The minimum number of carbon atoms in ketone to show position isomerism :-

Options

A. 3
B. 4
C. 5
D. 6

Answer

To determine the minimum number of carbon atoms in a ketone that can exhibit position isomerism, we need to analyze the structure of ketones and the concept of position isomerism step by step.

### Step-by-Step Solution:

1. **Understanding Ketones**:
- Ketones are organic compounds characterized by the carbonyl group (C=O) bonded to two carbon atoms. The general formula for a ketone is R-C(=O)-R', where R and R' are alkyl groups.

2. **Definition of Position Isomerism**:
- Position isomerism occurs when the arrangement of the carbon skeleton remains the same, but the position of the functional group (in this case, the carbonyl group) changes.

3. **Analyzing 3-Carbon Ketones**:
- For a ketone with 3 carbon atoms (C3), the only possible structure is:
- CH3-CO-CH3 (where the carbonyl group is on the second carbon).
- Since there is no other position to place the carbonyl group (it cannot be on the first or third carbon as those would lead to aldehydes), 3-carbon ketones do not show position isomerism.

4. **Analyzing 4-Carbon Ketones**:
- For a ketone with 4 carbon atoms (C4), the possible structure is:
- CH3-CO-CH2-CH3.
- The carbonyl group can only be placed on the second carbon (as placing it on the first or last carbon would again lead to aldehydes). Therefore, 4-carbon ketones also do not exhibit position isomerism.

5. **Analyzing 5-Carbon Ketones**:
- For a ketone with 5 carbon atoms (C5), we can have:
- CH3-CO-CH2-CH2-CH3 (carbonyl on the second carbon).
- CH3-CH2-CO-CH2-CH3 (carbonyl on the third carbon).
- Here, we can see that the carbonyl group can be placed on either the second or third carbon, leading to different compounds. Thus, 5-carbon ketones can exhibit position isomerism.

6. **Conclusion**:
- The minimum number of carbon atoms in a ketone required to show position isomerism is **5**.

### Final Answer:
The minimum number of carbon atoms in a ketone to show position isomerism is **5**.