Question

Which of the following compounds will be suitable for estimation of nitrogen by Kjeldahl's method?
Structures:

• A. I & V only
• B. I, II, III only
• C. II & V only
• D. III & IV only

Hint:

Remember the exceptions for Kjeldahl's method: Nitro compounds, Azo compounds, and Ring Nitrogen compounds cannot be estimated.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Kjeldahl's method estimates nitrogen by converting it into ammonium sulfate. This method fails for nitrogen contained in nitro groups, azo groups (diazo), or nitrogen present inside a ring (like pyridine), as these nitrogens cannot be easily converted to ammonium ions under the reaction conditions.
Step 2: Detailed Explanation:

I. Aniline: The \( -\text{NH}_2 \) group is attached to the benzene ring but is an amine. It can be converted to ammonium sulfate. Suitable.
II. Benzenediazonium chloride: Contains the azo/diazo group (\( -\text{N}_2^+ \)). Nitrogen escapes as gas upon heating. Not Suitable.
III. Nitrobenzene: Contains the nitro group (\( -\text{NO}_2 \)). Does not reduce to ammonia easily in this method. Not Suitable.
IV. Pyridine: Nitrogen is part of the aromatic ring. It is very stable and resists digestion. Not Suitable.
V. Benzylamine: The amino group is on the alkyl side chain. It behaves like an aliphatic amine and is easily digested. Suitable.
Suitable compounds are I and V.
Step 3: Final Answer:
Compounds I and V only.