A compound (A) reacts with thionylchloride to give a compound (B). (B) reacts with magnesium to form a Grignard reagent which is treated with acetone and the product is hydrolysed to give 2-methyl butan-2-ol. What are the structural formulae of (A) and (B) ?

Answer

To solve the problem step by step, we will identify the compounds (A) and (B) based on the provided reactions and the final product.

### Step 1: Identify the final product
The final product given is 2-methylbutan-2-ol. Its structural formula can be represented as:
\[ \text{CH}_3\text{C(OH)(CH}_3)\text{CH}_2\text{CH}_3 \]

### Step 2: Determine the structure of compound (B)
Since the final product is obtained from a Grignard reagent reacting with acetone, we can deduce that compound (B) must be an alkyl halide that can form the Grignard reagent. The Grignard reagent formed from (B) will be:
\[ \text{R-MgX} \]
where R is the alkyl group derived from (B).

### Step 3: Identify the Grignard reagent
The Grignard reagent is formed by the reaction of compound (B) with magnesium. Since we are treating it with acetone (which has the structure \(\text{CH}_3\text{C(=O)CH}_3\)), the alkyl group (R) must be ethyl (C2H5) to yield the final product of 2-methylbutan-2-ol.

Thus, compound (B) is:
\[ \text{C}_2\text{H}_5\text{Cl} \]
which is ethyl chloride.

### Step 4: Determine the structure of compound (A)
Compound (A) is the alcohol that reacts with thionyl chloride (SOCl2) to form compound (B). Since we have established that (B) is ethyl chloride, (A) must be the corresponding alcohol, which is ethanol (ethyl alcohol).

Thus, compound (A) is:
\[ \text{C}_2\text{H}_5\text{OH} \]
which is ethanol.

### Summary of the structural formulae
- Compound (A): Ethanol \(\text{C}_2\text{H}_5\text{OH}\)
- Compound (B): Ethyl chloride \(\text{C}_2\text{H}_5\text{Cl}\)

### Final Answer
- **Compound (A)**: Ethanol \(\text{C}_2\text{H}_5\text{OH}\)
- **Compound (B)**: Ethyl chloride \(\text{C}_2\text{H}_5\text{Cl}\)

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