Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. ELectronically, aldehydes are more recative than ketones because two alkyl groups reduce the electrophilicity of teh carbonyl more efficiently than in former.

For the preparation of phenylethanoic acid from benzyl alcohol, reagents required are

A. \(HBr\)

B. \(CO_2\)

C. \(KCN\)

D. \(\Delta / H_3O^+\)

E. \(AgNO_3\)

Choose the correct answer from the options given below:

A, C and D only

The correct answer is option 2. A, C and D only.

Let us break down the process of converting benzyl alcohol to phenylethanoic acid and understand why the specific reagents are required:

Step 1: Conversion of Benzyl Alcohol to Benzyl Bromide

Benzyl alcohol \((C_6H_5CH_2OH)\) is treated with hydrobromic acid \((HBr)\) to convert the hydroxyl group \((-OH)\) into a bromide group, forming benzyl bromide \((C_6H_5CH_2Br)\). This is a typical substitution reaction where the -OH group is replaced by a -Br atom.

Step 2: Conversion of Benzyl Bromide to Phenylacetonitrile

Benzyl bromide is then reacted with potassium cyanide \((KCN)\), which introduces a cyanide group \((-CN)\) in place of the bromide, forming phenylacetonitrile \((C_6H_5CH_2CN)\). This is a nucleophilic substitution reaction where the cyanide ion replaces the bromide ion.

Step 3: Hydrolysis of Phenylacetonitrile to Phenylethanoic Acid

Phenylacetonitrile is hydrolyzed under acidic conditions with heat to convert the nitrile group \((-CN)\) into a carboxylic acid group \((-COOH)\), resulting in phenylethanoic acid \((C_6H_5CH_2COOH)\). This is a two-step hydrolysis process where the nitrile first converts to an amide \((-CONH_2)\), and then to the carboxylic acid \((-COOH)\).

Therefore, the correct sequence of reagents for the synthesis of phenylethanoic acid from benzyl alcohol includes A (HBr), C (KCN), and D (Δ / H_3O^+). The correct option is: 2. A, C, and D only.