Question: 83. Identify the suitable reagent for the following conversion.
(1) (i) \mathrm{LiAlH_4}, (ii) \mathrm{H^+ / H_2O}
(2) (i) \mathrm{AlH(iBu)_2} (ii) \mathrm{H_2O}
(3) (i) \mathrm{NaBH_4}, (ii) \mathrm{H^+ / H_2O}
(4) (i) \mathrm{H_2 / Pd-BaSO_4}
Answer: Option (2)
Explanation:
The given conversion is the reduction of an ester,
\mathrm{C_6H_5COOCH_3} (methyl benzoate), to an aldehyde,
\mathrm{C_6H_5CHO} (benzaldehyde).
This requires a reagent that can partially reduce the ester to the aldehyde without further reducing it to the corresponding alcohol.
\mathrm{AlH(iBu)_2} is diisobutylaluminium hydride (DIBAL-H).
At controlled conditions, it reduces esters to aldehydes.
In the first step, DIBAL-H adds hydride to the ester to form an intermediate,
and on hydrolysis in the second step with \mathrm{H_2O},
the aldehyde \mathrm{C_6H_5CHO} is obtained.
Reagent \mathrm{LiAlH_4} in option (1) is a strong reducing agent and would reduce the ester completely to the primary alcohol \mathrm{C_6H_5CH_2OH},
not stopping at the aldehyde. Reagent \mathrm{NaBH_4} in option (3) is too mild to reduce esters under normal conditions.
Option (4), \mathrm{H_2 / Pd-BaSO_4}, is used mainly for partial reduction of acyl chlorides or alkynes, not esters, to aldehydes.
Therefore, only reagent (2) performs the desired selective reduction of the ester to the aldehyde,
so the correct answer is Option (2).