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 can analyze each step of the reaction process leading to the final product, which is 2-methylpropanol.

### Step-by-Step Solution:

1. **Identify the Final Product**: The final product of the reaction sequence is 2-methylpropanol. The structure of 2-methylpropanol can be represented as:
\[ \text{CH}_3\text{C}(\text{OH})\text{CH}_3 \]
This indicates that the final product has a branched structure with a hydroxyl (-OH) group.

2. **Reverse the Last Step**: The last step in the sequence involves hydrolysis of a Grignard reagent. The Grignard reagent that would yield 2-methylpropanol upon hydrolysis is:
\[ \text{RCH}_2\text{OMgBr} \]
where R is the alkyl group that will lead to the formation of 2-methylpropanol.

3. **Determine the Grignard Reagent**: To produce 2-methylpropanol, the Grignard reagent must be:
\[ \text{(CH}_3)_2\text{C}(\text{MgBr})\text{CH}_2 \]
This means the R group in the Grignard reagent is isopropyl (2-propyl).

4. **Identify the Preceding Step**: The Grignard reagent is formed from an alkyl iodide. The reaction to form the Grignard reagent from an alkyl iodide is:
\[ \text{R-I} + \text{Mg} \xrightarrow{\text{dry ether}} \text{R-MgI} \]
Therefore, we need to determine the alkyl iodide that would yield the Grignard reagent.

5. **Determine the Alkyl Iodide**: The alkyl iodide that would yield the Grignard reagent corresponding to isopropyl is:
\[ \text{(CH}_3)_2\text{C(I)}\text{CH}_2 \]
This indicates that the alkyl iodide is derived from the starting alcohol.

6. **Identify the Starting Alcohol**: The starting alcohol that would yield this alkyl iodide upon reaction with phosphorus and iodine (P + I2) is:
\[ \text{(CH}_3)_2\text{C(OH)}\text{CH}_2 \]
This is 2-propanol (or propane-2-ol).

### Conclusion:
The starting alcohol in this sequence of reactions is **2-propanol**.