STEREOCHEMISTRY OF QUATERNARY AMMONIUM COMPOUNDS

Abstract

This chapter describes the stereochemistry of quaternary ammonium compounds. The ammonium ion is calculated to be and is found experimentally to be tetrahedral in shape. This spatial representation holds for substituted ammonium ions except that large substituents are known to compress the angles somewhat. The quaternary ammonium salts are capable of optical activity. Tertiary amine oxides may also be resolved into optically active enantiomers. An evidence for the analogous behavior of tetrahedral carbon and tetrahedral nitrogen is obtained by preparing compounds containing a spiran-type nitrogen atom, which must, because of its four covalent bonds to carbon atoms, be involved in a quaternary salt. Analogous geometrical isomerism exists in structurally similar quaternary ammonium salts. Mills prepared quaternary salts from 4-phenyl-l-alkylpiperidines and from 4-hydroxy-l-alkylpiperidines by treating them respectively with alkyl halides. Quaternary ammonium salts, which are obtained as optically active enantiomers, may be racemized as a result of the equilibrium. The rate of racemization in water is low. Therefore, the quaternary salts are usually racemized in chloroform.