Which one of the following compounds does not show tautomerism ?

Options

A. `CH_3-CH_2-NO_2`
B. `CH_3-underset(CH_3)underset(|)overset(CH_3)overset(|)C-NO`
C. `CH_3-NH-overset(O)overset(||)C-CH_3`
D. `CH_3-overset(O)overset(||)C-underset(CH3)underset(|)(CH)-COOC_2H_5`

Answer

To determine which compound does not show tautomerism, we need to understand the conditions required for tautomerism to occur. Tautomerism is a type of structural isomerism that typically involves the migration of a hydrogen atom and a shift in the position of a double bond.

### Step-by-Step Solution:

1. **Understand Tautomerism**:
- Tautomerism is a form of isomerism where two isomers (tautomers) can interconvert, typically involving a hydrogen atom shift and a change in the position of a double bond.

2. **Identify Key Requirements**:
- For a compound to exhibit tautomerism, it must have:
- A double bond (or a triple bond).
- An alpha hydrogen (a hydrogen atom attached to the carbon adjacent to the double bond).

3. **Analyze Each Compound**:
- **Compound 1: CH3CH2NO2**
- Contains a NO2 group which can show tautomerism. It has an alpha carbon (CH2) with alpha hydrogen.
- **Compound 2: NO (Nitroso group)**
- Contains a nitrogen-oxygen double bond but lacks an alpha hydrogen. Therefore, it cannot show tautomerism.
- **Compound 3: C=O (Carbonyl group)**
- Contains a carbonyl group (C=O) with an alpha carbon that has alpha hydrogen. It can show tautomerism.
- **Compound 4: Another compound with a double bond and alpha hydrogen**
- Similar to the carbonyl group, it has an alpha hydrogen and can show tautomerism.

4. **Conclusion**:
- The compound that does not show tautomerism is **Compound 2 (NO)**, as it lacks an alpha hydrogen.

### Final Answer:
The correct answer is **Compound 2 (NO)**, which does not show tautomerism.