Researches on acetylenic compounds. Part LXVI. Base catalysed interconversions between pent-2-ynoic, penta-2,3-dienoic, and pent-3-ynoic acids—a mechanistic study

Abstract

The sodium salts of pent-2-ynoic (I), penta-2,3-dienoic (II), and pent-3-ynoic (III) acids are interconverted on heating in aqueous sodium hydroxide. The reaction was followed by g.c. analysis of the methyl esters after methylation of the acid mixtures with diazomethane at –70°. Rate and equilibrium constants for the individual steps were obtained, allowance being made for a competing hydration reaction. Solvent isotope effects k(D₂O)/k(H₂O) for the conversion of (I) into (II), 1·4, and of (II) to (III), 1·6, were found, and are interpreted as evidence for a carbanion intermediate. The isomerisations in deuterium oxide have also been studied by n.m.r., and the observation that pent-3-ynoate underwent deuteriation at C-2 faster than it isomerised to (II) is consistent with a carbanion mechanism for the isomerisation.