Question

The Kjeldahl method of Nitrogen estimation fails for which of the following reaction products?
(a) nitrobenzene with Sn/HCl
(b) benzonitrile (C₆H₅CN) with LiAlH₄
(c) benzyl cyanide (C₆H₅CH₂CN) with (i) SnCl₂ + HCl, (ii) H₂O
(d) aniline with NaNO₂/HCl

• A. (a), (c) and (d)
• B. (b) and (c)
• C. (c) and (d)
• D. (a) and (d)

Hint:

Kjeldahl fails for nitro, azo and diazonium compounds and when no nitrogen is left in the product; find which products fall in these classes.

Answer 1

The Correct Option is C

Kjeldahl digestion converts amino/amide-type nitrogen to (NH₄)₂SO₄; it fails for nitro, azo, diazo/diazonium compounds and for nitrogen in a ring.
(a) Nitrobenzene + Sn/HCl gives aniline (N as -NH₂): Kjeldahl works.
(b) Benzonitrile + LiAlH₄ gives benzylamine, C₆H₅CH₂NH₂: Kjeldahl works.
(c) Stephen reduction of benzyl cyanide gives phenylacetaldehyde, which contains no nitrogen, so there is nothing to estimate: fails.
(d) Aniline + NaNO₂/HCl gives benzenediazonium chloride, a diazo compound: Kjeldahl fails.
Hence it fails for (c) and (d).
Answer: (3) (c) and (d)