A carbonyl compound on reaction with dilute NaOH gives 2-ethyl-3-hydroxyhexanal. What is the name of the carbonyl compound? 

Butanal 

The correct answer is option 3. Butanal.

To determine the carbonyl compound that forms 2-ethyl-3-hydroxyhexanal upon reaction with dilute NaOH, we need to consider the aldol condensation reaction. In aldol condensation, two aldehyde or ketone molecules react in the presence of a base to form a β-hydroxy aldehyde or ketone.

Let  us break down the structure of 2-ethyl-3-hydroxyhexanal to identify the starting carbonyl compounds:

This suggests that the product was formed by the aldol condensation of two smaller carbonyl compounds.

Step-by-Step Analysis:

1. Identify the β-hydroxy aldehyde/ketone structure:

The structure 2-ethyl-3-hydroxyhexanal indicates that the hydroxyl group \((OH)\) is on the third carbon, and the ethyl group is on the second carbon, relative to the aldehyde group \((CHO)\).

2. Determine the fragments:

The product can be seen as a combination of two aldehyde molecules where one molecule adds to another at the alpha-carbon position.

3. Possible starting compounds:

If we deconstruct 2-ethyl-3-hydroxyhexanal, we get two potential aldehydes: One part of the molecule \((CH_3-CH_2-CHO) corresponds to butanal. Another part corresponds to a similar structure that would provide the ethyl and hydroxy groups.

4. Reaction type:

For an aldol condensation to form 2-ethyl-3-hydroxyhexanal, the original aldehyde should have been butanal \((CH_3CH_2CH_2CHO)\).

Here is how the reaction would proceed:

Two molecules of butanal \((CH_3CH_2CH_2CHO)\) react in the presence of dilute NaOH.

One molecule acts as a nucleophile, attacking the carbonyl carbon of the other molecule.

The result is the formation of the aldol product, which is 2-ethyl-3-hydroxyhexanal.

The carbonyl compound that reacts with dilute NaOH to give 2-ethyl-3-hydroxyhexanal is: Butanal