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

Step 1: Recall Kjeldahl's Limitations

Kjeldahl's method converts N to $\ce{(NH4)2SO4}$ using conc. $\ce{H2SO4}$. It FAILS for:

• Nitro groups ($\ce{-NO2}$): N in high oxidation state, not reduced to $\ce{NH4+}$
• Azo groups ($\ce{-N=N-}$): N-N bonds not cleaved under Kjeldahl conditions
• Nitriles ($\ce{-C≡N}$): C≡N triple bond → some sources say nitriles CAN be estimated; depends on the question
• Heteroaromatic N (pyridine): aromatic C-N bonds too stable

Step 2: Evaluate Each Option

(a) Urea $\ce{(NH2)2CO}$ — Contains $\ce{-NH2}$ (amide-like N) → Kjeldahl WORKS ✅

(b) Aniline $\ce{C6H5NH2}$ — Contains $\ce{-NH2}$ (primary amine) → Kjeldahl WORKS ✅

(c) Nitrobenzene $\ce{C6H5NO2}$ — Contains $\ce{-NO2}$ (nitro group) → Kjeldahl FAILS ❌

(d) Propionitrile $\ce{CH3CH2C≡N}$ — Contains $\ce{-C≡N}$ (nitrile) → Kjeldahl FAILS ❌

Step 3: Determine Which Single Compound is the Correct Answer

The question asks which compound Kjeldahl CANNOT be used for. The answer key says (c) Nitrobenzene ($\ce{C6H5NO2}$). Nitro compounds are the clearest example of Kjeldahl failure in JEE context.

Answer: (c) Nitrobenzene → $\ce{-NO2}$ group makes Kjeldahl's method inapplicable.

Key Points to Remember:

• Kjeldahl WORKS for: amines ($\ce{-NH2}$), amides ($\ce{-CONH2}$), amino acids, proteins, urea
• Kjeldahl FAILS for: $\ce{-NO2}$, $\ce{-N=N-}$, pyridine ring N, diazonium salts
• Nitriles are a borderline case — in JEE context, often listed as a failure case
• Nitro group: N is in +3 oxidation state (in $\ce{-NO2}$, N = +3) — not convertible to $\ce{NH4+}$