Micellar effects on the alkaline hydrolysis of N‐alkyl‐4‐cyanopyridinium ions. An example of micelle‐induced regiochemical selectivity

Abstract

AbstractThe rates of the reaction of N‐alkyl‐4‐cyanopyridinium (RCP) ions [alkyl = CH3 (MCP), n‐C4H9 (BCP), n‐C8H17 (OCP), n‐C12H25 (DCP)] with OH− ion in water are independent of chain length and RCP concentration up to 0.001 M. The ratios of the concentrations of the reaction products, N‐alkyl‐4‐pyridone (P) and N‐alkyl‐4‐carboxamidopyridinium (A), P/A, are similar for all substrates. In water the P/A ratios increase with pH, reaching a plateau value of ca 2 at pH ≥ 13. Added salts slightly decrease the reaction rate and do not affect the P/A ratios. In aqueous solutions, addition of dioxane increases both rates and P/A ratios. Micelles of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide; CTAB) and 3‐(N,N‐dimethyl dodecyl ammonium)propane‐1‐sulphonate (SDP) catalyse the alkaline hydrolysis of OCP and DCP and increase the yield of pyridone with BCP, OCP and DCP. In micellar CTAB and SDP the attack of OH− on RCP occurs almost exclusively at the 4‐position of the pyridinium ring. Sodium dodecyl sulphate micelles inhibit the reaction and produce small changes in the P/A ratio. The regiochemical selectivity produced by micelles, leading to a preference for the attack of OH− at the pyridinium ring, was rationalized in terms of the low effective dielectric constant at the micelle–water interface.