Reaction with Nitrous Acid ($$\text{HNO}_2$$)

Reaction

Primary Aliphatic Amines ($$1^\circ$$ Aliphatic Amine):
$$\text{R--NH}_2 + \text{HNO}_2 \xrightarrow{\text{NaNO}_2 + \text{HCl}, \, 273\text{--}278\text{ K}} [\text{R--N}_2^+\text{Cl}^-] \xrightarrow[\text{Decomposition}]{\text{H}_2\text{O}} \text{R--OH} + \text{N}_2\uparrow + \text{HCl}$$
Notation: Unstable diazonium salt releases $$N_2$$ gas quantitatively; alcohol is the major organic product alongside secondary alkenes/alkyl halides

Primary Aromatic Amines ($$1^\circ$$ Aromatic Amine / Diazotization):
$$\text{Ar--NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{273\text{--}278\text{ K } (0\text{--}5^\circ\text{C})} \text{Ar--N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}$$
Notation: Benzenediazonium chloride is stable only below $$5^\circ\text{C}$$ warming above $$5^\circ\text{C}$$ hydrolyzes it to phenol: $$Ar-OH$$ + $$\text{N}_2\uparrow$$ + $$HCl$$

Secondary Amines ($$2^\circ$$ Aliphatic & Aromatic Amines):
$$\text{R}_2\text{NH} + \text{HNO}_2 \xrightarrow{273\text{--}278\text{ K}} \text{R}_2\text{N--N=O} + \text{H}_2\text{O} \quad (\text{N-Nitrosamine})$$
$$Notation: Forms a yellow, water-insoluble oily liquid known as N-nitrosamine

Tertiary Amines ($$3^\circ$$ Amines):
$$\text{Aliphatic: }$$

$$\text{R}_3\text{N} + \text{HNO}_2 \xrightarrow{273\text{--}278\text{ K}} [\text{R}_3\text{NH}]^+\text{NO}_2^- \quad (\text{Water-Soluble})$$

$$\text{Aromatic: }$$

$$\text{Ar--NR}_2 + \text{HNO}_2 \xrightarrow{273\text{--}278\text{ K}} p\text{-Nitroso--Ar--NR}_2 + \text{H}_2\text{O} \quad (\text{Green Crystalline Solid via Electrophilic Substitution})$$

Notation:

• $$\text{N}_2 \text{ evolution}$$ = Quantitative indicator for primary aliphatic amines;
• $$273-278\text{ K}$$ = Strictly controlled ice-cold conditions ($$0-5^\circ\text{C}$$ ) necessary to preserve the diazonium salt structure.