ELECTROPHILIC AROMATIC-SUBSTITUTION .12. KINETIC-STUDIES OF THE TICL4-CATALYZED REACTIONS OF BENZYL-CHLORIDE, PARA-XYLYL CHLORIDE, AND PARA-NITROBENZYL CHLORIDE WITH BENZENE AND ANISOLE IN NITROMETHANE AND DICHLOROMETHANE

Through the use of vacuum line techniques, noncompetitive and competitive kinetic data have been obtained for the TiCl4-catalyzed reactions of benzyl chloride and p-methyl- and p-nitrobenzyl chloride with anisole and with benzene in solvents nitromethane and dichloromethane. In solvent nitromethane, the rate law for both p-xylylation and benzylation is R = k3[TiCl4]02[RCl] and for p-nitrobenzylation R = k4[TiCl4]02[RCl] [Arom], Consistent with our earlier toluene results, p-xylylation and p-nitrobenzylation fit Brown's selectivity relationship (BSR) while benzylation with benzyl chloride does not. Arrhenius/Eyring values for the benzylation of anisole, ΔH* = 52 (±8) kJ/mol and ΔS* = –74 (±29) J/(deg mol), equal those for toluene, as required for zero-order arene dependence. In solvent dichloromethane the rate law for benzyl chloride benzylation of anisole is R = k3[TiCl4]0[RC1] [anisóle] and for benzene R = k4[TiCl4]02[RCl][benzene]. The mechanism for the benzylation of benzene is different than that for anisole as evidenced by differing rate laws. Attempts to “force” a common mechanism by adding 2,4-dichloro-3,5,6-trimethylanisole were unsuccessful. It is likely that benzene and anisole are also benzylated through different mechanisms in a 50/50 anisole/benzene medium. Thus kA/kB ratios determined competitively in either medium may not be valid, i.e., may not be used to test the applicability of Brown's relationship. In dichloromethane the reactions of p-xylyl chloride with benzene and anisole follow the same rate law, R = k3[TiCl4]0[RCl] [Arom], Competitive p-xylylation runs yielded kA/kB ratios and anisole product isomer distributions that fit BSR, whereas noncompetitively determined kA/kB ratios do not. Noncompetitive kA's were shown to be anomalously low due to TiCl4-anisole complexation. All reactions fit BSR except benzylation with benzyl chloride in nitromethane; its mechanism is explained through Jencks’ model of carbocation activity. © 1990, American Chemical Society. All rights reserved.