Indicate the starting aldehyde or ketone from which each of the following compounds are formed by an aldol condensation reaction.
(a)2-Ethyl-3-hydroxy hexanal (b) 4-Hydroxy-4-methyl-2-pentanone

Answer

To determine the starting aldehyde or ketone for the given compounds formed through aldol condensation, we will analyze each compound step by step.

### Step-by-Step Solution:

**(a) For 2-Ethyl-3-hydroxyhexanal:**

1. **Identify the Structure:**
- The compound 2-Ethyl-3-hydroxyhexanal has a six-carbon chain with a hydroxyl group (OH) on the third carbon and an ethyl group on the second carbon. The aldehyde functional group is at the end of the chain.

2. **Determine the Aldol Reaction:**
- In an aldol condensation, two molecules of aldehyde or ketone react. The starting material must have an alpha hydrogen that can be removed to form an enolate ion.

3. **Possible Starting Material:**
- The compound can be formed from butanal (butyric aldehyde).
- When butanal (C4H8O) undergoes aldol condensation, it can react with itself. The reaction involves the formation of an enolate ion from one molecule of butanal, which then attacks the carbonyl carbon of another butanal molecule.

4. **Formation of the Product:**
- The aldol product formed will have a structure that matches 2-Ethyl-3-hydroxyhexanal after dehydration and rearrangement.

5. **Conclusion:**
- The starting aldehyde for 2-Ethyl-3-hydroxyhexanal is **butanal**.

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**(b) For 4-Hydroxy-4-methyl-2-pentanone:**

1. **Identify the Structure:**
- The compound 4-Hydroxy-4-methyl-2-pentanone has a five-carbon chain with a hydroxyl group on the fourth carbon and a methyl group also on the fourth carbon.

2. **Determine the Aldol Reaction:**
- This compound is a ketone, and for the aldol condensation, we need a ketone that has alpha hydrogens.

3. **Possible Starting Material:**
- The compound can be formed from acetone (2-propanone).
- When acetone (C3H6O) undergoes aldol condensation, it can react with itself. The reaction involves the formation of an enolate ion from one molecule of acetone, which then attacks the carbonyl carbon of another acetone molecule.

4. **Formation of the Product:**
- The aldol product formed will have a structure that matches 4-Hydroxy-4-methyl-2-pentanone after dehydration and rearrangement.

5. **Conclusion:**
- The starting ketone for 4-Hydroxy-4-methyl-2-pentanone is **acetone**.

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### Final Answers:
- (a) The starting aldehyde is **butanal**.
- (b) The starting ketone is **acetone**.