Why Diazonium Salts Must Be Kept Ice-Cold

Aniline treated with nitrous acid in an ice bath becomes benzenediazonium chloride, a salt so reactive that it must be used almost at once. That reactivity is its value: the diazonium group can be swapped for a halogen, a cyano group, an -OH group or hydrogen, or joined to another ring to make brightly coloured azo dyes. Nitrogen also appears in cyanides and isocyanides, two families with the same formula but very different properties.

This lesson covers the preparation and synthetic importance of diazonium salts, including coupling reactions, and the differences between cyanides and isocyanides.

Diazonium salts are made by treating a primary aromatic amine with nitrous acid at 273-278 K, and they are valuable because the diazonium group leaves easily as nitrogen gas, letting it be replaced by groups that are hard to attach to benzene directly, or kept in coupling reactions that make azo dyes.

Diazotisation:

$${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{H}}_2+\mathrm{NaN}{\mathrm{O}}_2+2HCl\to {\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}\mathrm{C}{\mathrm{l}}^{-}+\mathrm{NaCl}+2{\mathrm{H}}_2\mathrm{O}$$

- Nitrous acid forms in the solution from sodium nitrite and hydrochloric acid
- The low temperature is essential, because the salt decomposes on warming
- Aromatic diazonium ions are stabilised by resonance with the ring; aliphatic ones decompose at once

Replacement, with loss of nitrogen:

- Sandmeyer reaction — copper(I) chloride, bromide or cyanide gives chlorobenzene, bromobenzene or benzonitrile: ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}\mathrm{C}{\mathrm{l}}^{-}\stackrel{\mathrm{CuCl}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{N}}_2$
- Gattermann reaction — copper powder with HCl or HBr gives the same halobenzenes
- Potassium iodide gives iodobenzene without any catalyst
- Fluoroboric acid, then heating, gives fluorobenzene
- Warm water gives phenol
- Hypophosphorous acid or ethanol replaces the group with hydrogen, giving benzene

Retention of nitrogen — coupling reactions:

- The diazonium ion is a weak electrophile that attacks strongly activated rings, usually at the para position
- With phenol in alkaline solution, it gives orange 4-hydroxyazobenzene
- With aniline in mildly acidic solution, it gives yellow 4-aminoazobenzene
- The $-\mathrm{N}=\mathrm{N}-$ azo link joins two rings into an extended conjugated system that absorbs visible light

Worked example — 1,3,5-tribromobenzene. Brominate aniline to 2,4,6-tribromoaniline, diazotise it, then replace the diazonium group with hydrogen using ${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_2$ — a substitution pattern that direct bromination of benzene cannot give.

An everyday example. Many bright yellow, orange and red dyes used on Indian textiles are azo dyes, made by coupling diazonium salts with phenols or amines.

The substance. A diazonium salt places groups where direct substitution cannot — fluorine, iodine, -CN and -OH are hard or impossible to attach to benzene directly.

**Cyanides, or nitriles, $\mathrm{R}-\mathrm{C}\equiv \mathrm{N}$, have the alkyl group bonded to carbon, while isocyanides, $\mathrm{R}-\mathrm{N}\equiv \mathrm{C}$, have it bonded to nitrogen; nitriles hydrolyse to carboxylic acids and reduce to primary amines, whereas isocyanides hydrolyse to primary amines and reduce to secondary amines.

Structure:

- Nitrile — the alkyl group is attached to the carbon of the carbon-nitrogen triple bond
- Isocyanide — the alkyl group is attached to nitrogen, which carries a formal positive charge, while carbon carries a formal negative charge
- The cyanide ion is an ambidentate nucleophile that can attack through carbon or nitrogen

Preparation:

- Alkyl halides with KCN give mainly nitriles**, because the free cyanide ion attacks through carbon: $\mathrm{RX}+\mathrm{KCN}\to \mathrm{RCN}+\mathrm{KX}$
- Alkyl halides with AgCN give mainly isocyanides, because the covalent silver-carbon bond leaves only nitrogen free to attack: $\mathrm{RX}+\mathrm{AgCN}\to \mathrm{RNC}+\mathrm{AgX}$
- Primary amines with chloroform and alcoholic KOH give isocyanides
- Amides dehydrated with ${\mathrm{P}}_2{\mathrm{O}}_5$ give nitriles

Properties and reactions:

- Smell — nitriles smell pleasant; isocyanides smell very unpleasant
- Polarity — nitriles are more polar, with higher boiling points
- Hydrolysis — nitriles give carboxylic acids, $\mathrm{RCN}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{RCOOH}+\mathrm{N}{\mathrm{H}}_3$, while isocyanides give primary amines and methanoic acid, $\mathrm{RNC}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{RN}{\mathrm{H}}_2+\mathrm{HCOOH}$
- Reduction — nitriles give $\mathrm{RC}{\mathrm{H}}_2\mathrm{N}{\mathrm{H}}_2$; isocyanides give secondary amines, $\mathrm{RNHC}{\mathrm{H}}_3$
- Heating — isocyanides rearrange into the more stable nitriles

Worked example — one halide, two products. Bromoethane with KCN gives propanenitrile, which hydrolyses to propanoic acid; with AgCN it gives ethyl isocyanide, which hydrolyses to ethanamine and methanoic acid.

An everyday example. Acrylic fibres in winter sweaters sold across North India are made from acrylonitrile, a nitrile whose polar carbon-nitrogen triple bonds help give the fibres their strength.

The substance. The metal decides which end of cyanide attacks — the free ion from KCN uses its carbon, while AgCN, with carbon already bonded to silver, attacks through nitrogen.