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

Options

A. `CH_3CH_2-C-=N`
B. `C_6H_5NH_2`
C. `C_6H_5NO_2`
D. `NH_2CONH_2`

Answer

**Step-by-Step Solution:**

1. **Understanding Kjeldahl's Method:**
- Kjeldahl's method is used to estimate nitrogen content in organic compounds. The process involves heating the organic compound with concentrated sulfuric acid (H2SO4), which converts nitrogen into ammonium sulfate (NH4)2SO4.

2. **Reaction Overview:**
- The general reaction can be summarized as:
\[ \text{Organic Compound} + H2SO4 \rightarrow (NH4)2SO4 + \text{other products} \]
- After this, the ammonium sulfate is treated with excess sodium hydroxide (NaOH), which releases ammonia (NH3):
\[ (NH4)2SO4 + 2 NaOH \rightarrow Na2SO4 + 2 NH3 + 2 H2O \]
- The ammonia gas evolved is then absorbed in a known quantity of H2SO4, and the amount of H2SO4 consumed is used to determine the nitrogen content.

3. **Identifying Compounds that Cannot be Analyzed:**
- Certain compounds do not yield nitrogen in a form that can be analyzed by Kjeldahl's method:
- **Nitrogen in a Ring:** Compounds like pyridine, where nitrogen is part of a cyclic structure, do not yield nitrogen suitable for this method.
- **Nitro Compounds:** Compounds with a nitro group (NO2) do not yield nitrogen in a form that can be analyzed by this method.
- **Azo Compounds:** Compounds containing an azo group (–N=N–) also do not yield nitrogen suitable for this method.

4. **Evaluating Given Options:**
- Now, let’s evaluate the options provided in the question:
- **Option 1:** A compound with nitrogen not in a ring or nitro group (e.g., an amine) can be analyzed.
- **Option 2:** Aniline (C6H5NH2) is not in a ring and will give a positive result.
- **Option 3:** A compound with a nitro group (C6H5NO2) cannot be analyzed using Kjeldahl's method.
- **Option 4:** A compound with amine groups (like NH2C=O) can be analyzed.

5. **Conclusion:**
- The compound that cannot be estimated using Kjeldahl's method is the one containing the nitro group (C6H5NO2).

**Final Answer:**
- The correct answer is the compound with the nitro group (C6H5NO2).

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