Removing the amino group (H₃PO₂): practice and mechanism
First NaNO₂/acid at 0 °C turns the amino group into a diazonium salt (Ar–N₂⁺). N₂ is an excellent leaving group. H₃PO₂ replaces the N₂ group by H. This removes an amino group after it has done its directing job.
Example
Hypophosphorous acid reduces the diazonium salt and the group is replaced by hydrogen. Useful in synthesis: the NH₂ group first directs other substituents (o/p) and is removed afterwards.
Mechanism step by step
- 1
NaNO₂ and acid form the nitrosyl cation (NO⁺) at 0 °C. The amine attacks it; after proton shifts and loss of water the aryl diazonium ion Ar–N≡N⁺ forms.
- 2
Hypophosphorous acid transfers an electron, N₂ escapes and an aryl radical forms.
- 3
The radical takes an H atom from H₃PO₂: the former amino position now carries an H.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Phenol from a diazonium salt · Sandmeyer reaction (chloride)
Frequently asked questions
Why work at 0 °C?
Aryl diazonium salts decompose on warming (N₂ escapes) and can even explode when dry. In the cold they stay stable in solution long enough for the second step.
Why remove an amino group?
The NH₂ group is a strong o/p director. It is used to steer other substituents to particular positions (e.g. 1,3,5-tribromobenzene from aniline) and removed afterwards.