What represents the best method for converting a carboxylic acid to an aldehyde?

Options

A. Convert the acid to an acid chloride, and react the acid chloride with water.
B. Reduce the acid with `LiAlH_(4)`
C. Convert the acid to an acid chloride, and reduce the acid chloride with lithium aluminium tri (t - butoxy) hydride.
D. Convert the acid to an acid chloride, react the acid chloride with a Grignard reagent, and reduce the product with `LiAlH_(4)`.

Answer

To convert a carboxylic acid to an aldehyde, the best method involves the following steps:

### Step-by-Step Solution:

1. **Convert Carboxylic Acid to Acid Chloride**:
- Start with the carboxylic acid (R-COOH).
- React it with a reagent such as PCl5 (phosphorus pentachloride) to convert the carboxylic acid into an acid chloride (R-COCl).
- The reaction can be represented as:
\[ R-COOH + PCl5 \rightarrow R-COCl + POCl3 + HCl \]

2. **Reduce Acid Chloride to Aldehyde**:
- Now, take the acid chloride (R-COCl) and reduce it using lithium aluminium tri-tert-butoxy hydride (LiAlH(OtBu)3).
- This reagent selectively reduces acid chlorides to aldehydes without further reducing them to alcohols.
- The reaction can be represented as:
\[ R-COCl + LiAlH(OtBu)3 \rightarrow R-CHO + LiCl + Al(OtBu)3 \]

3. **Final Product**:
- The final product of this reaction is the aldehyde (R-CHO), which is the desired compound.

### Conclusion:
The best method for converting a carboxylic acid to an aldehyde is to first convert the carboxylic acid to an acid chloride and then reduce the acid chloride using lithium aluminium tri-tert-butoxy hydride.

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