Question

Identify A and B in each of the following reaction sequence:
(a) \[ CH_3CH_2Cl \xrightarrow{NaCN} A \xrightarrow{H_2/Ni} B \]
(b) \[ C_6H_5NH_2 \xrightarrow{NaNO_2/HCl} A \xrightarrow{C_6H_5NH_2} B \]

Hint:

Nucleophilic substitution with cyanide forms nitriles, which can be reduced to amines. Diazonium salts are highly reactive intermediates that can be converted to phenols by hydrolysis.

Answer 1

1. Identify A and B in the reaction sequence:

(a) Reaction:
CH₃CH₂Cl + NaCN → A → H₂/Ni → B

Solution:
In the first step, NaCN reacts with CH₃CH₂Cl (ethyl chloride) in a nucleophilic substitution reaction to form a nitrile (A). The reaction proceeds as follows:

CH₃CH₂Cl + NaCN → CH₃CH₂CN (ethyl cyanide, A)

In the second step, the nitrile group is hydrogenated using H₂ in the presence of a nickel catalyst to form an amine (B). The reaction proceeds as:

CH₃CH₂CN + H₂/Ni → CH₃CH₂NH₂ (ethylamine, B)

Identified compounds:
A = CH₃CH₂CN (ethyl cyanide), B = CH₃CH₂NH₂ (ethylamine)

(b) Reaction:
C₆H₅NH₂ + NaNO₂/HCl (0–5°C) → A → C₆H₅NH₂ + H⁺ → B

Solution:
In the first step, aniline (C₆H₅NH₂) reacts with sodium nitrite (NaNO₂) in the presence of hydrochloric acid (HCl) at 0-5°C to form a diazonium salt (A). The reaction proceeds as follows:

C₆H₅NH₂ + NaNO₂/HCl → C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride, A)

In the second step, the diazonium salt is reduced by adding a reducing agent like H⁺ to form aniline (C₆H₅NH₂) again, which is compound B. The reaction proceeds as:

C₆H₅N₂⁺Cl⁻ + H⁺ → C₆H₅NH₂ (aniline, B)

Identified compounds:
A = C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride), B = C₆H₅NH₂ (aniline)