Perkin reaction template

Acetate forms the enolate of acetic anhydride, which adds to benzaldehyde; the alkoxide is acetylated, E1cB loss of acetate gives the C=C, and hydrolysis of the anhydride gives (E)-cinnamic acid.

Mechanism step by step

An aromatic aldehyde and an anhydride with its sodium salt give a cinnamic acid

1. Enolate – Acetate removes an α-H of the anhydride
2. Addition – The enolate attacks the aldehyde carbon
3. Acetylation – The alkoxide attacks acetic anhydride
4. Collapse – C=O re-forms; acetate leaves
5. Enolate – Acetate removes an α-H next to the anhydride C=O
6. Elimination (E1cB) – Acetate leaves: the conjugated C=C
7. Hydrolysis – In the aqueous work-up hydroxide attacks the acetyl C=O
8. Cinnamate – C=O re-forms and cinnamate leaves; acid then gives cinnamic acid

Key points

• The base is the sodium salt of the acid that forms the anhydride (CH₃CO₂Na with (CH₃CO)₂O).
• Only aromatic aldehydes (no α-H) are used, so the aldehyde cannot form an enolate itself.
• The new C=C is E (trans): the phenyl and the carboxyl group end up on opposite sides.
• Gives cinnamic acids and coumarin (from salicylaldehyde).

Drawn with 272 atoms, 25 arrows, 14 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Furniss et al., Vogel's Textbook of Practical Organic Chemistry, 5th ed., Longman 1989 Textbook