Q.Identify $A$ and $B$ in the following reaction (1+1=2): bromocyclohexane $+ Mg \xrightarrow{\text{dry ether}} [A] \xrightarrow{H_2O/H^+} [B]$

Mg insertion into the C–Br bond of bromocyclohexane (in dry ether) gives the Grignard reagent [A]; its hydrolysis by aqueous acid replaces the $-MgBr$ group with $-H$ to give plain cyclohexane [B].

Formation of [A] — the Grignard reagent: Bromocyclohexane reacts with magnesium metal in anhydrous (dry) ether. Dry conditions are essential because Grignard reagents are extremely moisture-sensitive. Magnesium inserts into the carbon–halogen bond:

$C_6H_{11}Br + Mg \xrightarrow{\text{dry ether}} C_6H_{11}MgBr \;\; [A]$

[A] is cyclohexylmagnesium bromide, a Grignard reagent in which the cyclohexyl carbon bears a highly polarized, carbanion-like $C^{\delta-}-Mg^{\delta+}$ bond.

Hydrolysis of [A] — formation of [B]: Grignard reagents react instantly with any active-hydrogen compound, including water; the strongly nucleophilic/basic carbanion-like carbon is simply protonated, replacing $-MgBr$ with $-H$: …