Consider the following statements about the reactions of amines with nitrous acid.
(A) The nitrous acid used in these reactions is prepared in situ from a mineral acid and sodium nitrite.
(B) Primary aliphatic amines react with nitrous acid to give alcohols with the evolution of ammonia gas.
(C) The quantitative evolution of nitrogen from primary aliphatic amines with nitrous acid is used in the estimation of amino acids and proteins.
(D) Secondary and tertiary amines react with nitrous acid in the same manner as primary aliphatic amines.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
Nitrous acid is unstable, so it is made in the reaction mixture itself. The three classes of amines react with it differently; a primary aliphatic amine gives an unstable diazonium salt that breaks down to an alcohol and nitrogen gas:
\( \displaystyle \ce{NaNO2 + HCl -> HNO2 + NaCl} \)
\( \displaystyle \ce{R-NH2} \xrightarrow{\ce{NaNO2 + HCl}} \ce{[R-N2^+Cl^-]} \xrightarrow{\ce{H2O}} \ce{R-OH + N2 ^ + HCl} \)
(A) The nitrous acid used in these reactions is prepared in situ from a mineral acid and sodium nitrite. — Correct statement. It is made in situ, as shown in the first equation.
(B) Primary aliphatic amines react with nitrous acid to give alcohols with the evolution of ammonia gas. — Incorrect statement. The gas given off is nitrogen, not ammonia; both nitrogen atoms of the diazonium group leave as \( \ce{N2} \).
(C) The quantitative evolution of nitrogen from primary aliphatic amines with nitrous acid is used in the estimation of amino acids and proteins. — Correct statement. The nitrogen is given off quantitatively, so measuring it gives the amount of –NH2 groups.
(D) Secondary and tertiary amines react with nitrous acid in the same manner as primary aliphatic amines. — Incorrect statement. Secondary and tertiary amines react in a different manner; this difference is why the three classes behave differently with nitrous acid.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “Three classes of amines react differently with nitrous acid which is prepared in situ from a mineral acid and sodium nitrite.”
“(a) Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberate nitrogen gas quantitatively and alcohols. Quantitative evolution of nitrogen is used in estimation of amino acids and proteins.”
“Secondary and tertiary amines react with nitrous acid in a different manner.”