Nucleophilicity towards a saturated carbon atom: rate constants for the aminolysis of methyl 4-nitrobenzenesulfonate in aqueous solution. A comparison of the n and N + parameters for amine nucleophilicity

Abstract

Second-order rate constants (kNu) have been measured in aqueous solution (I= 0.1 mol dm –3, 25 °C) for the SN2 reactions of methyl 4-nitrobenzenesulfonate with ammonia, 41 primary amines, 20 secondary amines. 29 tertiary amines and 7 anionic nucleophiles. For the aminolysis reactions, Brønsted-type correlations of nucleophilicity with basicity require the classification of all amines in terms of strictly defined structural classes with βnuc in the range 0.15–0.39. Swain–Scott plots indicate that simple amines, water and other light-atom nucleophiles (hydroxide, azide and cyanide, anions) are five times more reactive than heavy-atom nucleophiles (thiosulfate, thiocyanate, iodide and bromide ions). For amine nucleophiles there is a close linear correlation (of slope 0.44, and including both primary and secondary amines) between log kNu for the aminolysis of methyl 4-nitrobenzenesulfonate and log kNu for amine addition to the 1-methyl-4-vinylpyridinium cation. This correlation demonstrates a close linear relationship between the Swain–Scott n parameter and Ritchie's N+parameter for amine nucleophiles in aqueous solution (N+= 2.1n– 4.3).