By which one of the following reactions ketones cannot be prepared ?

Options

A. Hydration of alkynes
B. Hydrolysis of gem-dihalides
C. Dry distillation of calcium carboxylates
D. Stephen's reaction

Explanation

To determine which reaction cannot prepare ketones, we need to analyze the provided options. Let's go through the possible reactions one by one.

### Step-by-Step Solution:

1. **Hydration of Alkynes**:
- When alkynes undergo hydration in the presence of an acid catalyst, they form ketones. For example, the hydration of 2-butyne yields 2-butanone (a ketone).
- **Conclusion**: This reaction can prepare ketones.

2. **Hydrolysis of Geminal Dihalides**:
- Geminal dihalides can be hydrolyzed to form ketones. For instance, hydrolysis of 1,1-dibromoethane can yield acetone (a ketone).
- **Conclusion**: This reaction can prepare ketones.

3. **Dry Distillation of Calcium Carboxylates**:
- Dry distillation of calcium carboxylates can lead to the formation of ketones. For example, the dry distillation of calcium acetate produces ketene, which can further react to form ketones.
- **Conclusion**: This reaction can prepare ketones.

4. **Stefan's Reaction**:
- Stefan's reaction involves the reaction of methyl cyanide (acetonitrile) with an aldehyde, producing an aldehyde (specifically acetaldehyde) as the product. This reaction does not yield ketones.
- **Conclusion**: This reaction cannot prepare ketones.

### Final Answer:
The reaction by which ketones cannot be prepared is **Stefan's reaction**.

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