Cannizzaro Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Cannizzaro Reaction The Cannizzaro reaction is a disproportionation reaction in which an aldehyde lacking alpha-hydrogens undergoes simultaneous oxidation and reduction in the presence of a strong base to yield a carboxylic acid (as its salt) and a primary alcohol. Typically, the disappearance of the aldehyde (often with a characteristic odor) and the formation of a primary alcohol and a carboxylate salt. The alcohol may be separable as an oily layer or by distillation. Often, there are no dramatic color changes. If a solid aldehyde (e.g., benzaldehyde derivatives) is used, the solid may dissolve over time as it reacts. The Cannizzaro reaction is overall thermodynamically favorable. The simultaneous oxidation of one aldehyde molecule and reduction of another is driven by the stability of the carboxylate anion product. ΔG is negative. Nucleophilic attack: A hydroxide ion attacks the carbonyl carbon of an aldehyde molecule, forming a tetrahedral intermediate (an anionic alkoxide). Hydride transfer: The tetrahedral intermediate expels a hydride ion. This hydride ion then attacks the carbonyl carbon of a second aldehyde molecule, simultaneously forming a carboxylate ion and an alkoxide ion (which rapidly protonates to form the alcohol). This is the rate-determining step. Proton transfer: In the final step, the alkoxide formed from the second aldehyde molecule rapidly picks up a proton from water or another acidic source (like a carboxylic acid, if formed first) to become the primary alcohol. The carboxylic acid, if formed, is immediately deprotonated by the strong base to yield the carboxylate salt. Confusing with Aldol Condensation: The crucial difference is the absence of alpha-hydrogens in Cannizzaro reactants, preventing enolization. Forgetting the disproportionation aspect: Students might only show reduction or oxidation, not both simultaneously. Incorrectly applying to aldehydes with alpha-hydrogens: This will lead to aldol products, not Cannizzaro products. Misidentifying products: Forgetting the formation of a carboxylate salt (not free acid) in basic conditions. In crossed Cannizzaro, aldehydes like formaldehyde (most reactive to nucleophilic attack) are preferentially oxidized to the carboxylate, while the less reactive aldehyde is reduced to the alcohol.