Cyclobutyl bromide on treatment with magnesium in dry ether forms an organometallic compound `(A)`. The organometallic compound `(A)` reacts with ethanal to give an alcohol `(B)` after mild acidification. Prolonged treatment of alcohol `(B)` with an equivalent amount of `HBr` gives 1-bromo-1-methylcyclopentane ( C) Write the structures of `(A)` and `(B)`, and explain how `(C)` is obtained from `(B)`.

Options

A. 1-Chloro-1-ethylcyclopentane
B. 1-Bromo-1-methycyclopentane
C. 3-Bromo-2-methycyclopentane
D. none of these

Explanation

To solve the problem, we will follow a systematic approach to determine the structures of the organometallic compound (A), the alcohol (B), and explain how (C) is obtained from (B).

### Step 1: Determine the Structure of (A)

- **Starting Compound**: Cyclobutyl bromide (C4H7Br) is a cyclic compound with four carbon atoms and a bromine atom attached.
- **Reaction with Magnesium**: When cyclobutyl bromide reacts with magnesium in dry ether, it forms a Grignard reagent. The general formula for a Grignard reagent is R-MgX, where R is an organic group and X is a halogen.

**Structure of (A)**:
\[ \text{(A) = Cyclobutyl-MgBr} \]
This can be represented as:
\[ \text{C}_4\text{H}_7\text{MgBr} \]

### Step 2: Reaction of (A) with Ethanal

- **Reaction**: The Grignard reagent (A) reacts with ethanal (acetaldehyde, CH3CHO). The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbon of the carbonyl group in ethanal.

- **Formation of Intermediate**: This reaction forms a tetrahedral intermediate, which upon mild acidification will yield an alcohol.

**Structure of (B)**:
\[ \text{(B) = 1-ethyl-1-cyclobutanol} \]
This can be represented as:
\[ \text{C}_4\text{H}_9\text{OH} \]

### Step 3: Conversion of (B) to (C)

- **Prolonged Treatment with HBr**: When alcohol (B) is treated with HBr, the hydroxyl group (-OH) is protonated to form water, leading to the formation of a carbocation.

- **Carbocation Rearrangement**: The initial carbocation formed is a secondary carbocation. However, it can undergo a rearrangement to form a more stable tertiary carbocation by shifting a hydrogen atom from the adjacent carbon to the carbocation center.

- **Formation of (C)**: The bromide ion (Br-) then attacks the carbocation to form 1-bromo-1-methylcyclopentane.

**Structure of (C)**:
\[ \text{(C) = 1-bromo-1-methylcyclopentane} \]
This can be represented as:
\[ \text{C}_6\text{H}_{11}\text{Br} \]

### Summary of Structures
- **(A)**: Cyclobutyl-MgBr
- **(B)**: 1-ethyl-1-cyclobutanol
- **(C)**: 1-bromo-1-methylcyclopentane