Identify Compound X Formed from Hydroboration Oxidation Sequence

To identify the structure of compound $\mathbf{X}$, let us analyze the reaction sequence step by step:

1. Identification of the Starting Material ($\text{C}_8\text{H}_6$): The degree of unsaturation (DU) for the compound $\text{C}_8\text{H}_6$ is calculated as: $\text{DU} = C + 1 - \frac{H}{2} = 8 + 1 - \frac{6}{2} = 6$ Since the molecule undergoes controlled hydrogenation and hydration reactions characteristic of alkynes attached to a benzene ring (which accounts for $4$ degrees of unsaturation), the structure of $\text{C}_8\text{H}_6$ is phenylacetylene ($\text{Ph}-\text{C}\equiv\text{CH}$).

2. Controlled Hydrogenation to Form $\text{C}_8\text{H}_8$: Phenylacetylene undergoes partial reduction in the presence of Lindlar's catalyst ($\text{Pd-BaSO}_4$ / $\text{H}_2$) to yield styrene ($\text{Ph}-\text{CH}=\text{CH}_2$): $\text{Ph}-\text{C}\equiv\text{CH} \xrightarrow{\text{H}_2, \text{Pd-BaSO}_4} \text{Ph}-\text{CH}=\text{CH}_2 \quad (\text{C}_8\text{H}_8)$

3. Hydroboration-Oxidation to Form Compound $\mathbf{X}$: Styrene ($\text{Ph}-\text{CH}=\text{CH}_2$) is subjected to hydroboration-oxidation using diborane ($\text{B}_2\text{H}_6$) followed by alkaline hydrogen peroxide ($\text{H}_2\text{O}_2 / \text{NaOH}$).

   • The hydroboration-oxidation reaction results in the overall addition of $\text{H}_2\text{O}$ across the carbon-carbon double bond with anti-Markovnikov regioselectivity.
   • Consequently, the hydroxyl group ($-\text{OH}$) attaches to the less substituted terminal carbon atom: $\text{Ph}-\text{CH}=\text{CH}_2 \xrightarrow[ii.\ \text{H}_2\text{O}_2, \text{NaOH}, \text{H}_2\text{O}]{i.\ \text{B}_2\text{H}_6} \text{Ph}-\text{CH}_2-\text{CH}_2\text{OH}$

Thus, compound $\mathbf{X}$ is 2-phenylethan-1-ol ($\text{Ph}-\text{CH}_2-\text{CH}_2\text{OH}$), which corresponds to structure (C).