In which of the following compounds , the C - Cl bond ioniosation shall give most stable carbonium ion ?

Options

A.
B.
C. `O_2NCH_2CH_2Cl`
D.

Answer

To determine which compound will give the most stable carbonium ion upon C-Cl bond ionization, we need to analyze the stability of the carbocations formed from each compound. Here’s a step-by-step breakdown:

### Step 1: Identify the Compounds
We have four compounds to analyze:
1. **Compound 1**: Tertiary alkyl chloride (e.g., tert-butyl chloride).
2. **Compound 2**: Benzyl chloride.
3. **Compound 3**: Nitroalkane (e.g., nitroethane).
4. **Compound 4**: Secondary alkyl chloride (e.g., isopropyl chloride).

### Step 2: Analyze the Stability of the Carbocations
1. **Compound 1 (Tertiary Alkyl Chloride)**:
- When the C-Cl bond breaks, a tertiary carbocation is formed.
- Tertiary carbocations are stabilized by hyperconjugation and inductive effects from adjacent alkyl groups.
- This carbocation has 9 alpha hydrogens contributing to its stability.

2. **Compound 2 (Benzyl Chloride)**:
- When the C-Cl bond breaks, a benzyl carbocation is formed.
- Benzyl carbocations are highly stable due to resonance. The positive charge can be delocalized over the aromatic ring.
- This resonance stabilization makes the benzyl carbocation very stable.

3. **Compound 3 (Nitroalkane)**:
- When the C-Cl bond breaks, a carbocation is formed that is adjacent to a nitro group.
- The nitro group is a strong electron-withdrawing group (−I effect) and destabilizes the carbocation by pulling electron density away from it.
- This makes the carbocation formed here very unstable.

4. **Compound 4 (Secondary Alkyl Chloride)**:
- When the C-Cl bond breaks, a secondary carbocation is formed.
- Secondary carbocations are less stable than tertiary ones but can still be stabilized by hyperconjugation and inductive effects from adjacent alkyl groups.
- This carbocation has 6 alpha hydrogens contributing to its stability.

### Step 3: Compare the Stability
- **Tertiary Carbocation**: Stable due to hyperconjugation and inductive effects.
- **Benzyl Carbocation**: Very stable due to resonance.
- **Nitroalkane Carbocation**: Unstable due to the electron-withdrawing effect of the nitro group.
- **Secondary Carbocation**: Moderately stable due to hyperconjugation.

### Conclusion
The most stable carbonium ion is formed from **Compound 2 (Benzyl Chloride)** due to resonance stabilization, which dominates over the inductive effects present in the other compounds.

### Final Answer
**Benzyl chloride (Compound 2) will give the most stable carbonium ion upon C-Cl bond ionization.**

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