The Cannizzaro reaction is a chemical reaction that involves the base-induced disproportionation of an aldehyde to yield a primary alcohol and a carboxylic acid. This reaction is unique to aldehydes that do not have alpha-hydrogens. Let's discuss the steps involved in the Cannizzaro reaction to identify the slowest step:
- The first step is the nucleophilic attack by a hydroxide ion (\(OH^-\)) on the carbonyl carbon of the aldehyde. This forms an alkoxide intermediate.
- The second step, which is the key step of this reaction, involves the transfer of a hydride ion from one alkoxide molecule to another molecule of the same aldehyde, generating an alkoxide ion and a carboxylate ion. This step is actually the slowest and rate-determining step of the Cannizzaro reaction.
- In the final step, the alkoxide ion formed is protonated to yield the alcohol and the carboxylate ion becomes the carboxylic acid upon protonation.
Thus, the slowest step in the Cannizzaro reaction is the hydride ion transfer.
Let's rule out the other options for clarity:
- Attack of \(OH^-\) on carbonyl: While crucial, this is a relatively fast step due to the strong nucleophilicity of hydroxide ions.
- Proton abstraction from acid: Does not occur as a slow step because acids generally protonate quickly.
- Deprotonation of alcohol: This step follows after the hydride transfer and is not the rate-determining step.
This examination of the mechanism confirms that the correct answer is hydride ion transfer.