Which of the following pair gives 1° carbonium ion by heterolysis of C-Br bond ?

Question

Accepted Answer

To understand why Option 3 is the correct answer, we need to analyze the structure of the compounds and the stability of the carbonium ions formed by the heterolysis of the C-Br bond.

### Explanation for Option 3: Neo-pentyl bromide, iso-butyl bromide

1. **Neo-pentyl bromide**: 
   - Structure: (CH₃)₃CCH₂Br
   - When the C-Br bond undergoes heterolysis, the resulting carbonium ion is a primary (1°) carbonium ion because the positively charged carbon is attached to only one other carbon atom.

2. **Iso-butyl bromide**:
   - Structure: (CH₃)₂CHCH₂Br
   - Similarly, the heterolysis of the C-Br bond in iso-butyl bromide results in a primary (1°) carbonium ion. The positively charged carbon is attached to only one other carbon atom.

Both neo-pentyl bromide and iso-butyl bromide form primary carbonium ions upon heterolysis of the C-Br bond, which is why Option 3 is correct.

### Explanation for why other options are incorrect:

1. **Option 1: Isopropyl bromide, isobutyl bromide**
   - **Isopropyl bromide**: (CH₃)₂CHBr forms a secondary (2°) carbonium ion, not a primary one, because the positively charged carbon is attached to two other carbon atoms.
   - **Isobutyl bromide**: As explained above, it forms a primary carbonium ion, but since isopropyl bromide does not, this option is incorrect.

2. **Option 2: Isobutyl bromide, sec-butyl bromide**
   - **Sec-butyl bromide**: CH₃CHBrCH₂CH₃ forms a secondary (2°) carbonium ion, not a primary one, because the positively charged carbon is attached to two other carbon atoms.
   - **Isobutyl bromide**: Forms a primary carbonium ion, but sec-butyl bromide does not, making this option incorrect.

3. **Option 4: Active amyl bromide, iso-propyl bromide**
   - **Active amyl bromide**: This is a less common name, but typically refers to a structure that forms a secondary or tertiary carbonium ion, not a primary one.
   - **Iso-propyl bromide**: As explained, forms a secondary carbonium ion.
   - Neither compound in this pair forms a primary carbonium ion, making this option incorrect.

In conclusion, Option 3 is the only pair where both compounds form primary carbonium ions upon the heterolysis of the C-Br bond, which is why it is the correct answer.