Alcohol`overset(P + I_2)(rarr)X overset(Mg, dry,"ether")(rarr)Y overset(HCHO)(rarr)Z overset(H_2O //H^+)(rarr)2`- methylpropanol
In this sequence of reaction, the starting alcohol is

Options

A. ethanol
B. propan-2-ol
C. propanol
D. butan-2-ol

Answer

To determine the starting alcohol in the given sequence of reactions, we will analyze each step carefully.

### Step 1: Identify the Reactions
The sequence of reactions is as follows:
1. Alcohol `ROH` reacts with phosphorus and iodine (`P + I2`) to form an alkyl iodide `RI`.
2. The alkyl iodide `RI` reacts with magnesium in dry ether to form a Grignard reagent `R-MgI`.
3. The Grignard reagent `R-MgI` reacts with formaldehyde (`HCHO`) to form an alcohol `R-CH2OH`.
4. The alcohol undergoes hydrolysis in the presence of water and acid (`H2O/H^+`) to yield a secondary alcohol, which is `2-methylpropanol`.

### Step 2: Analyze the Final Product
The final product is `2-methylpropanol`, which has the structure:
- **2-methylpropanol**: This compound has a total of 4 carbon atoms (C4) and an alcohol functional group (-OH) at the second carbon.

### Step 3: Determine the Structure of the Starting Alcohol
To find the starting alcohol `ROH`, we need to trace back from the final product:
- The Grignard reagent `R-MgI` must have been formed from an alkyl iodide `RI`, which in turn was derived from the starting alcohol `ROH`.
- The reaction of the Grignard reagent with formaldehyde (`HCHO`) adds one carbon to the structure. Therefore, if the product after this reaction is `R-CH2OH`, then `R` must be a carbon chain that, when combined with the carbon from formaldehyde, results in a total of 4 carbons in `2-methylpropanol`.

### Step 4: Identify the Alkyl Group
Since the final product is `2-methylpropanol`, we can deduce that:
- The Grignard reagent must have been derived from an alkyl iodide that has 3 carbons (C3) because the addition of one carbon from formaldehyde results in a total of 4 carbons.
- The alkyl iodide must have been formed from the starting alcohol, which is `propan-2-ol` (or `isopropanol`), because it has the structure necessary to yield the 3-carbon alkyl iodide upon reaction with phosphorus and iodine.

### Conclusion
Thus, the starting alcohol is **propan-2-ol** (or isopropanol).

### Final Answer
The starting alcohol is **propan-2-ol**.

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