ABSTRACT
As a class, the aliphatic amines encompass a large number of drugs, neurotransmitters, hormones, and other bioactive compounds [326]. In contrast to the oxyacids discussed in Chapter 5, the first dissociation in aliphatic and aromatic amines occurs from a charged species, an ammonium (–NH3+) group:
It is notable that in the gas phase the basicities of the methyl amines increase in a regular order with methyl substitution, as might be expected from simple considerations of induction and electron donation by the methyl groups, but in aqueous solution the basicities and acidities display an irregular pattern [327,328]. The aqueous acidities fall in the following order:
NH3 (CH3)3N (CH3)NH2 (CH3)2NH
pKa = 9.25 9.80 10.66 10.73
These values presumably result from the differing solvations of the methylammonium ions, where even small entropy differences can have a significant effect [329-336].
