Aqueous basicity and proton affinity of zwitterionic omega-(N-methylpiperidine)-alkanocarboxylates and omega-(N-piperidine)-alkanocarboxylic acids

The pK(a) values of 5 omega-(N-methylpiperidine)-alkanocarboxylate (N-methy lpiperidine betaines) and 5 omega-(N-piperidine)-alkanocarboxylic acid were determined by potentiometric titration of their hydrohalides with KOH. Sem iempirical geometry optimizations were performed for gaseous betaines. Four conformers were characterized and their PA values estimated. The PA values fulfilled the linear correlation with the aqueous pK(a) values estimated i n ref. 1. A linear correlation between the calculated heal of formation (De lta H-f) and the sum of the N ... O-1 and N ... O-2 distances, for the conf ormers containing the same number of CH2 groups, indicates that they are st abilized by the intramolecular electrostatic interactions between the posit ively charged nitrogen atom and oxygen atoms of the carboxylate group.