282 Peter Mulder et al. / Gas-phase thermolysis of tert-butyl hydroperoxide with benzene and chlorobenzene

Red. Trav. Chim. Pays-Bas 103, 282-287 (1984) 0034-1 86X/84/10282-06$2.00

Gas-phase thermolysis of tert-butyl hydroperoxide with benzene and chlorobenzene in the temperature range 200-300 O C

Peter Mulder and Robert Louw*

Gorlaeus Laboratories, The University of Leiden, P .O. Box 9502, 2300 RA Leiden, The Netherlands (Received February 20th, 1984)

Abstract. Thermolysis of tert-butyl hydroperoxide (1) in nitrogen with an excess of benzene in the temperature range 200-300 " C, leads to biphenyl (2) as the only observable benzene-derived product. With chlorobenzene, dichlorobiphenyls (4) are formed, together with chlorobiphenyl (5), ca. 12% on 4, and phenol (20-25% on 4); chlorophenols are not produced. The use of ca. equimolar quantities of iodine based on 1 results in formation of iodobenzene (3) and chloroiodobenzenes (7), respectively, at the expense of biaryl. In air, chlorobenzene and 1 produce chlorophenols (S), biaryl then being a minor product. Hydroxyl radicals produced from 1 abstract hydrogen from benzene to give phenyl radicals (Ph') which, in nitrogen, arylate benzene. From isomer distributions of 7 and of 4 it is inferred that hydrogen abstraction from chlorobenzene leads to chlorophenyl radicals, with a ratio o/m/p N 24/52/24. With chlorobenzene, @so substitution, PhCl + 'OH -+ PhOH, also takes place. In the presence of oxygen or iodine, formation of biaryls involves irreversible addition of (ch1oro)phenyl radical, the intermediate adduct radicals reacting with oxygen or iodine. In nitrogen, however, the first step in arylation is reversible. Apart from undergoing 0-0 bond homolysis, 1 is attacked by 'Me (formed from Me,CO') and, to some extent, by aryl radicals. A large arene/l intake ratio (> loo), or the addition of either iodine or air suppresses - or even prevents - free-radical-induced decomposition of 1.

Introduction

In our previous paper' on the gas-phase thermolysis of tert- -butyl hydroperoxide (1) - carried out at ca. 250°C in an atmosphere of nitrogen - we reported that the primary steps involved formation of 'OH and 'Me radicals (Eqns. 1 and 2). The fate of these radicals depended on the composition of the reaction mixture. In "neat" pyrolysis, induced decomposition (Eqn. 3) greatly contributed to the overall conversion of 1 whereas, in the presence of an excess of toluene, reactions (3a,b) were largely replaced by steps (4a,b).

Me3COOH (1) -+ Me,CO'+'OH (1)

Me,CO' -+ Me,CO+'Me (2)

1 + X' -+ Me,COO'+HX (3)

PhCH, + X' -+ PhCH,'+HX (4)

C6H.5 + x' -+ P h ' + HX (5)

(1) Eqns. 3-6: X = (a) OH; (b) Me; (c) Ph; (d) H; (e) C1.

Hydroxyl and 'Me radicals differ greatly in their rates of hydrogen abstraction. Typically, the activation energy for abstraction from sec-R-H is 0.9 kcal/mol for 'OHZa but 10.2 kcal/mol for 'Mezb. Abstraction from benzene by 'OH is also rapid and exothermic to the extent of ca. 10 kcal/m01~.~. A recent study of the decay rate of 'OH in benzene vapour showed, for example, k,, to be ca. 2 . 10' M - ' . s - ' at 25OoC5. Abstraction (5b) is endothermic by ca. 5 k ~ a l , , ~ ; since the energy of activation for the reverse reaction is 7.5-1 1 kcal/mo16, E,, will be at least 13 kcal/mol. In solution, both 'OH and 'Me are known to add to benzene (Eqn. 6) and its derivatives, with oxidation (Eqn. 6B) leading to phenols and methylbenzenes, respectively. In air, at

ambient temperatures, addition of 'OH (6,a) constitutes the first step in the slow oxidative breakdown of benzene7. With increasing temperature, this addition becomes reversible (6,a, 6 -,a), the slower abstraction step (5a) gaining in im- p ~ r t a n c e ~ . ~ . As part of our studies on gas-phase homolytic aromatic sub- s t i t u t i o n ~ ~ . ~ - especially those involving 'OH9 - we have carried out the thermolysis of 1 with benzene vapour in an

' P. Mulder and R. Louw, Red. Trav. Chim. Pays-Bas 103, 148 (1984).

'=R. Atkinson, K . R. Darnall, A . C . Lloyd, A . M. Winer and J. N. Pitts, Jr., Adv. Photochem. 11, 375 (1979);

bJ. A . Kerr in "Free Radicals", Vol. I, Ed. J . A. Kochi, Wiley, New York, 1973, p. 15. W . Dorrepaal and R. Louw, Int. J. Chem. Kinet. 10, 249 (1978).

4aK. W . Egger and A . T. Cocks, Helv. Chim. Acta 56, 1516 (1973);

' ,F. P. Tully, A . R. Ravishankara, R. L . Thompson, I . M. Nicovich, P. C . Shah, A . M. Kreuffer and P. H. Wine, J. Phys. Chem. 85, 2262 (1981);

bS. W . Benson, "Thermochemical Kinetics", Wiley, 1968.

bM. K . Eberhardt, J. Am. Chem. SOC. 103, 3876 (1981); ' R . A . Perry, R. Atkinson and J. N. Pitts, Jr., J. Phys. Chem. 81,296

(1977). 6aF. J . Duncan and A. F. Trotman-Dickenson, J. Chem. Soc. 4672

(1962); W . Fielding and H. 0. Pritchard, J. Phys. Chem. 66, 821 (1966).

laR. A . Kenley, J. E. Davenport and D. G . Hendry, J. Phys. Chem. 85, 2740 (1981);

bR. Atkinson, W. P. L . Carter, K . R . Darnall, A . M. Winer and J. N. Pitts, Jr., Int. J. Chem. Kinet. 12, 779 (1980).

' , K . van Nierop and R. Louw, Red. Trav. Chim. Pays-Bas 96, 176 (1977); W . F. M . Vonk and R. Louw, ibid. 96, 59 (1977);

Perkin Trans. 1635 (1973);

(1973). P. Mulder and R. Louw, Tetrahedron Lett. 23, 2605 (1982).

' R . Louw, J. W . Rothuizen and R. C . C . Wegman, J. Chem. SOC.

dR. Louw and H . J. Lucas, Recl. Trav. Chim. Pays-Bas 92, 55

Recueil. Journal of the Royal Netherlands Chemical Society, 103/10, October 1984 283

T ( " C ) 'I (sId Ph, (2) Exp. no.c T(OC) 5 (sId

212 107 0.007 1-6 220 109 233 103 0.030 1-7 249 103 252 99 0.089 1-8 278 98 272 95 0.269 1-9 298 94 278 98 0.248 I- 108 277 189

atmosphere of nitrogen. The effect of added iodine, known to react very rapidly with carbon free radicals", is described. Analogous runs have been performed with chlorobenzene. The changes brought about by admitting oxygen are also reported.

Results

CH4 C2H6 (ZMe)'

0.164 0.0 18 0.20 0.457 0.102 0.66 0.624 0.290 1.20 0,546 0.335 1.22 0.534 0.257 1.05

Major products from 1 were CH, and C2H6. Acetone was also formed but not quantitatively detemined. At > 270°, at which temperatures 1 is almost completely converted', the value of (ZMe) corresponds to a quantitative yield. The ratio CH,/C2H6 is seen to decrease from ca. 9 at 220" (run 1-6) to ca. 1.6 at higher temperatures (run 1-9). In Table 11, results are summarized for runs at 278°C with varying inflow rates of 1 and of benzene. In series A, production of 2 parallels the inflow of 1 and shows little tendency to level off (Fig. 1). In series B 1 , the yield of 2 first increases with the benzene/l intake ratio (Fig. 2 ) , but then levels off for intake ratios > 200. An accompanying check on the production of CH, and C2H6 (Table 11, B2) shows that

11-1 11-2 11-3 11-4 (=I-5) 11-5

6) Me,COOH/N,/benzene

Using the flow system and techniques described earlier', we first studied product formation in the temperature region in which 1 decomposes smoothly. With a constant feed com- position, the results shown in Table I were obtained. GLC analysis showed that phenol was not formed, the only ob- servable product stemming from benzene being biphenyl (2).

Table I Thermolysis of Me,COOH (1) and benzene".

405 400 405 440 525

l o D . M . Golden and S. W. Benson, Chem. Rev. 69, 125 (1969).

benzene in

67 138 335

Exp. no.b

5 (s) CH4 C2H6 (XMe)

98 0.624 0.290 1.20 86 0.571 0.256 1.08 64 0.503 0.143 0.79

1 in

1.19 1.19 0.68 0.34

I- 1 1-2 1-3 1-4 1-5'

Products (mmol/h) 5 (s)

Ph2 (2) PhI (3) CH4 C2H6 (XPh)/ 1

94 0.14 0.60 0.02 0.001 0.62 95 0.27 (0) 0.62 0.29 0.23

125 0.090 0.39 <0.01 <0.001 0.7 1 170 0.032 0.22 <0.01 <0.001 0.75

" Flow rates of Ph,, CH4 and C2H6 are given in mmol/h; inflow of 1 1.19; benzene 67 mmol/h. mmol/h. mmol/h; no benzene present (see ref. I ) .

N, 457 mmol/h. N, 440 8 1 1.05 mrnol/h; N, 280 Av. residence time. (XMe) = (CH,) + 2(C2H6), in mmol/h. ' 1 1.04 mmol/h; N, 440 mmol/h.

Table II Thermolysis of llbenzene at 278" C ; effect of varying concentrations".

A . Benzene inflow = 67 mmol/h

Ph, (2) out

0.167 0.403 0.708 1.04 1.42

105 106 105 98 84

0.037 0.1 12 0.165 0.248 0.242

Exp.no. I N,in I benzenein Ph, out

11-6 11-7 11-8 II-9b

33.5 134 334 334

102 86 77 78

0.1 I 5 0.393 0.693 0.313b

450 440 312 312

B2. Inflow of 1 = 1.19 mmol/h

11-10 ( G 1-8) 440 11-1 1 i 440 11-12 440

a Data on in- and out-flow in mmol/h.

Table I11 Thermolysis of 1 and benzene at 275" C; effect of iodine".

Exp. no. i N, in

460 457 330 225

111-2' 111-3 111-4

' Inflows: benzene 67; I, 1.0 mmol/h. (XPh) = (2) + (3). ' Compare runs 1-5 and 1-8; no iodine present.

284

- s : z E 2 Y

a

T 0.3

0.2

0.1

0

Peter Mulder et al. / Gas-phase thermolysis of tert-butyl hydroperoxide with benzene and chlorobenzene

a high. benzene/l intake ratios (CMe) is significantly below 100% based on 1; this is very probably due to reduction of the residence time T, leaving an increasing fraction of 1 unconverted. The CH,/C,H, ratio increases f roh 2.1 (exp. 11-10) to 3.5 (exp. 11-12).

4 5.

3 .

?.

I

0.5 1 .o 1.5 - Inflow of t-BuOOH [mmol/h]

Formation of biphenyl (2) at constant benzene inflow (see Table 11-A).

2 a

1 70

50

3 0

10

0

.* . 9

3 00 600 900

+ PhH/t-BuOOH intake ratio

Yield of biphenyl (% on inflow of 1) as a function of the ben- zenell intake ratio (see Table II-B).

(ii) Effect of added iodine

Iodine was fed into the reactor as a solution in benzene. From the examples given in Table I11 it can be seen that added iodine greatly affects the product composition. Compared with run 1-5, for example, biphenyl production is reduced and a new product, iodobenzene (3), is formed. In the presence of I,, the overall yield of benzene-derived products is over 70% based on ( l ) , up greatly from the ca. 23% observed in its absence. Formation of CH, and C,H, is almost com- pletely suppressed. Methyl iodide was qualitatively observed to be an important product, but, due to its volatility, it could not be accurately determined. In run 111-1, conversion of I, was nearly complete, but in run I114 a substantial proportion of iodine was present in the emuent. Due to the presence of some unreacted 1, accurate determination of I, in the outflow was not possible.

(iii) 1 and chlorobenzene

With chlorobenzene, CIC,H,C,H,Cl (4) is the major prod- uct; PhC,H,CI (5) and phenol (6) are also formed, but chloro- phenols are definitely absent (Table IVA).Note that the yield of 4 + 5 is ca. 12% at temperatures > 270°C. The ratios 5/4 and 6/(4 + 5) show little temperature dependence. With added iodine (run IV-7), IC,H,Cl (7) is an important product. Replacement of nitrogen by air (run IV-4) greatly diminishes the formation of biaryls, the main now being chlorophenols (8). Formation ofall possible isomers of4 ,5 ,7 and 8 is consonant with homolytic mechanisms (Table IVB). Note that oxygen and iodine affect the isomeric composition of 4.

Discussion

(i) 1 and benzene

Compared with the results on neat pyrolysis', benzene ap- pears to have little influence on the production of CH, and C,H, (Table I). The absence of toluene proves that addition of 'Me to benzene (Eqn. 6b) is ineffective, either because of its reversibility" or due to the slowness of addition compared with alternative reactions, (3b) and (7).

2 'Me + C,H, (7)

2 'Ph +Phz(2) (8) In line with these results, experiments using tert-butyl peroxide in place of 1, in nitrogen and benzene at 210"C, yielded neither toluene nor 2". The latter observation sup- ports the suggestion that H abstraction by 'Me (Eqn. 5b) is also negligible. Thus, formation of 2 will result from H ab- straction by 'OH (Eqn. 5a), followed by (6c) and loss of H from the intermediate phenylcyclohexadienyl radicals (Ic)*.

* Since under the present conditions, phenol is not formed from 1 and benzene/N,, (reversible) addition of 'OH to benzene to yield Ia (Eqn. 6), followed by loss of H, is insignificant compared with H abstraction (3a).

I l a R . A . Jackson, J . Chem. SOC. Chern. Commun. 573 (1974); bA. Amano, 0. Horie and N. H. Hanh, Int. J . Chem. Kinet. 8,321

(1976). P. Mulder and R. Louw, unpublished observations.

l 3 See M. K . Eberhardr, J . Phys. Chem. 78, 1799 (1974).

286 Peter Mulder et al. / Gas-phase thermolysis of tert-butyl hydroperoxide with benzene and chlorobenzene

a benzene/l intake ratio of ca. 100 (Fig. 2), the yield of 2 is about one half of the "maximum" value of ca. 70%; in other words, reactions (3c) and (6c) then proceed at about equal rates. However, since 1 is largely decomposed under the conditions used, the actual benzene/l ratios are much larger than the intake ratios. Therefore, at 278"C, the ratio k,,/k6, appears to be well above 100. It should be pointed out that phenylation (6c) involves initial addition of Ph' and that this first step, (6,c), is reversible, especially above 250°C (see also section iii). This would make k6, an "overal" rate constant and kdAc > k6,. In any case, the selectivity of Ph' towards 1 and benzene is intermediate between those of 'OH (little or no reaction with 1) and 'CH, (no reaction with benzene) - vide supra. Indeed, Ph' is more reactive6= and, in H abstraction, less selective than Me' 19.20.

Due to 'Me-, 'Ph- and, presumably, 'H-induced decompo- sition, reaction (3) accounts for the major part of the con- version of 1. As in "neat" pyrolysis of l ' , the resulting Me,COO' radicals will react with each other (Eqn. 9), fol- lowed by regeneration of 'Me (reaction 2). Adduct radicals Ic may also be involved in termination, presumably by reaction (10).

2 Me,COO' -+ (tetraoxide) + 0, + 2 Me,CO'

Me,COO' + I c+ 1 + 2 (9)

(10)

(ii) Effect of iodine

Formation of CH, and C,H6 is almost completely suppressed by small amounts of I, (Table 111); this must be due to reaction ( 1 I), for which k , , is as high as 10" M - . s - (ref. 10).

Me' + I, -+ Me1 + I'

Ph' + I, -+ PhI (3) + I ' (11)

(12) Although k , , 2 k , , (refs. 10,20), iodine is much less effective in preventing formation of biphenyl. This substantiates our conclusion that 2 does not arise via reaction (8), but rather via addition of 'Ph to benzene (reaction 6). When the 12/1 intake ratio is > 1, the total yield of products derived from Ph' ((ZPh), Table 111) is 70-75 %, consonant with the results depicted in Fig. 2 for very high C6H6/1 ratios. Apparently, few Ph' radicals succeed in attacking 1 (Eqn. 3c) under these conditions.

That formation of biphenyl still occurs to some extent is probably due to reaction of intermediate radicals Ic with iodine (Eqn. 13). Successful competition between reaction (13) and the reversal of Ic (step 6,c, yielding Ph' and ben- zene) produces an intermediate (9) which is likely to lose HI (step 13B).

Ph, P

(9)

Reaction (1 3,) is ca. 7 kcal/mol endothermic*. By analogy with reaction ( l l ) , reaction (13,) will have negligible "extra" activation energy; hence, log k, , , N

10-7/0. With [I2] N M, as is the case in our experi- ments, v,,, = k, , , . [Ic] . [I,] N [Ic] M . s - ' at 275°C.

* The bond dissociation enthalpy DH"(iPr-I) = 53 kcal/mo14; hence DH"(R-I) in (9) will be 53-25 = 28 kcal/mol, if the re- sonance energy of Ic is 25 k c a l / m ~ l ~ ~ . l ~ . DH"(1-I) = 35 (ref. 4).

This value is very close to that calculated for the rate of reversal of Ic, v6- / \ , = k6-,; [Ic]: assigning the parameters log A / s - ' = 14, E = 30 kcal/mol (ref. 1) to this rate constant, k6-nc = lo2.' at 275°C. If reversal of Ic to Ph' + C6H6 is neglected, the product ratio

Using log k , , = 10 and log k6/\, = 8.7-4/0 (ref. 20) = 7.lat 275"C, and taking [I,]/[C6H6] = 1/200 (one-third of the in- take ratio so as to simulate experimental conditions in our tank-flow reactor), the product ratio (3)/(2) is calculated to be ca. 4. This value agrees well with actual ratios (Table HI).

(3)/(2) is governed by v12/v6Ac = { k12 ' [I21 } / { k6Ac ' [C6H61 }.

(iii) Chlorobenzene

As with benzene, 'OH will abstract H from PhCl to give 0-, m- andp-CIC,H,' radicals (reaction 5') , the latter producing 4 (Eqn. 6'). However, judging from the formation of 5, @so substitution (reaction 6") also seems possible". The small proportion of C1' atoms thus formed will abstract H from PhCl and/or from 1 (reaction 3e). The yield of 4, under comparable conditions, is about fifty percent of that of bi- phenyl from benzene (compare runs IV-5 + 6 with 1-4 + 5). This is due to a retarding effect of CI on hydrogen abstraction by electrophilic 'OH (Eqns. 5c,5'). A more detailed account of substituent effects will be published shortly".

PhCl + 'OH -+ o-,m-,p-C1C6H,' + H,O (5')

PhCl + 'C6H,C1 -+

r 1

PhCl + 'OH

C6H,C1 + I, + o,m,p-IC6H,C1 (7) (12')

'C6H,CI + 0, -+ C1C6H,00' + + CIC,H,OH (8) (14)

In the presence of oxygen(run IV-4), adduct radicals I ' are likely to react with 0, to give 4 and HO,'. Judging from data obtained with other (alkyl) radicals, such a disproportio- nation will be rather fast, with a rate constant of ca. lo6 M - ' . s - at 300 K2a*22. This reaction will then prevent re- versal (6LA); as a consequence, @so substitution (6") be- comes relatively less important, as is seen from the decrease of the 5/4 product ratio to about 0.02 (Table IVA). Formation

2o R. G. Kryger, J . P. Lorand, N. R. Stevens and N. R. Herron, J . Am.

21aP. Mulder, forthcoming Thesis, Leiden; Chem. SOC. 99, 7589 (1977).

bP. Mulder and R. Louw, J. Chem. SOC. Perkin Trans 11, in the press.

22 S. W. Benson, Adv. Chem. Ser. 76, 143 (1968).

Recueil, Journal of the Royal Netherlands Chemical Society, 103/10, October 1984 285

P-,P-?

4.9 5.6 6.2 4.1 7.1 7.6 3.9

Table IV Thermolysis of 1 with chlorobenzene".

5

0- m- P-

42.1 36.6 21.3 33.5 43.9 22.6 27.7 49.2 23.1

55.6 23.7 20.7 16.3 59.4 24.3 12 59 29

- - -

A . Products

o-,o-I o-,m-' 0-,p-' m-,m-' m-,p-'

Exp. no. CIC,HC,HCI (4)

PhC,H,CI (5)

PhOH (6) (6)/(4 + 5 )

0.23,

0.24

IV- 1 IV-2 IV-3 IV-4b.C IV-5 IV-6 IV-7"

212 232 252 254 272 291 295

111 106 102 98 99 94 99

0.0057 0.0175 0.057 0.0056 0.127 0.109 0.147

0.00073 0.0024 0.0067

50.0001 0.013 0.010 0.0026

0.13 0.14 0.12

50.02 0.10 0.09 0.018

< 0.002 0.0045 0.0 16 0.043' 0.030 0.0029 0.082

B . Isomer Distributions (%)

IV-I IV-2 IV-3 IV-4 IV-5 IV-6 IV-7

9.0 7.5 7.1

14.8 5.9 5.4

14.4

34.4 32.0 28.1 34.1 24.4 22.3 34.4

16.0 15.4 14.5 17.3 14.1 12.9 18.7

17.4 18.9 21.5 15.9 23.6 25.2 13.1

18.1 20.6 22.6 13.7 24.8 26.6 15.5

Inflow: N, 450; 1 1.19; PhCl 57 mol/h. Inflow N, 335; 0, 135; 1 1.17 mmol/h. Also formed: 0.183 mmol/h of CIC,H,OH (8); Inflow: N, 430; 1 0.94; PhCl 57; I, 1.0 mmol/h; also formed: 0.165 mmol/h of IC6H,CI (7); olmlp = 25,2149.8125.0.

olmlp = 24.2/52.1/23.7. (6 + 8)/(4 + 5).

due to the increasing fraction of 1 remaining unconverted. Phenylation (reaction 6c) may entail formation of free H atoms (step 6+)***. These H atoms are likely to abstract hydrogen from 1 (Eqn. 3d), since the analogous reaction" with MeOOH is fast, log k/M- ' . s - N 8-1.9/8. H abstraction from benzene (Eqn. 5d) must be slow since it is endothermic to the extent of 5 k ~ a l ' ~ . ' ~ ; E,, is reported to be ca. 10 kcal/mo18d,'yb. From Fig. 2 and Table I1 B 1, it can be seen that variation of the benzene/l intake ratio, in the range of 0 5 3 % of 1 on benzene, has a large effect on the yield of biphenyl (2). With

Combination of Ph' (Eqn. 8), which has been advanced as the mechanism of formation of 2, not only in solution (compare ref. 13) but also in gas-phase reaction^'^, cannot possibly be important. Should 2 have been formed in this way, the prod- uction of C,H,in nearly equal amounts would call for the formation of toluene by cross-combination of Me' and Ph', which is not the case. (Upon thermolysis of 1 in toluene, Me' and benzyl radicals are formed, which give rise to C,H,, bibenzyl and ethylbenzene, the latter compounds being formed in a ratio of about 2/1.)' Reaction (8) is insignificant due to the fairly high rate of phenylation of benzene (reaction 6c). The relative rate is:

** This (maximum) value - common for concentrations of transient free radicals - is obtained as follows: Assuming reactions (5a) and (6c) to be responsible for formation and disappearance of [Ph-1, vSa = v , ~ , or [Ph'] = (kSa/k6J. ['OH]. The steady-state concentration of 'OH is approximated by k , . [ l ] N k,; [C,H,]. ['OH]. Hence, [Ph'] = (k,/k, ,) . [1]/ [C,H,]. With log k , / s - ' = 15.3-41.4/8 (ref. 1) and log k6c = 8-4/8 (ref. 8d,15), kl/k6= N lo- ' M. The ratio [l]/[C,H,], around 250°C, is I lo-'.

*** With log k,,/s-' N 13.5-32/8 (ref. 17), at >550 K, uni- molecular decomposition ofIc will successfully compete with its disproportionation, e.g. with 'Me or with Me,COO' (Eqn. 10, vide infra).

14aA. D . Kirk and J. H . Knox, Trans. Faraday SOC. 56,1296 (1960); bA. Lin and E. Kehat, Ind. Eng. Chem. Prod. Res. Dev. 8, 436 (1969);

" M . 0. Delcourt-Euverte and J. Drillat, Bull. SOC. Chim. Fr. 5 (1970). C. T. Brooks, S . J. Peacock and B . G . Reuben, J. Chem. SOC. Faraday Trans. I 75, 652 (979). C. Anastasi, I. W . M. Smith and D . A . Parkes, Trans. Faraday SOC. 74, 1693 (1978).

l7 D . G. L. James and R. D . Suart, Trans. Faraday SOC. 64, 2752 (1968).

I' F. glemr and P. Warneck, Int. J. Chem. Kinet. 11, 267 (1977). 19aR. F. Bridger and G. A . Russell, J. Am. Chem. SOC. 85, 3754

bT. Bkrces and J. Dombi, Int. J. Chem. Kinet. 12, 183 (1980). (1964);

Using logk,,/M-'.s-' = 8-4/8 (see refs. 8d,15; 8 = 2.303 1 R . Tin kcal/mol), taking [C,H,] N 3. lo-, M (a value typical for our runs at ca. 250°C, Table I ) , and as- signing ks a usual value for recombination of radicals, 1O'O M - ' . s - ' , V ~ / V , ~ N 106.[Ph']. Since [Ph'] I M**,

On the basis of known values for k,, (ca. 2.2. 1OY)l6 and for k,, (2.10' M - ' . s - ' ) , k,,/k,, is ca. 10 at 250°C. Hence, attack of 1 by 'OH will be significant if the benzene/l ratio is, say, below 50, whereas nearly all 'OH will react with benzene if this ratio is above loo**. Given our experimental conditions, intake ratios and degrees of conversion of 1, the latter situation holds for most of our experiments. Methyl radicals either combine to give C2H, (Eqn. 7) or abstract H from 1 (Eqn. 3b). Judging from the yields of CH, given in Table I , the latter reaction can account for about fifty percent of the decomposition of 1. In the series 1-6 through 1-9, the ratio [CH,]/[C,H,] decreases; with increasing tem- perature, the rate of primary dissociation of 1 (Eqn. 1) increases and gains in importance since less (1) will be avail- able for H abstraction (3b). Accordingly, in the series 11-10 through 11-12 (Table I1 B2), the [CH,]/[C,H,] ratio increases

vs/v6c I 10 - 4.

Recueil, Journal of the Royal Netherlands Chemical Society, 103110, October 1984 287

of biaryl is a totally minor reaction, since 'C,H,CI now prefers to react with 0, (Eqn. 14), ultimately producing chlorophenol (8) from the intermediate arylperoxy radicals. Oxygen may even interfere at an earlier stage, namely by reaction with adduct radicals of arene and 'OH (Ia, Eqn. 6; I"', Eqn. 6"'). Reactions involving 'OH and 0, at various temperatures will be discussed in detail elsewhere,'.

In the presence of I,, the production of 4 and 5 is also diminished, and the 514 product ratio is similarly much lower (Table IVA, exp. 7). These effects, including formation of IC,H,CI (7), are analogous to those observed for benzene (section ii) and can be similarly rationalized, i.e. by reaction (12') and by conversion of I' - see reaction (6') - with I,. The site selectivity for H abstraction (5') will be reflected by the isomer distribution of 7 formed in steps (12'). Rates of iodine transfer are (almost) diffusion controlled2' so it is unlikely that individual isomers of 'C,H4C1 will react with substantially different rates. In the absence of I, , 'C,H,CI will be formed in the same isomer ratio (exp. IV-7: o/m/p = 24/52/24). If it is assumed that the three isomeric 'C,H,Cl radicals arylate PhCl (Eqn. 6 ' ) with identical rates and site selectivity, the isomer dis- tributions of 4, of runs IV-5 + 6 then imply arylation with the site selectivity of o/m/p N 23/48/29. Such a distribution is very similar to that observed for phenylation of PhCl in the gas phase above 400°C23; a reaction clearly involving rever- sible addition of Ph'. For irreversible phenylation at lower temperatures, in solution, the isomer pattern is ca. 6012511 523,24. Remarkably, the distribution of 4, as reported for runs IV-4 + 7, tallies with arylation according to such a pattern if the isomer ratio of 'C,H,CI is still o/m/p = 24/52/24. These changes in isomer distribution thus underscore the fact that, above ca. 25OoC, arylation in l/arene mixtures involves reversible addition or Ar', whereas, in the presence of 0, or I,, step 6; leads irreversibly to biaryl.

At lower temperatures, 4 (and 5) show higher proportions of ortho isomers. Since it is unlikely that the isomer ratio of 'C,H,CI, generated by H abstraction by 'OH (Eqn. 5 ' ) , is strongly temperature dependent, the shift in the isomer dis- tribution of 4 must be due to a change in site selectivity for arylation (6'). The isomer compositions for runs IV-1 + 2 point to isomer distributions for arylation of ca. 45/35/20 and 40/38/22, respectively. This shift towards the "low tempera- ture" distribution points to a diminishing importance of the back reaction (6LA). If the isomer distribution of 'C,H,Cl remains unaltered, say, 24/52/24, the analogous change in the distribution of 5 is not easily understood if @so substitution is the only, or major, pathway leading to 5. Phenylation (6"") would be a satisfactory rationale, but this calls for a mode of formation of Ph' from chlorobenzene. Chlorine abstraction by 'C,H,CI, reaction (1 5) , must be excluded since we failed to detect formation of dichlorobenzene, although hydrogen atoms could well fulfil this function; we have noticed that in thermolysis of PhCl with hydrogen reaction (16) is an impor- tant chain-carrying step25, k, , being about as large as the rate constant for reaction of H' with MeOOH".

Ph' + PhCl 4 - ( 5 ) (6 " " )

PhCl + 'C,H4C1 + Ph' + C,H4C1, (15)

PhCl + H' -+ Ph' + HCl (16)

- H

We therefore suggest that the major part of 5 is formed via reaction (6"") rather than via (6"); in air, free H atoms will be scavenged by 0,, the low 514 product ratio of _N 0.02 then being due to reactions (6") and (6'). The isomer distribution of ClC,H,OH (8) is close to that of IC,H,CI (7), see Table IVA. This would support the supposition that reaction (14) is a major pathway to (8), although process (6"'), reversible addition of 'OH followed by disproportionation with 0,, may well involve a comparable isomer distribution. As mentioned previously, the features and mechanisms of hy- droxylation/oxidation of arenes will be outlined in more detail elsewhere2'.

Experimental

Chemicals Me3COOH (1) - Fluka, Technical grade, 80% - was distilled twice in vacuo (b.p. 32"C/18 mm). Iodometric titration' showed a purity of 98 f 1%. Tert-butyl peroxide (Fluka); benzene, chlorobenzene and pyridine (J. T. Baker; (p.a.); bis(trimethylsi1yl)trifluoroacetamide and chloro- trimethylsilane (Merck; (p.a.); iodine (BDH, Analar); tank oxygen and nitrogen (Loosco 99.99%) were used without further purifi- cation. The apparatus and methods of thermolysis have been previously describedsd. The reported data are based on runs performed at a minimum in duplicate; product analyses were reproducible to within 5 % .

Analysis

Methane and ethane: by GLC'. Aromatic compounds: 2 ,4 and 5, iodobenzene (3), phenol (6) and chlorophenols (8) - analyzed as Me,Si derivatives (vide infra) - on a capillary SE-30 column, iodochlorobenzenes (7) on a capillary Carbowax (Ca-5) column. GLC conditions: H, 0.4 atm; flow ca. 1 ml/min; Tinj. = T,,,, = 220°C; temp. prog. 100°C (8 min.), 8"C/min., 200°C (10 min.). The relative concentrations of products were calculated from integrated area ratios using p-C,H,CI, as an internal standard.

Silylation of phenols

To dried (molsieve 3 A) aliquots of reaction mixtures (ca. 2 ml) containing chlorophenols the following reagents were subsequently added: pyridine (0.2 ml), bis(trimethylsi1yl)trifluoracetamide (0.1 ml) and chlorotrimethylsilane (0.1 ml). Conversion was completed within 5 min as shown by similar treatment of mixtures of 0-, m- and p-CIC,H,OH. On GLC, 0-, m- and p-CIC,H,OSiMe, were well separated in contrast to the parent phenols.

23 R. Louw and J . W. Rothuizen, Tetrahedron Lett. 3807 (1967). 24 M. J . Perkins, in "Free Radicals", Ed. J . K . Kochi, Wiley, 1973,

25aR. Louw, J . H. M. Di jh and P. Mulder, Chem. Ind. 159 (1983); bR. Louw, J . H. M. Dijh , J. A. Manion and P. Mulder, forthcoming publication.

Vol. 11, p. 246.