Which among the following is most reactive to give nucleophilic addition?

Options

A. `FCH_(2)CHO`
B. `ClCH_(2)CHO`
C. `BrCH_(2)CHO`
D. `ICH_(2)CHO`

Explanation

To determine which among the given substituted aldehydes (A, B, C, and D) is most reactive towards nucleophilic addition, we need to analyze the effect of the substituents on the carbonyl carbon.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- A: CH2C(O)H with a fluorine substituent.
- B: CH2C(O)H with a chlorine substituent.
- C: CH2C(O)H with a bromine substituent.
- D: CH2C(O)H with an iodine substituent.

2. **Understand the Carbonyl Group**:
The carbonyl group (C=O) is polarized due to the electronegativity of oxygen. The carbon atom in the carbonyl group becomes electron-deficient (partially positive charge) and is susceptible to nucleophilic attack.

3. **Effect of Substituents**:
The reactivity of the carbonyl compound towards nucleophilic addition is influenced by the nature of the substituents attached to the carbonyl carbon.
- **Electron-Withdrawing Groups**: These groups increase the positive charge on the carbonyl carbon, enhancing its reactivity towards nucleophiles.
- **Electron-Donating Groups**: These groups decrease the positive charge on the carbonyl carbon, reducing its reactivity.

4. **Analyze the Substituents**:
- Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) are all electron-withdrawing groups (–I effect). However, they differ in their electronegativity and the strength of their –I effect.
- The order of –I effect strength is: F > Cl > Br > I. This means that fluorine is the strongest electron-withdrawing group among them.

5. **Determine Reactivity**:
Since the reactivity towards nucleophilic addition is directly proportional to the positive charge on the carbonyl carbon, the compound with the strongest –I effect will be the most reactive.
- Therefore, compound A (with fluorine) will have the highest positive charge on the carbonyl carbon, making it the most reactive towards nucleophilic addition.

### Conclusion:
The most reactive compound towards nucleophilic addition is **A (with fluorine)**.

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