Description: The Cannizarro reaction is a “disproportionation” reaction between two equivalents of an aldehyde, resulting in a carboxylic acid and an alcohol. One equivalent of aldehyde is oxidized, and the other is reduced.
Notes: The product reflects two equivalents of the aldehyde reacting with each other. Strong base (such as NaOH or KOH) is required along with considerable heat. Note that one aldehyde has been oxidized to an ester, and the other reduced to an alcohol.
The most important constraint in this reaction is that the aldehyde lacks a hydrogen on the alpha carbon (in other words, that it is not enolizable). If an enolate can be formed, the aldol reaction will occur instead.
Notes: Example 1 shows the products formed without an acid workup (note that we have the conjugate base of the carboxylic acid). Example 2 shows addition of acid at the end (acid workup) which yields the carboxylic acid. Note that in the first two cases, the aldehyde is simply heated with strong base. No aldol reaction is possible because an enolate cannot be formed from either of these aldehydes. In contrast, the bottom aldehyde does not undergo the Cannizarro – it will proceed through enolization and subsequent aldol reaction instead.
Mechanism: In the first step of this mechanism, hydroxide ion attacks the carbonyl carbon of the aldehyde (Step 1, arrows A and B). Then, in the key step, the C-O π bond is reformed at the same time as the C1-H [“hydride”] is transferred to a second equivalent of aldehyde. [Step 2, arrows C, D, and E]. This results in oxidation of C-1 [forming a carboxylic acid] and reduction of C-2 [forming an alkoxide (deprotonated alcohol)]. Also note that acid-base equilibrium favours formation of a carboxylate and the alcohol (Step 3, arrows F and G) [stronger acid + stronger base –> weaker acid and weaker base]. Acid workup [H+] gives the carboxylic acid.
Notes: The Cannizarro is not a very general reaction nor a particularly useful one, and its use is being de-emphasized in textbooks.