Electrophilic Aromatic Substitution. 8. A Kinetic Study of the Friedel-Crafts Benzylation Reaction in Nitromethane, Nitrobenzene, and Sulfonate. Substitutient Effects in Friedel-Crafts Benzylation

Article abstract of DOI:10.1021/ja00335a029

Kinetic studies have been made of the reaction of benzyl chloride with benzene and/or toluene catalyzed by TiCl4 in nitromethane and in nitrobenzene and catalyzed by SbCl5 in nitromethane and sulfolane.The reaction are zero order in aromatic, first order in benzyl chloride, and either second order in TiCl4 or first order in SbCl5.Competitively determined k_T/k_B values range from 2.1 to 3.1 and product toluene isomer percentages were ortho 42-46percent, meta 4.7-6.4percent, and para 48-52percent for all four reactions.The reactions do not fit Brown's selectivity relationship (BSR), giving slope b values in the range 0.6-0.8.Competitive or noncompetitive kinetic studies are reported for Friedel-Crafts benzylation reactions with p-chloro, 3,4-dichloro, p-methyl, and p-nitrobenzyl chlorides in nitromethane.The AlCl3-catalyzed reactions of p-chloro- and 3,4-dichlorobenzyl chloride are both zero order in aromatic and do not follows BSR.However, the TiCl4-catalyzed p-xylyl chloride reaction fits Brown's relationship even though it is also zero ordered in aromatic.The p-nitrobenzyl chloride reaction catalyzed by AlCl3 is first order in aromatic hydrocarbon and also obeys Brown's relationship.These mechanistic differences are rationalized in terms of Jencks' approach: the xylyl cytion is diffusionally equilibriated and shows relatively high substrate selectivity, the less stable p-chloro- and 3,4-dichlorobenzyl cations react faster than they diffuse thus exhibiting little selectivity, while with p-nitrobenzyl chloride an S_N2 displacement mechanism is preferred.