Kinetics and isotope effects in proton transfers from 3-bromo- and 3-nitro-(+)-camphor

Abstract

The rates of mutarotation of 3-bromo-(+)-camphor and 3-bromo-3-deuterio-(+)-camphor have been measured in solutions of sodium hydroxide in 15–70% v/v dimethyl sulphoxide–water, and in 20 and 45% w/w dioxan–water. The rate increases sharply with dimethyl sulphoxide content, and the primary hydrogen isotope effect shows indications of a maximum at about 40% dimethyl sulphoxide. The same reaction was studied in glycine buffer solutions in 15% dimethyl sulphoxide and in the two dioxan–water mixtures. There is clear evidence for general base catalysis by the glycine anion, compared with which the effect of hydroxide ions is abnormally small.

The rates of bromination of 3-nitro-(+)-camphor and of 3-deuterio-3-nitro-(+)-camphor were measured in a variety of aqueous buffer solutions. The reaction is of zero order with respect to bromine and is catalysed by the basic buffer components. The pK of 3-nitro-(+)-camphor was found to be 3.54, and the isotope effect exhibits a marked maximum when the pK of the buffer acid is close to this value. The reactions of both isotopic species conform fairly well to Brönsted relations, but it is pointed out that the existence of a maximum in K^H/K^D must imply a deviation from a linear relation for at least one of the isotopic species, and the experimental results lend some support to this view.