MP Board · Class 12 · Chemistry · AminesDiscuss the basic strength of aliphatic and aromatic amines in aqueous phase. Explain how the nature of substituents affects their basicity with appropriate examples. Also, write the chemical reactions of benzenediazonium chloride with (i) $\text{H}3\text{PO}2$ and (ii) $\text{CuCN/KCN}$.

Basic Strength of Amines in Aqueous Phase\nAmines act as Lewis bases due to the presence of an unshared pair of electrons on the nitrogen atom. In the gaseous phase, the basic strength of aliphatic amines increases with an increase in the number of alkyl groups due to the electron-releasing inductive effect (+I effect) of alkyl groups. However, in the aqueous phase, the basic strength depends upon three main factors:

1. Inductive Effect (+I Effect): Alkyl groups release electrons, increasing electron density on nitrogen and hence increasing basicity.
2. Solvation Effect: Protonated amines form hydrogen bonds with water molecules. Greater stabilization through hydration (solvation) increases basic strength. Smaller cations get more hydrated.
3. Steric Hindrance: Bulky alkyl groups hinder the approach and hydration of the protonated amine, decreasing basicity. \nCombining these factors, the order of basicity for methyl-substituted amines in aqueous solution is: $(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N}$\nFor ethyl-substituted amines, the order is: $(\text{C}_2\text{H}_5)_2\text{NH} > (\text{C}_2\text{H}_5)_3\text{N} > \text{C}_2\text{H}_5\text{NH}_2$

Basicity of Aromatic Amines\nAromatic amines like aniline are much weaker bases than ammonia and aliphatic amines. This is because:

• In aniline, the lone pair of electrons on the nitrogen atom is delocalized over the benzene ring through resonance.
• As a result, the electron density on nitrogen decreases, making it less available for protonation.
• Furthermore, the anilinium ion formed after accepting a proton is less stable relative to neutral aniline due to resonance constraints compared to aliphatic systems.

Chemical Reactions of Benzenediazonium Chloride

(i) Reaction with $\text{H}_3\text{PO}_2$ (Hypophosphorous acid):\nBenzenediazonium chloride reacts with hypophosphorous acid in the presence of copper catalysts to form benzene along with the reduction of diazonium group. $$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{H}_3\text{PO}_2 + \text{H}_2\text{O} \xrightarrow{\text{Cu}^+} \text{C}_6\text{H}_6 + \text{N}_2 + \text{H}_3\text{PO}_3 + \text{HCl}$$

(ii) Reaction with $\text{CuCN/KCN}$ (Sandmeyer Reaction):\nBenzenediazonium chloride reacts with cuprous cyanide in the presence of potassium cyanide to yield benzonitrile. $$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{KCN} \xrightarrow{\text{CuCN}} \text{C}_6\text{H}_5\text{CN} + \text{KCl} + \text{N}_2$$