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 compound will react at the slowest rate with a nucleophile, we need to analyze the electron-donating or electron-withdrawing effects of the substituents on the carbonyl carbon. The more electron-donating groups present, the less positive charge density on the carbonyl carbon, making it less susceptible to nucleophilic attack.

### Step-by-Step Solution:

1. **Identify the Compounds**: We have four options to analyze:
- Option 1: Methanol (CH3OH)
- Option 2: Butanone (C4H8O)
- Option 3: 2-Pentanone (C5H10O)
- Option 4: Pentan-3-one (C5H10O)

2. **Analyze Electron Effects**:
- **Methanol**: The hydroxyl group (-OH) is not a strong electron-donating group. The carbonyl carbon is relatively positive, making it more reactive towards nucleophiles.
- **Butanone**: This ketone has two alkyl groups (ethyl and methyl) which can donate electron density, but not as effectively as larger groups.
- **2-Pentanone**: This ketone has a larger alkyl group (isopropyl) which can provide more electron density than butanone, making it less reactive than butanone.
- **Pentan-3-one**: This compound has two ethyl groups, which are strong electron-donating groups. The electron density shared by both ethyl groups significantly reduces the positive charge on the carbonyl carbon, making it the least reactive towards nucleophiles.

3. **Determine the Order of Reactivity**:
- The order of reactivity from fastest to slowest is:
- Methanol (fastest)
- Butanone
- 2-Pentanone
- Pentan-3-one (slowest)

4. **Conclusion**: The compound that will react at the slowest rate with a nucleophile is **Pentan-3-one** (Option 4).