Give the expected products for the aldol condensations of (c) pentan-3-one.

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Identify the alpha-hydrogens in pentan-3-one. Alpha-hydrogens are the hydrogens attached to the carbon atoms adjacent to the carbonyl group. In pentan-3-one, the alpha-hydrogens are on carbons 2 and 4.

Determine the enolate ion formation. Under basic conditions, one of the alpha-hydrogens is removed by a base, forming an enolate ion. The enolate ion is stabilized by resonance between the alpha-carbon and the carbonyl oxygen.

Perform the nucleophilic attack. The enolate ion acts as a nucleophile and attacks the carbonyl carbon of another molecule of pentan-3-one, forming a beta-hydroxyketone intermediate.

Dehydrate the beta-hydroxyketone. Under heating or continued basic conditions, the beta-hydroxyketone undergoes elimination of water (E1cB mechanism), forming an α,β-unsaturated ketone as the final product.

Write the final product structure. The product will be a conjugated enone, specifically 4-methylpent-3-en-2-one, resulting from the aldol condensation of pentan-3-one with itself.

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Aldol Condensation

Aldol condensation is a reaction between aldehydes or ketones that contain alpha-hydrogens, leading to the formation of β-hydroxy carbonyl compounds (aldols) followed by dehydration to yield α,β-unsaturated carbonyl compounds. This reaction is significant in organic synthesis as it allows for the formation of larger carbon skeletons and introduces functional groups.

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Crossed Aldol

Enolate Ion Formation

In aldol condensation, the first step involves the formation of an enolate ion from a carbonyl compound, which occurs when a base abstracts an alpha-hydrogen. This enolate ion acts as a nucleophile, attacking the carbonyl carbon of another molecule, leading to the formation of the aldol product. Understanding this step is crucial for predicting the products of the reaction.

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02:26

Formation of Enolates

Dehydration Reaction

After the aldol product is formed, it can undergo dehydration, where a molecule of water is eliminated, resulting in the formation of an α,β-unsaturated carbonyl compound. This step is essential as it stabilizes the product and enhances its reactivity, making it a key feature of aldol condensations. Recognizing the conditions that favor dehydration is important for predicting the final products.

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