Preliminary Studies of the Mechanism of Metal-Halogen Exchange. The Kinetics of Reaction of n-Butyllithium with Substituted Bromobenzenes in Hexane Solution

Article abstract of DOI:10.1021/ja00366a024

Initial measurements of the rate of reaction of bromobenzene with n-butyllithium in hexane solution have shown the exchange to be first order in bromobenzene and first order in n-butyllithium, with an activation energy of 12 kcal mol^-1 (52 kJ mol^-1).A Hammett relationship for the reaction of substituted bromobenzenes with n-butyllithium suggests negative charge character in the transition state (ρ ca 2).The addition of a Lewis donor (p-methylanisole) to the hexane solution was found to result in an increase in the rate of exchange, but did not affect the Hammett reaction constant.Several transition-state structures are considered; available evidence suggests that the exchange may be concerted, with either a four-centered structure, or S_N2-type attack of the n-butyl anion at the bromine of the aryl bromide