Kinetics and mechanism of reactions of acetyl and chloroacetyl chlorides with methanol and phenols in acetonitrile

Includes bibliographical references.Any study of the reaction mechanism for the interaction of acyl halides with various nucleophiles trill be complicated by the delicate balance between ionic mechanisms and addition intermediate mechanisms. The structure of the substrate, the type of nucleophile, and the solvent are important factors to be considered. The kinetics of the reactions of acetyl and chloroacetyl chlorides with methanol in acetonitrile have been measured by following the rate of the production of hydrogen chloride using titration against a toluene solution of triethylamine. A rate expression of the following type was obtained. Rate - k2(RCOOC1) (MeOH) + k3(RCOC1) (MeOH)2, where R = CH3, CH2Cl, or CHC12. The observed equation indicates that both second and third order kinetics are operative, and a study on the effect of both phenol and tetraethylammonium chloride added to the system aided in the determination of the most plausible mechanism for the methanolysis of the three acyl chlorides. The order of reactivity of acetyl and chloroacetyl chlorides varies as CH3COC1 < CH2C1COC1 < CHC12COCl, which is consistent with an addition intermediate mechanism being the major reaction path. A detailed comparison of the rate differences with those for related systems suggested that, for acetyl chloride, there might well also be a contribution from an ionic mechanism to the overall reaction scheme. The same acyl chlorides reacted with various phenols in the same solvent, and the kinetic results suggested a predominantly ionic mechanism for acetyl chloride and, especially in the presence of added tetraethylammonium chloride an addition intermediate mechanism was proposed to be the major component far the phenolysis of chloroacetyl chloride. The rate of phenolysis of acetyl chloride was greater than for chloroacetyl chloride, which is opposite trend to that for methanolysis. A comparison of the rates of attack by p-methoxyphenol, phenol and p-nitrophenol shewed that the two substituted phenols reacted faster with acetyl chloride in acetonitrile than phenol itself, but not by any larger factor. This is consistent with a "leveling effect" in the nucleophilicities for attack upon an acylium-chloride ion pair, coupled with a greater effect being exerted upon the ionization process by the more polar substituted phenols.M.S. (Master of Science