanchard's suggestion (10) that ortho substitutio~l in nitrobenzene and benzonitrile results from preliminary radical complexing a t the substituent (the region of highest electron density), fo1lo~~-ed bj- ready rearrangement to ortho product.

Further comment on the occurrence of concurrent benzoyloxylation and phenylation of anisole is pertinent. I t has been noted previously (12, 19, 52) that application of the competitive method can lead to meaningful results only where side reactions are unimpor- tant. This criterion is not satisfied for anisole, so that previously quoted relative reactivities (14, 19) call have little significance.

Grateful acli~lowledglnellt is ~ n a d e to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research, and also to the National Research Council of Canada, for an equipment grant.

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CAhrADIAN JOURhTAL OF CHEMISTRY. VOL. 40. 1982

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