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 (k_Nu) have been measured in aqueous solution (I= 0.1 mol dm ^–3, 25 °C) for the S_N2 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 β_nuc 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 k_Nu for the aminolysis of methyl 4-nitrobenzenesulfonate and log k_Nu 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).