Question

In how many of the following molecules, nitrogen can be estimated completely by using Kjeldhal method?

• A. 2
• B. 3
• C. 4
• D. 5

Answer 1

Answer: (C)

4 molecules

Answer 2

Solution:

1. Molecule 1: Contains two –NH₂ groups bonded to a carbonyl carbon (a urea‐like moiety). Under Kjeldahl digestion, amide–N (as in urea) is converted to NH₄⁺.
2. Molecule 2: Benzylamine (–CH₂NH₂) is an aliphatic primary amine; it is fully converted to ammonium.
3. Molecule 3: The nitrogen is part of a pyridine ring. Aromatic ring‐nitrogen (as in pyridine) is resistant to complete conversion under standard Kjeldahl conditions.
4. Molecule 4: Contains a diazo (–N=N–) linkage; such nitrogen is not converted to ammonia.
5. Molecule 5: Has two nitrogen atoms—a pyridine nitrogen (not completely digested) and an aliphatic methylamino group. Since one nitrogen remains unconverted, the total nitrogen is not estimated completely.
6. Molecule 6: The –NO₂ (nitro group) does not yield NH₄⁺ in Kjeldahl digestion.
7. Molecule 7: Although it has an –NH₂ group attached, its pyrrole ring nitrogen (due to aromatic stabilization) is not fully converted.
8. Molecule 8: An aliphatic amine (N-cyclohexylmethylamine) is completely digested.
9. Molecule 9: Cyclohexylmethylamine (–CH₂NH₂) is also completely digested.

Thus, only molecules 1, 2, 8, and 9 produce ammonia quantitatively on digestion.

Quick Core Explanation:

• Molecules with aliphatic amine or urea (amide) groups (1, 2, 8, 9) yield NH₄⁺ completely under Kjeldahl digestion.
• Heterocyclic, diazo, and nitro–nitrogen (in 3, 4, 5 [partially], 6, 7) are not completely converted.