Mechanisms of Solvolyses of Acid Chlorides and Chloroformates.Chloroacetyl and Phenylacetyl Chloride as Similarity Models

摘要

Rate constants and product selectivities(S =([ester product]/[acid product])x([water]/,[alcohol solvent])are reported for solvolyses of chloroacetyl chloride(3)at -10 deg C and phenylacetyl chloride(4)at 0 deg C in ethanol/ and methanol/water mixtures.Additional kinetic data are reported for solvolyses in acetone/water,2,2,2-trifluoroethanol(TFE)/water,and TFE/ethanol mixtures.Selectivities and solvent effects for 3,including the kinetic solvent isotope effect(KSIE)of 2.18 for methanol,are similar to those for solvolyses of p-nitrobenzoyl chloride(1,Z = NO_2);rate constants in acetone/ water are consistent with a third-order mechanism,and rates and products in ethanol/ and methanol/water mixtures can be explained quantitatively by competing third-order mechanisms in which one molecule of solvent(alcohol or water)acts as a nucleophile and another acts as a general base(an addition/elimination reaction channel).Selectivities increase for 3 as water is added to alcohol.Solvent effects on rate constants for solvolyses of 3 are very similar to those of methyl chloroformate,but acetyl chloride shows a lower KSIE,and a higher sensitivity to solvent-ionizing power,explained by a change to an S_N2/S_N1(ionization)reaction channel.Solvolyses of 4 undergo a change from the addition/elimination channel in ethanol to the ionization channel in aqueous ethanol(<80% v/v alcohol).The reasons for change in reaction channels are discussed in terms of the gas-phase stabilities of acylium ions,calculated using Gaussian 03(HF/6-31G(d),B3LYP/6-31G(d),and B3LYP/6-311G(d,p)MO theory).