MP Board · Class 12 · Chemistry · AminesDiscuss the basic strength of amines in aqueous solution. Compare the basicity of aliphatic amines and ammonia, and explain the combined effect of inductive effect, steric hindrance, and hydration effect on the basicity of methyl-substituted and ethyl-substituted amines.

Basic Strength of Amines in Aqueous Solution

\nAmines act as Lewis bases because they contain a nitrogen atom with an unshared pair of electrons, which can be donated to an acid. In aqueous solutions, the basic strength of amines is determined by the ease with which they accept a proton from water to form substituted ammonium cations.

1. Basicity Order of Aliphatic Amines vs. Ammonia\nAliphatic amines are stronger bases than ammonia. This is due to the electron-releasing inductive effect ($+I$ effect) of the alkyl groups. The alkyl groups push electrons towards the nitrogen atom, increasing the electron density on the nitrogen, making the lone pair more readily available for sharing with a proton.

2. Factors Governing Basicity in Aqueous Medium\nThe basic strength in aqueous solution depends upon the simultaneous action of three main factors:

• Inductive Effect ($+I$ effect): Greater number of alkyl groups increases the electron density on nitrogen, thereby increasing basicity ($3^\circ > 2^\circ > 1^\circ$).
• Hydration Effect (Solvation stabilization): The substituted ammonium cations formed after accepting a proton get stabilized by hydrogen bonding with water molecules. Greater the stability of the ammonium cation (more hydrogen bonding), stronger is the base. The extent of hydration decreases with an increase in the size of the alkyl group ($1^\circ > 2^\circ > 3^\circ$).
• Steric Hindrance: Bulky alkyl groups hinder the approach of a proton to the nitrogen atom and also crowd the ammonium ion, decreasing its stability ($1^\circ > 2^\circ > 3^\circ$).

3. Combined Order of Basicity\nTaking all these factors into account, the experimental base strength order in aqueous medium is found to be:

• For Methyl-substituted amines: Secondary ($2^\circ$) > Primary ($1^\circ$) > Tertiary ($3^\circ$); i.e., $(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N}$.
• For Ethyl-substituted amines: Secondary ($2^\circ$) > Tertiary ($3^\circ$) > Primary ($1^\circ$); i.e., $(\text{C}_2\text{H}_5)_2\text{NH} > (\text{C}_2\text{H}_5)_3\text{N} > \text{C}_2\text{H}_5\text{NH}_2$. \nThus, secondary amines generally exhibit maximum basicity due to the optimal balance between inductive, steric, and hydration effects.