Problem 229 - A Complete Resource Book in Chemistry for JEE Main

Question

In cannizaro reaction given below \(\mathrm{Ph} \mathrm{CHO} \stackrel{\odot}{\stackrel{\circ}{O} \mathrm{OH}}{\longrightarrow} \mathrm{Ph} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{Ph} \ddot{\mathrm{C}} \mathrm{O}_{2}\) the slowest step is: (a) the transfer of hydride to the carbonyl group (b) the abstraction of proton from the carboxylic group (c) the deprotonation of \(\mathrm{Ph} \mathrm{CH}_{2} \mathrm{OH}\) (d) the attack of : \(\mathrm{OH}\) at the carboxyl group

Step-by-Step Solution

Answer

(a) the transfer of hydride to the carbonyl group is the slowest step in the Cannizzaro reaction.

Step 1: Understand the Cannizzaro Reaction

The Cannizzaro reaction involves the disproportionation of an aldehyde lacking an α-hydrogen in the presence of a strong base, resulting in one molecule being reduced to an alcohol and another being oxidized to a carboxylic acid. This reaction typically involves the hydride ion transfer step as a crucial part of the mechanism.

Step 2: Identify Key Steps in the Cannizzaro Reaction

The two main steps in the Cannizzaro reaction are the nucleophilic attack of hydroxide ion on the carbonyl group and the transfer of a hydride ion from one aldehyde molecule to another. This hydride transfer is the key step responsible for reduction and oxidation.

Step 3: Evaluate Possible Slow Steps

In the options given, focus on the hydride transfer because it involves the breaking and forming of important bonds, making it a potential rate-determining step. On the other hand, proton transfer steps are generally fast and involve weaker interactions.

Step 4: Conclude the Slowest Step

Given that the hydride ion transfer involves significant bond changes, it is the most complex and thus the slowest step in the reaction.

Key Concepts

Hydride Ion Transfer

The hydride ion transfer plays a central role in the Cannizzaro reaction. This is the step where a hydride ion (represented as \(H^-\)) is transferred from one aldehyde molecule to another. During this transformation, one molecule of the aldehyde is reduced to form an alcohol, while the other is oxidized to create a carboxylate ion.
This process is essential because the hydride ion is a powerful reducing agent, making it capable of converting the aldehyde into its corresponding alcohol. By donating a negative hydrogen ion, it simultaneously allows for the creation of the carboxylate ion, demonstrating a unique balance between oxidation and reduction.

• The hydride ion transfer involves significant bond breaking and forming.
• It occurs in a two-part process, involving first the attack by the hydroxide ion, then the transfer of the hydride ion.

Understanding this transfer is crucial since it's the step that underlies the overall redox nature of the Cannizzaro reaction.

Disproportionation of Aldehyde

The Cannizzaro reaction is a fascinating example of a chemical process known as disproportionation. Disproportionation involves a single species undergoing both oxidation and reduction simultaneously. In the case of the Cannizzaro reaction, the species in question is an aldehyde lacking an α-hydrogen atom.
During the reaction, two molecules of the aldehyde are transformed: one molecule is reduced, forming an alcohol, while the other is oxidized to form a carboxylate ion. This type of reaction is unique because it doesn't require any external oxidizing or reducing agents—the aldehyde itself serves as both!

• One molecule acts as a reducing agent while another acts as an oxidizing agent.
• This reaction is most common with formaldehyde and benzaldehyde, where α-hydrogens are absent.

The disproportionation nature of this reaction provides insight into the dual capabilities of aldehydes, acting both as electron donors and acceptors within the same reaction system.

Mechanism of Cannizzaro Reaction

The mechanism of the Cannizzaro reaction begins with a hydroxide ion attacking the carbonyl carbon of the aldehyde, which leads to an alkoxide ion intermediate. This initial step is crucial as it sets the stage for the subsequent hydride ion transfer.
The alkoxide ion, a negatively charged oxygen species, is set up perfectly to facilitate the next key step. From this intermediate state, the hydride ion transfer occurs.

• The nucleophilic hydroxide attack is swift and leads to the unstable alkoxide.
• The alkoxide then donates a hydride ion to the carbonyl carbon of a neighboring aldehyde.

After this, the aldehyde that received the hydride ion is reduced to an alcohol, while the alkoxide gets further oxidized to create a carboxylate ion. Understanding this mechanism highlights the delicate balance of nucleophilic attack, intermediate formation, and crucial hydride transfer in achieving successful disproportionation of aldehydes.