Write the reactions of Aromatic Primary - aliphatic secondary and tertiary amines with 2 N HCl and 5% NaNO2 in an ice bath at 5 °C.

Answer 1

First, what the reagents actually are. You are not adding HNO2 from a bottle - it is unstable, so you generate it in the flask:

NaNO2 + HCl -> HNO2 + NaCl
HNO2 + H+ -> NO+ (nitrosonium) + H2O

NO+ is the real electrophile. Each amine class then behaves differently because of how many N-H bonds it has.

1) AROMATIC PRIMARY (aniline, ArNH2) - diazotization:
ArNH2 + NaNO2 + 2 HCl -> ArN2+ Cl- + NaCl + 2 H2O
You get a benzenediazonium chloride. The nitrogen attacks NO+, then a sequence of proton transfers and loss of water gives the diazonium ion. Clear solution, no gas evolved.

2) SECONDARY (R2NH) - N-nitrosation:
R2NH + HNO2 -> R2N-N=O + H2O
The nitrogen has only one H to lose, so it stops at the N-nitrosamine. These separate as a yellow oil. Handle with care - N-nitrosamines are potent carcinogens.

3) ALIPHATIC TERTIARY (R3N) - no reaction of note:
R3N + HCl -> R3NH+ Cl-
There is no N-H at all, so there is no proton to lose and no stable nitroso product. The amine just dissolves as its ammonium salt. No oil, no gas.

WHY THE ICE BATH - this is the part the question is really testing. Aryl diazonium salts are only stable near 0-5 C. Warm the flask past about 5 C and it decomposes:
ArN2+ + H2O -> ArOH + N2 + H+
You lose your diazonium to phenol and bubbles of nitrogen. Since diazonium salts are the gateway to Sandmeyer reactions and azo dyes, you keep them cold until you use them.

Worth knowing for contrast: aliphatic primary amines also form diazonium ions, but alkyl diazonium salts are unstable even at 0 C. They lose N2 immediately to give a carbocation, so you get a messy mixture of alcohols and alkenes with vigorous gas evolution. Aryl works and alkyl does not because the aryl diazonium is stabilized by the ring.

As a lab test you can tell all three apart by eye: clear solution with no gas is 1 aromatic, a yellow oil is secondary, and quiet dissolution with nothing else happening is 3 aliphatic. Adding beta-naphthol to the first one gives a bright orange-red azo dye, which confirms the diazonium.