5.1 The Basicity of Amines
Reaction of an amine with a proton produces an ammonium salt，but a primary or secondary amine can also react as an acid if treated with sufficiently strong base. Basicity① is quantified as a value called the pKb，which is determined by the reaction of the amine with water to generate hydroxide ion. The pKa value of the corresponding ammonium salt （the conjugate acid of the amine） is more frequently used in quantitative comparisons，however. With ammonia as a reference，a primary amine is more basic as a result of the inductive，electron-donating effect of the alkyl group. As more alkyl groups are attached，steric bulk forces a more planar structure on the nitrogen atom，which lowers the amine’s basicity.
Cyclic amines are more basic than their acyclic counterparts because of decreased steric hindrance on pyramidalization of the nitrogen atom. Hybridization of the nitrogen atom also affects base strength，so nitrogen heterocycles are often less basic than their saturated analogs. The basicity of anilines is influenced by resonance effects，too. The electron pair on the nitrogen atom interacts with the p electrons of the benzene ring in aniline derivatives，lowering their basicity compared with that of an alkylamine.