Question 1

What must a carbonyl compound have to act as the donor in an aldol reaction?

Accepted Answer

The donor carbonyl compound must have at least one alpha-hydrogen. Base removes this hydrogen to form an enolate whose alpha-carbon acts as the nucleophile. The acceptor carbonyl compound does not need an alpha-hydrogen.

Question 2

Which atoms form the new carbon-carbon bond in aldol addition?

Accepted Answer

The new C-C bond joins the alpha-carbon of the donor enolate to the carbonyl carbon of the acceptor. The acceptor oxygen is not the attack site; its C=O bond opens and that oxygen becomes the beta-hydroxyl oxygen after protonation.

Question 3

Why does aldol condensation favor an alpha,beta-unsaturated aldehyde?

Accepted Answer

Heating promotes dehydration of the beta-hydroxy carbonyl product. In the basic E1cB pathway, an alpha-hydrogen is removed first to form a resonance-stabilised carbanion or enolate, followed by loss of hydroxide. The resulting C=C bond is conjugated with the carbonyl group.

Question 4

What is the self-aldol product of two acetaldehyde molecules?

Accepted Answer

Self-aldol addition of two molecules of acetaldehyde gives 3-hydroxybutanal, CH3CH(OH)CH2CHO. On heating it loses one molecule of water and forms crotonaldehyde, CH3CH=CHCHO.

Question 5

Why is formaldehyde a useful acceptor in a crossed aldol reaction?

Accepted Answer

Formaldehyde has no alpha-carbon and therefore no alpha-hydrogen, so it cannot form an enolate. It can still act as an electrophilic carbonyl acceptor when acetaldehyde supplies the enolate, helping produce the crossed aldol product HOCH2CH2CHO.

Aldol Condensation: Two Carbonyls, One New C-C Bond

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