Solvent Effects in the Reactions of Free Radicals and Atoms. IV. Effect of Aromatic Solvents in Sulfuryl Chloride Chlorinations by GLENA

Home , Photochlorination, Sulfuryl chloride

5002 GLENA. RUSSELL \.-01. so

[COXTRIEUTIOV FROM THE GEUERAL ELECTRICRESE 9RCTI T,AEoR 4TORl.l Solvent Effects in the Reactions of Free Radicals and Atoms. IV. Effect of Aromatic Solvents in Sulfuryl Chloride Chlorinations BY GLENA. RUSSELL RECEIVEDJANUARY 16, 1958

Photochemical chlorination and sulfuryl chloride chlorination give different products in the chlorination of branched-chain hydrocarbons. This has been interpreted as being due to equilibrium (1) and to the lower reactivity and greater selectivitv of the SOzCl radical relative to the chlorine atom. In aromatic solvents little difference is noted between the products of photochlorination and sulfuryl chloride chlorination, possibly because in both types of chlorination most of the chlorine atoms are complexed with the solvent, equations 2 and 3.

It has been reported that the sulfuryl chloride where the subscripts i and f represent the initial chlorination of branched-chain hydrocarbons is and final hydrocarbon concentrations. From equa- more selective than photochlorination with molec- tion A it is calculated that in sulfuryl chloride ular chlorine.2 For example, the products of the chlorination at 95' the relative reactivity of t- photochlorination of 2,3-dimethylbutane at 55' butylbenzene to toluene is 0.81 to 1.0, while in indicate that a t.-hydrogen atom is 3.7 times as photochlorination at 80' the relative reactivity is reactive as a p.-hydrogen atom. Sulfuryl chloride 0.72 to 1. The difference between these ratios is chlorination is much more selective. At 55' sul- not significant and may be due to the different tem- furyl chloride reacts with 2,3-dimethylbutane in the peratures employed. The sulfuryl chloride chlo- presence of illumination to produce 1- and 2-chloro- rination which was performed at 95' would be ex- 2,3-dimethylbutane in a ratio suggesting that the pected to be somewhat more selective if performed t.-hydrogen atom is 10 times as reactive as a p.- at 80'. Further evidence that photochlorinatiorl hydrogen atom2b and sulfuryl chloride chlorination often yield the Sulfuryl chloride chlorinations are not always same products has been furnished by Walling and more selective than photochlorinations. The com- Miller who found that p-chlorotoluene was 0.69 as petitive chlorination of cyclohexane and toluene is reactive as toluene at '70' in both photochlorina- only slightly more selective when sulfuryl chloride tion and sulfuryl chloride chlorination.3 is used in place of molecular chlorine. At 80°, us- The foregoing results present a puzzling picture. ing sulfuryl chloride, a cyclohexane hydrogen atom In some cases sulfuryl chloride is a more selective is 3.5 times as reactive as a toluene hydrogen atom chlorination reagent than is molecular chlorine while in photochlorination the relative reactivities while in other cases no difference between sulfuryl are 2.7 to 1.2a When the relative reactivities of chloride and molecular chlorine is noted. How- the hydrogen atoms of toluene and t-butylbenzene ever, the recent findings that aromatic solvents can are considered, no difference in reactivity is noticed exert a pronounced effect upon the products of a when sulfuryl chloride or chlorine is used as the photochlorination reacti~n~b,~also suggests an ex- chlorinating agent. Details of these chlorinations planation to this anomaly. are summarized in Table I. Sulfuryl chloride chlorination and photochlorination using molecular chlorine have been observed TABLEI to yield widely different products only in the chlo- COMPETITIVE CHLORINATION OF TOLUENEAND &BUTYL- rination of branched-chain hydrocarbons in the BENZENE presence of aliphatic solvents. The most logical Photochlozination, Sulfuryl c>loride, Reactants0 80 95 interpretation of this discrepancy is that the hydro- t-Butylbenzene 0.659 0.672 gen-abstraction reactions are different in the two Toluene .707 .856 chlorination reactions, In photochlorination the Sulfuryl chloride* .121 hydrogen abstraction reaction is Chlorine ,106 C1. + RH --f R. + HCl Productsn Benzyl chloride 0.063 0.069 while in sulfuryl chloride chlorination hydrogen ab- Neophyl chloride' .043 ,044 straction involves both chlorine atoms and the t-Butylbenzened .616 .628 SOzCl radicaLZa Toluened .644 .787 so2 + c1. lg? S02Cl. (1) All quantities in moles. 1% by weight of benzoyl per- oxide added. Total alkyl chlorides minus benzyl chloride. SOgCl. + RH +SO2 + HCl f Re By difference. Further evidence for the existence of the SO2CI The relative reactivities of toluene and t-butyl- radical has been provided by Kharasch and Zavi~t.~ benzene were calculated from the data of Table I Competitive chlorinations of aralkyl hydrocar- by the equation bons in the liquid phase do not indicate an ap- react. of t-BuCaHs - log [I-BuCBH~]~- log [t-BuC&]r preciable difference between photochlorination and react. of CH&Xf€5 log [CHaC&]i - log [CHacbH~lr sulfuryl chloride chlorination. Since it is now (A) known that in the presence of aromatic hydro- (1) Directive Effects in Aliphatic Substitutions. XIII. (3) C. Walling and B. RIiller, ibid., 79, 4181 (19.57). (2) (a) G. A. Russell and H. C. Brown, THISJOURNAL, 77, 4031 (4) G A. Russell, ibid., 79, 2977 (1DR7). (1955); (b) G. A. Russell, ibid.. 80, 4987 (1958). (5) AI. S. Kharasch and A. F. Zavist, ibid.. 73, SJGl (1851). Sept. 20, 1958 SOLVENTEFFECTS IN SULFTJRYLCHLORIDE CHLORINATIONS 5003 carbons chlorine atoms are mainly cornple~ed~b~~ rium-isotope effect (KH!~D = 2.1 at 70') is due pri- marily to the reactivity of the toluene-chlorine C1. + Ar Ar + C1. (2) atom n-complex. The chlorination of a-dl-toluene it is suggested that in the presence of a large excess in an aliphatic solvent gives lower deuterium-iso- of aromatic hydrocarbon equilibrium 1 is replaced tope effects in both photochlorination and sulfuryl by equilibrium 3. chloride chlorination.6 A larger deuterium-isotope S0,Cl. + Ar I- SO2 + Ar+Cl. (3) TABLEI11 Thus, in the presence of aromatic solvents chlorine atoms in both photochlorination and sulfuryl SUMMARYOF DEUTERIUM-ISOTOPE EFFECTS chloride, chlorinations are mainly complexed by IN CHLORINATION Chlorinat- the aromatic solvent and the products of either ing Temp., chlorination will be determined by the reactivity Substrate Solvent agent OC. kdkn of the complexed chlorine atom. (CHa)aC-H(D) Isobutane C1s + hu - 16 1 .3-1.6" (CHdaC-H(D) Chlorobenzene CIr + hu - 15 1.6-2.10 As a test of this interpretation, 2,3-dimethyl- CsHsCHrH(D) Toluene Clz + hu 80 Z.Oh butane was chlorinated with sulfuryl chloride in the CsHsCHrH(D) Toluene CIS + hu 70 2.lC presence of benzene and t-butylbenzene. The re- CsHsCHrH(D) Toluene sozc12 70 2.1c sults, summarized in Table 11, demonstrate that in CsHsCHrH(D) Carbon tetrachloride SOxCIa 77 1.4d CsHsCHrH(D) Carbon tetrachloride Clr + hu 77 1.3d the presence of aromatic solvents the products of CsHsCHz-H (D) Toluene Clr + hv 110 1.5d photochlorination and sulfuryl chloride chlorina- CsHsCHr H(D) Toluene sozc1, 110 1 .7d tion become similar. Furthermore, as the solvent Ref. 2b. *Ref. 7. Ref. 3. Ref. 6 effect of the aromatic hydrocarbon in photochlorination becomes larger, the difference between rela- effect is observed in sulfuryl chloride chlorinations tive reactivities observed in photochlorination and in carbon tetrachloride solution than for photo- sulfuryl chloride chlorination decreases (Table 11, chlorinations performed in this solvent. In the column 4). At 55' sulfuryl chloride is 2.7 times as photochlorination of a-dl-toluene at 77' the deu- selective a chlorination agent as is chlorine in the terium-isotope effect, kH/kD, is reported to be 1.30 absence of any solvent except 2,3-dimethylbutane f 0.01, while sulfuryl chloride chlorination, in- itself. In the presence of 4 M benzene, sulfuryl volving the more selective SOzCl radical, gives a chloride is only 1.9 times as selective even though deuterium-isotope effect of 1.42 + 0.02.6 These sulfur dioxide is more soluble in benzene. In the results indicate: (a) different intermediate radicals presence of 4 M t-butylbenzene, sulfuryl chloride is in sulfuryl chloride chlorinations C1. and SO2Cl. only 1.3 times as selective as photochlorination and photochlorinations C1. in the presence of ali- while in the presence of 8 M benzene the difference phatic solvents; (b) different intermediate radicals in selectivity is still less, being about 1.1 to 1. in photochlorinations performed in aliphatic sol- TABLEI1 vents (free chlorine atoms) and aromatic solvents (solvated chlorine atoms); (c) the same radical PHO'IOCHLORINATlON AND SULFURYL CHLORIDE CIILORINA- intermediates in sulfuryl chloride chlorinations and TION OF 2,3-DI~THYLBUTANB photochlorinations (solvated chlorine atoms) per- -Relative reactivity (t./p.)- Sulfuryl formed in aromatic solvents. Photo- chloride Temp., chlorina- Sulfuryl Photo- Solvent 'C. tion chloride chlorination Experimental 2,3-Dimethylbutane 55 3.7 10 2.70 Competitive Chlorinations of Toluene and t-Butylbenzene. Benzene, 4 M 55 14.5 27.5 1.91 -The halogenation procedures were similar to those described previously for photochlorination7 and sulfuryl chlo- t-Bntylbenzene, 4 M 55 24 31.7 1.32 ride chlorination.28 Products of the chlorination were Benzene, 8 M 55 32 36 1.12 analyzed for total alkyl chloride by the Rauscher methods 2,3-Dimethylbutane 25 4.2 12 2.85 and for benzyl chloride in the presence of neophyl chloride Benzene, 8 25 49 53 1.08 by reaction with a 1: 1 piperidine-ethanol solution.' M Sulfuryl Chloride Chlorination of 2,J-Dimethylbutane.- Further evidence supporting this interpretation Apparatus described previously was employed.*b To 2,3-dimethylbutane (25 ml.), or a mixture of 2,3-dimethyl- is available from studies of the deuterium-isotope butane and appropriate solvent, was added 0.5 ml. of effects in these reactions. Walling and Miller re- freshly distilled sulfuryl chloride. The mixture was de- pmt the same deuterium-isotope effect when a-dl- gassed by a stream of nitrogen at the desired reaction tem- toluene is photochlorinated or chlorinated with sul- perature under a Dry Ice reflux condenser. After 5 minutes of degassing the sample was illuminated while still in the furyl chloride at 70' in the absence of a solvent thermostated water-bath. Nitrogen was continuously other than the hydrocarbon itself.aJ This deute- swept through the apparatus. After about 0.5 hour sulfur dioxide no longer was evolved. Analysis of the chlorination (6) K. B. Wiberg and L. H. Slaugh, THISJOURNAL, 80, 3033 (1958), report a significant difference in deuterium-isotope effect mixture for 1- and 2-chloro-2,3-dimethylbutaneby gas-liquid observed in the photochlorination and sulfuryl chloride chlorination chromatography already has been described.lb of a-&-toluene in toluene solution at 110". At higher temperatures, SCHENECTADY,N. Y. interaction between chlorine atoms and the aromatic solvent should be less important, but so should interactions between sulfur dioxide (7) H. C.Brown and G. A. Russell, ibid., 74, 3996 (1952). and chlorine atoms. (8) W. H. Rauscher, Ind. Eng. Chem., Anal. Ed., 9, 296 (1937).