Question

Consider the following: Benzene diazonium chloride, Nitrobenzene, Pyridine, Aniline, Benzylamine, urea. How many of the above compounds are not suitable for the estimation of nitrogen by Kjeldahl's method?

• A. 3
• B. 2
• C. 4
• D. 1 \bigskip

Hint:

Kjeldahl method is NOT applicable to: \[ -NO_2 \] Nitro compounds, Diazonium salts, Many azo compounds, Certain heterocyclic nitrogen compounds. These are frequently asked exceptions in competitive examinations.

Answer 1

The Correct Option is A

Concept: Kjeldahl's method is used for estimating nitrogen present in organic compounds. However, it is not applicable when nitrogen is present in certain special forms because such nitrogen is not converted quantitatively into ammonium sulfate during digestion. The method is not suitable for:
• Nitro compounds
• Azo compounds
• Diazonium salts
• Nitrogen present in some ring systems

Step 1: Examine Benzene diazonium chloride.
It contains nitrogen as a diazonium group. Hence Kjeldahl's method is not suitable.

Step 2: Examine Nitrobenzene.
Nitrogen is present in nitro form. Therefore Kjeldahl's method cannot be used.

Step 3: Examine Pyridine.
Nitrogen is present in a heterocyclic aromatic ring. It is not estimated satisfactorily by Kjeldahl's method.

Step 4: Examine the remaining compounds.
Aniline, benzylamine and urea can be analyzed using Kjeldahl's method. Therefore unsuitable compounds are: \[ \text{Benzene diazonium chloride} \] \[ \text{Nitrobenzene} \] \[ \text{Pyridine} \] Total number \[ =3 \] Hence, \[ \boxed{3} \] \[ \boxed{\text{Option (A)}} \]