Q.Explain the basic nature of amines with suitable example.

In aqueous solution, aliphatic amine basicity follows: $R_2NH > R\text{-}NH_2 > R_3N > NH_3$; option (d) correctly places $NH_3$ lowest and $R_2NH$ highest.

In the gas phase, increasing alkyl substitution increases basicity purely by the +I (electron-donating) effect, giving $R_3N > R_2NH > RNH_2 > NH_3$.

However, in aqueous solution basicity also depends on how well the resulting ammonium cation is stabilised by hydrogen bonding with water (solvation). A tertiary amine's ammonium ion ($R_3NH^+$) has only one N–H bond available for hydrogen bonding and also suffers steric hindrance, so it is poorly solvated despite the strong inductive effect of three alkyl groups.

Balancing the inductive effect and the solvation effect, the observed order of basic strength for aliphatic amines in water is:

$R_2NH > R\text{-}NH_2 > R_3N > NH_3$

Here $NH_3$ is the weakest base (least alkyl substitution, no +I effect) and $R_2NH$ is the strongest (best balance of inductive effect and solvation). This is consistent with option (d): $NH_3 < R\text{-}NH_2 < R_2NH > R_3N$.

Amines are basic because the nitrogen atom has a readily available lone pair of electrons that can be donated to accept a proton.

In an amine, nitrogen is bonded to carbon and/or hydrogen atoms using three of its five valence electrons in covalent bonds, while the remaining two electrons exist as a lone pair on nitrogen. This lone pair is not tied up in any bond and is available for donation.

When an amine encounters a proton ($H^+$) or any electron-deficient species (a Lewis acid), the nitrogen lone pair is donated to form a new bond ($R_3N: + H^+ \rightarrow R_3NH^+$), which is precisely the definition of basic behaviour (a Bronsted base accepts a proton; a Lewis base donates an electron pair). The tetrahedral (pyramidal) structure of amines and nitrogen's electronegativity do not by themselves confer basicity - it is specifically the availability of this non-bonding lone pair that makes the nitrogen atom nucleophilic/basic.