Chlorobenzene reacts with Mg in dry ether to give a compound A, which further reacts with ethanol to give B. Identify B.

Chlorobenzene reacts with magnesium in the presence of dry ether to form phenylmagnesium chloride, which is a Grignard reagent. This is compound A.

$C_6H_5Cl + Mg \xrightarrow{\text{dry ether}} C_6H_5MgCl$

Grignard reagents are highly reactive towards compounds containing active or acidic hydrogen atoms. Ethanol contains an acidic hydrogen atom attached to the oxygen atom. When phenylmagnesium chloride reacts with ethanol, it abstracts the acidic proton to form benzene. This is compound B.

$C_6H_5MgCl + C_2H_5OH \rightarrow C_6H_6 + Mg(OC_2H_5)Cl$

Thus, the final product B is benzene.

Answer: Benzene