JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Practical and Analytical Chemistry

Kjeldahl's method cannot be used to estimate nitrogen for which of the following compounds?

(A)(see structure)
(B) $C X_{6} H X_{5} NH X_{2}$
(C) $C X_{6} H X_{5} NO X_{2}$ ✓ Correct
(D) $CH X_{3} CH X_{2} - C \equiv N$

Answer: (C)

Solution

Kjeldahl's method works only when the nitrogen can be turned into ammonium sulphate on digestion with hot concentrated

H X_{2} SO X_{4}

. Nitrogen held in a nitro group, an azo group or an aromatic ring is not reduced to ammonia this way. Nitrobenzene,

C X_{6} H X_{5} NO X_{2}

, carries its nitrogen in a nitro group, so its nitrogen escapes estimation, whereas aniline, the nitrile and urea all surrender their nitrogen as ammonia and can be estimated.

JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Single correct option

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