steric hinderance, etc., which affect the basic strength of amines. Just ponder over. You may get the answer in the following paragraphs. Structure-basicity relationship of amines Basicity of amines is related to their structure.
Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine. (a) Alkanamines versus ammonia Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity. Due to the electron releasing nature of alkyl group, it (R) pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid.
Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups. This trend is followed in the gaseous phase.
The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH . The trend is not regular in the aqueous state as evident by their p K b values given in Table . . In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules.
The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows: Decreasing order of extent of H-bonding in water and order of stability of ions by solvation. Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order.
Secondly, when the alkyl group is small,