Kinetics of bimolecular nucleophilic substitution reactions in aqueous alcoholic media

Abstract

Bimolecular nucleophilic substitution reactions involving both aliphatic and aromatic halides have been studied with a view of explaining the contrasting behaviour observed in rates of methoxylation and ethoxylation in corresponding aqueous alcoholic media. 1. Experimental evidence suggested and literature data confirmed that there is a difference in balance of the alkoxide-hydroxide equilibrium in alkaline aqueous methanol and ethanol respectively, the ethoxide ion being more extensively hydrolysed than the methoxide ion. 2. The observed decrease in rates of ethoxylation in ethanol-water mixtures is not due to increased solvent polarity. It arises from a change of nucleophilic species brought about by increased hydrolysis of the ethoxide ion to the weaker hydroxide-ion nucleophile. 3. Methoxylation reactions have been exonerated as being the sole examples of bimolecular nucleophilic substitution reactions displaying anomalous kinetic behaviour in aqueous alcoholic media. Rates of reactions involving the thiophenoxide ion and, perhaps, phenoxide ion also increase when the solvent is changed from the absolute to an aqueous alcohol. Ion association and hydrogen-bonding in carbanions have been suggested possible causes of the phenomena. 4. The position regarding methoxylation reactions has been clarified. An increased rate of bimolecular alkaline methoxylation of all non-ionic organic substrates is to be expected when the solvent is changed from absolute to aqueous methanol. The decrease in rate observed in more water-rich aqueous methanol is due to increased hydrolysis of the methoxide ion to the weaker hydroxide ion nucleophile.