Benzene is treated with oleum to produce compound (X) which when further heated with molten sodium hydroxide followed…

Step 1: Benzene + oleum → X

Benzene reacts with fuming sulfuric acid (oleum) via electrophilic aromatic substitution (SEAr): $\ce{C6H6 + H2SO4 (oleum) -> C6H5SO3H}$ (benzenesulfonic acid) = X

Step 2: Benzenesulfonic acid + molten NaOH, then H3O+ → Y

Fusion of benzenesulfonate salt with NaOH at high temperature (alkali fusion) is the industrial synthesis of phenol: $\ce{C6H5SO3Na + 2NaOH ->[ melt ] C6H5ONa + Na2SO3}$

Acidification: $\ce{C6H5ONa + HCl -> C6H5OH + NaCl}$

So Y = Phenol ($\ce{C6H5OH}$)

Step 3: Phenol + Zn → Z

When phenol is treated with zinc (Zn dust), the $\ce{-OH}$ group is removed (reduction/deoxygenation): $\ce{C6H5OH + Zn ->[ \Delta ] C6H6 + ZnO}$

Z = Benzene ($\ce{C6H6}$)

Step 4: Common wrong approach

Students sometimes think Zn oxidises phenol or produces cyclohexanol. Remember: Zn dust causes reduction — it removes the phenolic -OH to give benzene. This is distinct from catalytic hydrogenation (which would give cyclohexane).

Key Points to Remember:

• Benzene → oleum → benzenesulfonic acid → alkali fusion → sodium phenoxide → acid → phenol.
• Phenol + Zn dust → benzene (Zn reduces and removes -OH).
• Zn/HCl with diazonium salts also gives benzene — similar deoxygenation concept.