Key Points

Amines are derivatives of ammonia ($NH_3$) with a pyramidal structure and $sp^3$ hybridization. They are classified as primary ($R-NH_2$), secondary ($R_2NH$), and tertiary ($R_3N$) based on the number of alkyl or aryl groups attached to the nitrogen atom.

Amines act as Lewis bases due to the lone pair of electrons on the nitrogen atom. Aliphatic amines are generally stronger bases than ammonia due to the electron-releasing (+I) effect of alkyl groups, while aromatic amines are weaker because the lone pair is delocalized into the benzene ring by resonance.

The basicity of alkylamines in aqueous solution is determined by a combination of the inductive effect, solvation effect, and steric hindrance. For ethyl substituted amines, the order of basic strength is $(\text{C}_2\text{H}_5)_2\text{NH} > (\text{C}_2\text{H}_5)_3\text{N} > \text{C}_2\text{H}_5\text{NH}_2 > \text{NH}_3$.

This reaction converts a primary amide into a primary amine containing one carbon atom less than the amide. The general reaction is $R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$.

This method is used for the preparation of pure primary aliphatic amines. Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

Primary amines (both aliphatic and aromatic) when heated with chloroform ($CHCl_3$) and ethanolic potassium hydroxide (KOH) form foul-smelling isocyanides or carbylamines. This reaction is a characteristic test for primary amines.

This test distinguishes primary, secondary, and tertiary amines using benzenesulphonyl chloride ($C_6H_5SO_2Cl$). Primary amines form a sulphonamide soluble in alkali, secondary amines form an insoluble sulphonamide, and tertiary amines do not react.

Primary aliphatic amines react with nitrous acid ($HNO_2$) to form unstable diazonium salts which liberate nitrogen gas and form alcohols. Primary aromatic amines react at low temperatures (273-278 K) to form stable arenediazonium salts.

The $-NH_2$ group is a powerful activating, ortho- and para-directing group. Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.

To prevent polysubstitution and oxidation, the high reactivity of aniline is controlled by protecting the amino group via acetylation with acetic anhydride. The resulting acetanilide ($C_6H_5NHCOCH_3$) is less activating, allowing for controlled monosubstitution.

The process of converting a primary aromatic amine into a diazonium salt by treating it with nitrous acid ($NaNO_2 + \text{dil. } HCl$) at a low temperature (273-278 K) is called diazotisation. The general reaction is $Ar-NH_2 + NaNO_2 + 2HCl \rightarrow Ar-N_2^+Cl^- + NaCl + 2H_2O$.

This reaction is used to replace the diazonium group ($-N_2^+$) with Cl, Br, or CN by treating the diazonium salt solution with the corresponding cuprous(I) salt. For example, $ArN_2^+Cl^- \xrightarrow{CuCl/HCl} ArCl + N_2$.

This reaction is a modification of the Sandmeyer reaction where the diazonium group is replaced by Cl or Br using copper powder and the corresponding halogen acid. The yield is generally lower than in the Sandmeyer reaction.

Arenediazonium salts react with electron-rich aromatic compounds like phenols or anilines to form brightly colored azo compounds containing the azo linkage ($-N=N-$). This electrophilic substitution reaction is the basis for the formation of azo dyes.

Diazonium salts are versatile intermediates in organic synthesis. They allow the introduction of functional groups like F, Cl, Br, I, CN, OH, and $NO_2$ into the aromatic ring, which are often difficult to introduce by direct substitution.