Question

A and B in the following reaction are

• A. acetaldehyde and iodoform
• B. acetone and iodoform
• C. ether and iodoform
• D. acetone and methanol

Answer 1

Answer: (B)

acetone and iodoform

Answer 2

The reaction shown is a classic example of the iodoform reaction (a type of haloform reaction).

The starting material is propan-2-ol (isopropyl alcohol):

This compound contains a methyl carbinol group (-CH(OH)CH₃), which is a prerequisite for a positive iodoform test.

The reaction proceeds in two main steps under the given conditions (I₂/NaOH):

Step 1: Oxidation of Propan-2-ol to Acetone (Compound A)

Alcohols containing the -CH(OH)CH₃ group are first oxidized to the corresponding methyl ketones in the presence of halogen and base. Propan-2-ol is oxidized to acetone (propanone):

$$\text{CH}_3\text{CH(OH)}\text{CH}_3 \xrightarrow{\text{I}_2/\text{NaOH}} \text{CH}_3\text{COCH}_3$$

Therefore, A is acetone.

Step 2: Haloform Reaction of Acetone to form Iodoform (Compound B) and Sodium Acetate

Acetone (Compound A), being a methyl ketone (containing the -COCH₃ group), then undergoes further reaction with I₂/NaOH. This involves complete iodination of the methyl group attached to the carbonyl, followed by cleavage of the tri-iodomethyl group.

$$\text{CH}_3\text{COCH}_3 \xrightarrow{\text{I}_2/\text{NaOH}} \text{CH}_3\text{COONa} + \text{CHI}_3$$

The products are sodium acetate (the salt of acetic acid) and iodoform. The reaction scheme shows the formation of a carboxylic acid (or its salt, given the basic conditions) which is acetic acid, and compound B. Therefore, B is iodoform (CHI₃).

The complete reaction sequence is:

$$\text{CH}_3\text{CH(OH)}\text{CH}_3 \xrightarrow{\text{I}_2/\text{NaOH}} \text{CH}_3\text{COCH}_3 \text{ (A)} \xrightarrow{\text{I}_2/\text{NaOH}} \text{CH}_3\text{COONa} + \text{CHI}_3 \text{ (B)}$$

Based on this analysis, A is acetone and B is iodoform.

Explanation of the solution:

Propan-2-ol undergoes oxidation to acetone (A) in the presence of I₂/NaOH. Acetone (A) then reacts further under the same conditions to yield sodium acetate and iodoform (B) via the haloform reaction.