Nucleophilic substitution at acyl carbon of a carboxylic acid derivative generally proceeds by.

Options

A. Addition-elimination mechanism
B. Elimination-addition mechanism
C. `S_(N)1` mechanism
D. `S_(N)2` mechanism

Answer

To determine the mechanism of nucleophilic substitution at the acyl carbon of a carboxylic acid derivative, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
- Nucleophilic substitution at the acyl carbon of carboxylic acid derivatives can occur through different mechanisms. The main types are addition-elimination and elimination-addition.

2. **Understand the Structure of Carboxylic Acid Derivatives**:
- A common carboxylic acid derivative is an acyl chloride, represented as R-C(=O)-Cl. Here, R is an alkyl or aryl group, and the acyl carbon is the carbonyl carbon (C=O).

3. **Nucleophile Attack**:
- In the reaction, a nucleophile (which has a negative charge or a lone pair of electrons) will attack the electrophilic acyl carbon. The carbonyl carbon is electrophilic due to the partial positive charge it carries (δ+), while the oxygen in the carbonyl has a partial negative charge (δ-).

4. **Formation of a Tetrahedral Intermediate**:
- When the nucleophile attacks the acyl carbon, it forms a tetrahedral intermediate. The bond between the carbon and the oxygen in the carbonyl group will shift, resulting in a negatively charged oxygen (alkoxide).

5. **Elimination of the Leaving Group**:
- The negatively charged oxygen in the tetrahedral intermediate will then facilitate the elimination of the leaving group (in this case, Cl). This step regenerates the carbonyl group and results in the final product.

6. **Final Product Formation**:
- After the elimination of the leaving group, the final product is formed. In the case of acyl chloride reacting with an alcohol, the product would be an ester.

7. **Conclusion**:
- The overall mechanism can be summarized as an addition followed by an elimination, which is characteristic of the addition-elimination mechanism.

### Final Answer:
The nucleophilic substitution at the acyl carbon of a carboxylic acid derivative generally proceeds by **addition-elimination mechanism**.

---