Question
How do we decide which name reaction applies to a given carbonyl compound? Summarise the Aldol, Cannizzaro, Perkin, Claisen, and Reformatsky reactions with their conditions and applicability.
Solution — Step by Step
Requirement: The aldehyde or ketone must have at least one alpha-hydrogen (H on the carbon adjacent to C=O).
Conditions: Dilute NaOH or dilute HCl (base or acid catalysed).
What happens: Two molecules combine — one acts as nucleophile (enolate from alpha-H), the other as electrophile (carbonyl carbon). Product is a beta-hydroxy aldehyde/ketone (aldol). On heating, it loses water to give an alpha,beta-unsaturated carbonyl (aldol condensation).
Example:
Requirement: The aldehyde must lack alpha-hydrogens (like HCHO, benzaldehyde, trimethylacetaldehyde).
Conditions: Concentrated NaOH.
What happens: One molecule gets oxidised to a carboxylate and the other gets reduced to an alcohol. It is a disproportionation reaction.
Example:
Requirement: An aromatic aldehyde (like benzaldehyde) reacting with an acid anhydride.
Conditions: Sodium salt of the acid (e.g., CH3COONa) as base, heat.
What happens: The anhydride provides the alpha-H (through its enolate), which attacks the aromatic aldehyde. Product is an alpha,beta-unsaturated acid (cinnamic acid derivative).
Example:
Requirement: An ester with alpha-hydrogens.
Conditions: Strong base like sodium ethoxide ().
What happens: One ester molecule’s alpha-H forms an enolate that attacks another ester’s carbonyl. Product is a beta-keto ester.
Example:
graph TD
A{Does carbonyl have alpha-H?} -->|Yes| B{What is the substrate?}
A -->|No| C[Cannizzaro reaction]
B -->|Aldehyde/Ketone + dil. NaOH| D[Aldol condensation]
B -->|Ester + NaOEt| E[Claisen condensation]
B -->|ArCHO + anhydride| F[Perkin reaction]
B -->|Aldehyde/Ketone + Zn/BrCH2COOR| G[Reformatsky reaction]
C --> H[conc. NaOH required]
D --> I[beta-hydroxy carbonyl product]
E --> J[beta-keto ester product]Why This Works
The central question for all these reactions is: does the compound have an alpha-hydrogen?
- Alpha-H present: Aldol, Claisen, Perkin, or Reformatsky — the alpha-H forms an enolate that attacks another carbonyl
- No alpha-H: Cannizzaro — disproportionation is the only option under basic conditions
The Reformatsky reaction uses an organozinc reagent (, formed from Zn + alpha-bromo ester) to add to an aldehyde or ketone. It gives a beta-hydroxy ester — similar to Aldol but uses zinc chemistry instead of enolate chemistry.
Alternative Method
A quick “when to use” table for exam revision:
| Reaction | Substrate | Reagent/Conditions | Product |
|---|---|---|---|
| Aldol | Aldehyde/ketone with alpha-H | Dil. NaOH/HCl | Beta-hydroxy carbonyl |
| Cannizzaro | Aldehyde WITHOUT alpha-H | Conc. NaOH | Acid salt + Alcohol |
| Perkin | ArCHO | Acid anhydride + acid salt | Alpha,beta-unsaturated acid |
| Claisen | Ester with alpha-H | NaOEt | Beta-keto ester |
| Reformatsky | Aldehyde/ketone | Zn + alpha-bromo ester | Beta-hydroxy ester |
Common Mistake
The most common error is attempting an Aldol reaction on benzaldehyde or formaldehyde. Benzaldehyde has no alpha-hydrogen — it undergoes Cannizzaro, not Aldol. HCHO also has no alpha-H and gives Cannizzaro. If a question gives “benzaldehyde + dil. NaOH” and asks for the product, the answer is Cannizzaro products (sodium benzoate + benzyl alcohol), NOT an aldol product.