Aldol condensation template

Two acetaldehydes give crotonaldehyde: enolate formation, addition, protonation, deprotonation and loss of hydroxide.

Mechanism step by step

Aldol addition followed by base-promoted loss of water (E1cB) to an enal

1. Enolate – Hydroxide removes an α-H; the charge moves onto O
2. Addition – The enolate attacks a second aldehyde
3. Protonation – Water gives the aldol (β-hydroxy aldehyde)
4. Second enolate – On heating, another α-H is removed
5. Loss of OH⁻ – E1cB: the C=C forms, conjugated with C=O

Key points

• Room temperature stops at the aldol; heating drives the condensation.
• Conjugation of C=C with C=O makes the enal the stable product.
• Crossed aldols work best when one partner has no α-H (e.g. benzaldehyde).
• Intramolecular versions make five- and six-membered rings.

Drawn with 75 atoms, 18 arrows, 7 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 26 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020 Textbook