CBSE · Class 12 · Chemistry · AminesExplain the basic strength of amines in aqueous solution. Compare the basicities of primary, secondary, and tertiary aliphatic amines with methyl and ethyl substituents, and account for the observed order.

Introduction\nAmines behave as organic bases due to the presence of an unshared pair of electrons on the nitrogen atom, which allows them to accept a proton. In the gaseous phase, the basic strength of aliphatic amines follows the inductive effect order: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$) > Ammonia. However, in an aqueous solution, the situation is more complex due to the interplay of three distinct factors.

Factors Affecting Basicity in Aqueous Solution

1. Inductive Effect (+I Effect): Alkyl groups are electron-releasing groups. More the number of alkyl groups attached to nitrogen, higher is the electron density on nitrogen, making it easier to donate the lone pair and thus increasing basic strength. This factor alone favors the order: $3^\circ > 2^\circ > 1^\circ > NH_3$.
2. Solvation Effect (Hydration): The substituted ammonium cations formed after accepting a proton get stabilized by hydrogen bonding with water molecules. Greater the stability of the substituted ammonium cation through solvation, stronger is the corresponding amine. The extent of stabilization decreases with an increase in the size of the alkyl group: $1^\circ > 2^\circ > 3^\circ$.
3. Steric Hindrance: Bulky alkyl groups sterically crowd the nitrogen atom, hindering the approach and attack of a proton as well as preventing effective solvation of the resulting cation. This steric hindrance is maximum in tertiary amines and minimum in primary amines.

Basicity Order for Methyl Substituted Amines\nCombining the inductive, solvation, and steric effects for methylamines in water, the secondary amine is the strongest base because it achieves an optimal balance between +I effect and steric/solvation constraints.

• Order: Secondary ($2^\circ$) > Primary ($1^\circ$) > Tertiary ($3^\circ$) > Ammonia
• Specifically: $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$

Basicity Order for Ethyl Substituted Amines\nIn the case of ethyl substituted amines, the bulkier ethyl group introduces greater steric hindrance around the nitrogen atom in tertiary amines, affecting their hydration significantly.

• Order: Secondary ($2^\circ$) > Tertiary ($3^\circ$) > Primary ($1^\circ$) > Ammonia
• Specifically: $(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$\nThis clearly demonstrates that both electronic and spatial factors dictate the actual basic behavior of amines in aqueous media.