Chirality of amines

Tertiary amines of the type NHRR’ and NRR’R” are not chiral: although the nitrogen atom bears four distinct substituents counting the lone pair, the lone pair of electrons can “flip” across the nitrogen atom and invert the other molecules. The energy barrier for just such a Walden inversion of the stereocenter with a lone pair of electrons is relatively low, e.g. ~7 kcal/mol for a trialkylamine, therefore it is difficult to obtain reliably chiral products using tertiary amines. Because of this low barrier, amines such as NHRR’ cannot be resolved optically and NRR’R” can only be resolved when the R, R’, and R” groups are constrained in cyclic structures. Quaternary amine structures, e.g. H₃C-N⁺-RR’R”, are chiral and are readily optically resolved.