Chemistry · Amines · NEET
Two reagents do this job. Hypophosphorous acid (H3PO2, also written as phosphinic acid) is the common one. Ethanol (C2H5OH) also works. Both act as reducing agents: they give a hydrogen atom to the ring carbon while the -N2+ group leaves as N2 gas. Reaction: Ar-N2+ Cl- + H3PO2 + H2O gives Ar-H + N2 + H3PO3 + HCl.
Because the full journey started from an aromatic amine (Ar-NH2). You first turned the amine into the diazonium salt (Ar-N2+) using NaNO2/HCl at 273-278 K, then you remove that nitrogen as N2. Overall the -NH2 group has been taken off the ring and replaced by -H. Removing the amino group is called deamination, even though the actual step uses the diazonium salt.
Deamination gives the plain hydrocarbon (Ar-H), for example aniline gives benzene. You get phenol (Ar-OH) only when you warm the diazonium salt with water above 283 K. So H3PO2 or ethanol gives -H (benzene), while warm water gives -OH (phenol). Do not mix these two up in NEET.
The -NH2 group is used only as a temporary handle to direct other groups to the correct ring position, then it is taken off. For example, to put a substituent at a spot that is hard to reach directly, you use the amine to steer the reaction, and after the target group is placed you deaminate to leave -H there. This is the main use of deamination in NEET multi-step synthesis.
Both replace -N2+ by -H, but with ethanol a side reaction can also give an aryl ethyl ether (Ar-O-C2H5) in some cases, while H3PO2 gives a cleaner Ar-H. For NEET, remember both give the hydrogen replacement; H3PO2 is the preferred and safer answer when only one option is asked.
Choose the correct sequence of reagents in the conversion of 4-nitrotoluene to 2-bromotoluene.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Benzene (C6H6) is formed, along with N2 gas. The -N2+ group is replaced by -H.
The diazonium salt is made cold at 273-278 K. The deamination step with H3PO2 or ethanol is then carried out; the key point for NEET is the reagent, not a special temperature like phenol formation.
Yes. H3PO2 and ethanol act as reducing agents. They supply a hydrogen atom to the ring and are themselves oxidised (H3PO2 becomes H3PO3).
No. Aliphatic diazonium salts are unstable and decompose instantly, so the clean aromatic deamination route does not apply to them. Deamination via H3PO2 is used for aromatic diazonium salts.
It lets you use the -NH2 group as a temporary position-director, then remove it. Many multi-step conversion problems end with H3PO2 to strip off the nitrogen and leave -H at that carbon.
