Reaction of Amines with Nitrous Acid

Nitrous acid ($\text{HNO}_2$), always generated in situ from $\text{NaNO}_2$ and a

mineral acid ($\text{HCl}$ or $\text{H}_2\text{SO}_4$) because it is unstable when

isolated, reacts with each class of amine in a chemically distinct way -- making this

reaction a powerful diagnostic for distinguishing $1^\circ$, $2^\circ$ and $3^\circ$

amines, and for distinguishing aliphatic from aromatic amines within each class.

Primary aliphatic amine. Diazotization gives an aliphatic diazonium salt,

$\text{R}-\text{N}_2^+$, but unlike its aromatic counterpart this species is extremely

unstable even in the cold: it decomposes essentially instantly, losing nitrogen gas

(observed as brisk effervescence) and giving a mixture of products dominated by the

alcohol formed via carbocation capture by water, $\text{R}-\text{OH}$ (plus some alkene

and alkyl halide from the same unstable carbocation intermediate). Because it cannot be

isolated or even meaningfully observed as a discrete species, this reaction is

synthetically far less useful than the aromatic case.

Primary aromatic amine. Diazotization at $0$–$5^\circ\text{C}$ gives a STABLE

arenediazonium salt, $\text{Ar}-\text{N}_2^+$ (Section 25.14) -- stable specifically

because the positive charge on the terminal nitrogen can be delocalised into the

aromatic ring by resonance, an option simply not available to an aliphatic diazonium ion.

This stability is precisely what makes the aromatic diazonium salt so synthetically

valuable (Sections 25.15–25.17).

Secondary amine (aliphatic or aromatic). With only ONE hydrogen left on nitrogen,

a secondary amine cannot form a diazonium-type species at all; instead nitrous acid

$N$-nitrosates the nitrogen directly, giving a yellow, oily $N$-nitrosamine,

$\text{R}_2\text{N}-\text{N}=\text{O}$ (for $N$-methylaniline: $\text{C}_6\text{H}_5$

$\text{N}(\text{CH}_3)-\text{NO}$), with no loss of nitrogen gas.

Tertiary amine. A tertiary ALIPHATIC amine has no $\text{N}-\text{H}$ to lose at all, …