Nitrogen can be estimated by Kjeldahl's method for which of the following compounds?

Step 1: Recall Limitations of Kjeldahl's Method

Kjeldahl's method CANNOT estimate nitrogen in compounds where nitrogen is:

• In a nitro group ($\ce{-NO2}$): too oxidised, does not convert to $\ce{(NH4)2SO4}$
• In a nitrile group ($\ce{-C≡N}$): ring-strain nitrile nitrogen doesn't convert well
• In an azo group ($\ce{-N=N-}$): N–N bonds are not cleaved under Kjeldahl conditions
• In heteroaromatic rings (pyridine, quinoline): aromatic C–N bonds too stable
• In diazonium salts: unstable and reactive differently

Kjeldahl's method WORKS for:

• Amines ($\ce{-NH2}$, $\ce{-NHR}$, $\ce{-NR2}$)
• Amides ($\ce{-CONH2}$, $\ce{-CONHR}$)
• Amino acids

Step 2: Evaluate Each Compound

(a) Nitrobenzene $\ce{C6H5NO2}$ — Contains $\ce{-NO2}$ group → Kjeldahl's method FAILS ❌

(b) Benzylamine $\ce{C6H5CH2NH2}$ — Contains $\ce{-NH2}$ (primary amine) → Kjeldahl's method WORKS ✅

(c) Pyridine — N in heteroaromatic ring → Kjeldahl's method FAILS ❌

(d) Azobenzene $\ce{C6H5-N=N-C6H5}$ — Contains $\ce{-N=N-}$ azo group → Kjeldahl's method FAILS ❌

Step 3: Conclusion

Answer: (b) Benzylamine — only this compound has nitrogen in a form (amine) that Kjeldahl's method can estimate.

Key Points to Remember:

• Kjeldahl works for: amines, amides, amino acids, proteins
• Kjeldahl FAILS for: $\ce{-NO2}$, $\ce{-N=N-}$, $\ce{-C≡N}$, aromatic ring N (pyridine)
• Benzylamine: $\ce{C6H5CH2NH2}$ — primary aliphatic amine attached to benzyl group
• Mnemonic for Kjeldahl failures: "NAP" — Nitro, Azo, Pyridine-type