Kinetic evidence for a novel, Grob-like substitution/fragmentation mechanism for the reaction of nucleophiles with trialkylammoniomethyl halides

摘要

Kinetic evidence is presented in favor of a concerted, bimolecular mechanism for the unusual nucleophilic substitution/fragmentation reactions of iodide ion with (halomethyl)trimethylammonium salts. The relative reactivities of allyl, ethyl and methyl groups for this Grob-like reaction are also determined. Activation parameters are presented for the thermal reactions of 1-iodo-N,N,N-trimethylmethaniminium iodide (log A = 8.7, E_a = 19.0 kcal mol~(-1)) and 1-iodo-N-allyl-N,N-dimethylmethaniminium iodide (log A = 14.6, E_a = 25.7 kcal mol~(-1)) in acetonitrile, the latter reaction having an observed second order rate constant 39 times larger than the former at 70 deg C. The results are compared with the classic data of Hughes, Ingold and Conant for S_N2 reactions of alkyl halides.