(a) (i)

When (cyclohexanone) is treated with HCN, it gives

Explanation:

With pure HCN, the reaction occurs very slowly. Therefore, it is catalyzed by a base.

HO^- + H—CN → :CN^-‑ + H₂O

Now, the nucleophile (:CN^-) attack the carbonyl carbon, and the H⁺ released above attack the negative oxygen:

Note: We can understand the above reaction through the following reaction:

(ii)

(iii) CH₃—CH=CH—CHO

Nitriles can be reduced directly to aldehydes by using DIBAL-H

(diisobutylaluminium hydride)

(b) (i) To distinguish between Butanal (aldehyde) and Butan-2-one (ketone), we will perform a chemical test by using Tollen’s reagent:

Tollens’ reagent is an ammoniacal solution of silver nitrate and is prepared by adding NH₄OH solution to AgNO₃ solution till the the precipitate of Ag₂O first formed just redissolves. When butanal (aldehyde) is heated with Tollens’ reagent, the latter is reduced to metallic silver which deposits on the walls of the test tube as bright silver mirror.

During this reduction, the following reaction occur:

⇒ Formation of Tollens’ reagent:

2AgNO₃ + 2NH₄OH → Ag₂O + 2 NH₄NO₃ + H₂O

⇒ Now, butanal is treated with Tollens’ reagent:

CH₃—CH=CH—CHO + 2[Ag(NH₃)₂]⁺ + 3OH^- → CH₃CH₂CH₂COO^- + 2Ag + 4NH₃ + 2H₂O

The silver (Ag) thus deposited shines like a mirror. The formation of silver mirror confirms the presence of butanal.

On the other hand, butanone(ketone) does not give this test. It does not form silver with Tollens’ reagent.

(ii) Benzoic acid and Phenol

Benzoic acid and Phenol can be distinguished as follows:

⇒ NaHCO₃ test: Benzoic acid being a stronger acid decomposes NaHCO₃ to evolve CO₂.

C₆H₅COOH + NaHCO₃→ C₆H₅COONa + CO₂ + H₂O

Phenol being a weak acid does not give this test.

C₆H₅OH + NaHCO₃→ No evolution of CO₂

⇒ FeCl₃ test: Phenol gives a violet colouration with neutral FeCl₃ solution.

C₆H₅OH + FeCl₃→ Violet colouration

Benzoic acid gives a buff coloured precipitate of ferric benzoate.

C₆H₅COOH + FeCl₃→ (C₆H₅COO)₃Fe + 3HCl

OR

(a) (i) Etard reaction:

Toluene is oxidized to benzaldehyde with a solution of chromyl chloride (CrO₂Cl₂) in CS₂ or CCl₄. The brown chromium complex thus precipitated is separated and decomposed with dilute acids to give benzaldehyde.

The above reaction is called Etard reaction.

(ii) Stephen Reaction:

When a solution of a nitrile is reduced with stannous chloride (SnCl₂) in the presence of hydrogen chloride gas (HCl) at a room temperature, imine hydrochloride is precipitated. This upon hydrolysis with boiling water give aldehydes.

The above reaction is called Stephen reaction.

(b)(i) Benzoic acid to Benzaldehyde

Step 1: Benzoic acid is treated with SOCl₂, to form Benzoyl chloride (C₆H₅—COCl) by the replacement of OH by Cl atom.

Step 2: Now, Benzoyl chloride is treated with H₂/Pd and BaSO₄, to form benzaldehyde. This process is called Rosenmund reduction.

(ii) Acetophenone to Benzoic acid

Step 1: Acetophenone reacts with I₂/NaOH to form sodium acetate. This reaction is known as Iodoform test due to the formation of a yellow precipitate of iodoform in this reaction.

Step 2: Now, sodium acetate undergoes hydrolysis to form benzoic acid.

(iii) Ethanoic acid to 2-Hydroxyethanoic acid.

Step 1: Ethanoic acid reacts with chlorine or red phosphorus to give 2-chloro ethanoic acid. This reaction is called Hell- Volhard Zelinsky (H.V.Z.) reaction

Step 2: Now, 2-chloro ethanoic acid reacts with KOH to form 2-hydroxyethanoic acid by the replacement of Cl by OH atom.