Which of the following will react at the slowest rate with a nucleophile ?

Options

A. Methanal
B. Butanone
C. 2-Pentanone
D. 3-Pentanone

Answer

To determine which of the given compounds reacts at the slowest rate with a nucleophile, we need to analyze the electrophilicity of the carbonyl carbon in each compound. The more electrophilic the carbonyl carbon, the faster it will react with a nucleophile.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- The compounds given are:
- Methanol (CH₃OH)
- Butanone (C₄H₈O)
- 2-Pentanone (C₅H₁₀O)
- 3-Pentanone (C₅H₁₀O)

2. **Draw the Structures**:
- Methanol: H-C(=O)-OH
- Butanone: CH₃-C(=O)-CH₂-CH₃
- 2-Pentanone: CH₃-C(=O)-CH₂-CH₂-CH₃
- 3-Pentanone: CH₃-CH₂-C(=O)-CH₂-CH₃

3. **Analyze Electrophilicity**:
- The electrophilicity of the carbonyl carbon is influenced by the presence of electron-donating groups (which decrease electrophilicity) and electron-withdrawing groups (which increase electrophilicity).
- Methanol has no alkyl groups attached to the carbonyl carbon, making it more electrophilic.
- Butanone has one alkyl group (ethyl) which has a slight +I (inductive) effect, reducing electrophilicity.
- 2-Pentanone has two alkyl groups (methyl and ethyl) which further decrease the electrophilicity of the carbonyl carbon due to their +I effect.
- 3-Pentanone has the most significant +I effect because it has two alkyl groups (ethyl and propyl) on either side of the carbonyl carbon, making it the least electrophilic.

4. **Conclusion**:
- Since 3-pentanone has the most electron-donating groups around the carbonyl carbon, it will have the lowest electrophilicity and thus will react at the slowest rate with a nucleophile.
- Therefore, the answer is **3-Pentanone** (Option D).