Chemistry - Which compound reacts faster in the Cannizzaro Reaction?

Solution 1:

The mechanism of the Cannizzaro reaction is illustrated below.

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The first step involves attack by the nucleophilic hydroxide ion on the positively polarized carbonyl carbon to form a tetrahedral intermediate. Once the tetrahedral intermediate is formed, substituents on the aromatic ring can have little resonance interaction with the former carbonyl carbon because it is now $\ce{sp^3}$ hybridized. So we need to look at the step leading to the tetrahedral intermediate - the first step.

However, if we examine some of the resonance structures for the starting carbonyl compounds (where the substituents on the aromatic ring can interact with the carbonyl through resonance) we see that the nitro compound places positive charge adjacent to the already positive polarized carbonyl carbon - a destabilizing situation (remember, destabilizing something means making it higher energy, more reactive). On the other hand, the methoxy compound places negative charge adjacent to the already positive polarized carbonyl carbon - a stabilizing situation.

The destabilized carbonyl in the nitro compound will be more reactive towards nucleophilic attack.

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Solution 2:

Looking at the reaction after the fact (always easier!): Two factors favoring compound a are (1) the equilibrium between the aldehyde and its adduct with hydroxide ion will more strongly favor the adduct when it has an electron-withdrawing p-nitro group; (2) once that intermediate is formed, it will be faster to undergo hydride transfer to another molecule of the aldehyde, when the latter has an electron-withdrawing group (nitro).