Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity. See fig.1.
`=>` Due to the electron releasing nature of alkyl group, it (`color{red}(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 `color{red}(+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 > `color{red}(NH_3)`.
● The trend is not regular in the aqueous state as evident by their `color{red}(pK_b)` values given in Table 13.3.
● In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group `color{red}(+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 : See fig.2.
Decreasing order of extent of `color{red}(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, like `color{red}(–CH_3)` group, there is no steric hindrance to `color{red}(H)`-bonding.
`=>` In case the alkyl group is bigger than `color{red}(CH_3)` group, there will be steric hinderance to `color{red}(H)`-bonding. Therefore, the change of nature of the alkyl group, e.g., from `color{red}(–CH_3)` to `color{red}(–C_2H_5)` results in change of the order of basic strength.
`=>` Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
`=>` The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows :
`color{red}((C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3)`
`color{red}((CH_3)_2NH > CH_3NH_2 > (CH_3)_3 N > NH_3)`
Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity. See fig.1.
`=>` Due to the electron releasing nature of alkyl group, it (`color{red}(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 `color{red}(+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 > `color{red}(NH_3)`.
● The trend is not regular in the aqueous state as evident by their `color{red}(pK_b)` values given in Table 13.3.
● In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group `color{red}(+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 : See fig.2.
Decreasing order of extent of `color{red}(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, like `color{red}(–CH_3)` group, there is no steric hindrance to `color{red}(H)`-bonding.
`=>` In case the alkyl group is bigger than `color{red}(CH_3)` group, there will be steric hinderance to `color{red}(H)`-bonding. Therefore, the change of nature of the alkyl group, e.g., from `color{red}(–CH_3)` to `color{red}(–C_2H_5)` results in change of the order of basic strength.
`=>` Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
`=>` The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows :
`color{red}((C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3)`
`color{red}((CH_3)_2NH > CH_3NH_2 > (CH_3)_3 N > NH_3)`