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

Question

In cannizaro reaction given below \(\mathrm{Ph} \mathrm{CHO} \stackrel{\odot}{\stackrel{\circ} \mathrm{OH}}{\longrightarrow} \mathrm{Ph} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{Ph} \ddot{\mathrm{C}} \mathrm{O}_{2}\), 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.

Step 1: Understanding the Cannizaro Reaction

The Cannizaro reaction involves the transfer of a hydride ion from one molecule of aldehyde to another, directly converting an aldehyde into both an alcohol and a carboxylate ion without any change in the number of carbon atoms, typically in the presence of a strong base.

Step 2: Analyzing the Steps Involved

In the Cannizaro reaction, some key steps are involved: (a) The presence of hydroxide ions initiates the nucleophilic attack on the carbonyl carbon. (b) Transfer of hydride occurs from one aldehyde molecule to another (c) Formation of a carboxylate ion and an alkoxide ion proceeds, and (d) Protonation of the alkoxide ion occurs leading to alcohol formation.

Step 3: Identifying the Slowest Step

The rate-determining step, or slowest step, in the Cannizaro reaction is the transfer of the hydride ion from the alkoxide ion to the carbonyl group of another aldehyde molecule. This is because hydride transfer, being an internal redox reaction, requires significant energy and is typically the slowest step in the reaction sequence.

Step 4: Conclusion Based on Options

Among the given options, (a) 'the transfer of hydride to the carbonyl group' accurately describes the slowest step of the Cannizaro reaction. This step involves substantial energy and is consistent with the mechanistic understanding of this reaction.

Key Concepts

Understanding Hydride Transfer in the Cannizaro Reaction

In the context of the Cannizaro reaction, hydride transfer is a crucial step where a hydride ion (H⁻) is transferred from one aldehyde molecule to another. This is an unusual feature, as it involves no change in the number of carbon atoms, setting it apart from other typical oxidation-reduction reactions. For a better grasp, imagine two aldehyde molecules: molecule A acts as a hydride donor, and molecule B as the recipient. During the reaction, molecule A donates a hydrogen atom along with its pair of electrons to the carbonyl group of molecule B.
This step converts one of the aldehyde molecules into an alkoxide ion, which further aids in forming the carboxylate and alcohol products.

• Key Points:
• Hydride transfer involves the movement of a hydrogen atom with two electrons.
• This leads to the formation of both alcohol and carboxylate products.
• It is one of the identifying steps of the Cannizaro reaction and requires notable energy input.

Rate-Determining Step in Cannizaro Reaction

The rate-determining step is an essential concept in chemical kinetics, signifying the slowest step in a sequence that dictates the overall reaction rate. In the Cannizaro reaction, the rate-determining step is the hydride transfer from an alkoxide ion to another aldehyde molecule's carbonyl group. This internal redox step typically demands significant energy due to the breaking and forming of bonds within the same molecular framework.
Understanding this step is vital because it determines how quickly the reaction proceeds overall. If you think of the reaction as a multi-lane highway, this slow transfer is the bottleneck, controlling the traffic, or in this case, the reaction pace.

• The importance of the rate-determining step lies in its influence on the reaction kinetics.
• It often requires careful adjustment of reaction conditions to optimize.

The Role of Nucleophilic Attack in the Cannizaro Reaction

A nucleophilic attack is a critical step in many organic reactions, where a nucleophile seeks to donate a pair of electrons to an electrophile, forming a new bond. In the Cannizaro reaction, it begins with the hydroxide ion (OH⁻) attacking the carbonyl carbon of an aldehyde. This attack is foundational in breaking the carbon-oxygen double bond partially, allowing molecular rearrangements essential for further steps.
The nucleophilic attack creates an alkoxide intermediate, priming it for the subsequent hydride transfer. Understanding this initial step is crucial because it sets the stage for all ensuing transformations in the reaction mechanism.

• Essentials of Nucleophilic Attack:
• This step introduces the nucleophilic entity (OH⁻) into the reaction framework.
• It is characterized by the formation of a new bond and structural rearrangement.
• It sets up the molecular environment needed for successful hydride transfer.