Question:
In the Cannizzaro reaction, the slowest step is:
In the Cannizzaro reaction, the slowest step is:
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In the Cannizzaro reaction, the rate-determining step is hydride ion transfer from the tetrahedral intermediate to another aldehyde molecule.
Updated On: Apr 16, 2026
- attack of \(OH^-\) on carbonyl
- hydride ion transfer
- proton abstraction from acid
- deprotonation of alcohol
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The Correct Option is B
Solution and Explanation
Concept:
The Cannizzaro reaction is a disproportionation reaction of aldehydes without \(\alpha\)-hydrogen atoms (e.g., formaldehyde, benzaldehyde) in the presence of concentrated alkali. One molecule of aldehyde is reduced to alcohol, and another is oxidized to carboxylic acid.
Step 1: Mechanism overview. The reaction proceeds through the following steps:
• Nucleophilic attack of \(OH^-\) on the carbonyl carbon, forming a tetrahedral intermediate (hemiacetal anion).
• Hydride ion (\(H^-\)) transfer from this intermediate to another aldehyde molecule.
• Proton abstraction from the carboxylic acid by the alkoxide to form carboxylate salt.
Step 2: Identify the rate-determining step. The rate-determining step (slowest step) is the hydride ion transfer. This step involves breaking the C-H bond and transferring the hydride to another aldehyde. It has a high activation energy because hydride is a poor leaving group and the transfer requires proper orbital alignment.
Step 3: Why not the other steps?
• (A) Attack of \(OH^-\) on carbonyl — fast, as \(OH^-\) is a strong nucleophile.
• (C) Proton abstraction from acid — occurs after the slow step, typically fast.
• (D) Deprotonation of alcohol — occurs after the slow step, fast.
Step 4: Conclusion. Since hydride transfer has the highest activation energy, it is the slowest step and determines the overall reaction rate.
Step 1: Mechanism overview. The reaction proceeds through the following steps:
• Nucleophilic attack of \(OH^-\) on the carbonyl carbon, forming a tetrahedral intermediate (hemiacetal anion).
• Hydride ion (\(H^-\)) transfer from this intermediate to another aldehyde molecule.
• Proton abstraction from the carboxylic acid by the alkoxide to form carboxylate salt.
Step 2: Identify the rate-determining step. The rate-determining step (slowest step) is the hydride ion transfer. This step involves breaking the C-H bond and transferring the hydride to another aldehyde. It has a high activation energy because hydride is a poor leaving group and the transfer requires proper orbital alignment.
Step 3: Why not the other steps?
• (A) Attack of \(OH^-\) on carbonyl — fast, as \(OH^-\) is a strong nucleophile.
• (C) Proton abstraction from acid — occurs after the slow step, typically fast.
• (D) Deprotonation of alcohol — occurs after the slow step, fast.
Step 4: Conclusion. Since hydride transfer has the highest activation energy, it is the slowest step and determines the overall reaction rate.
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