Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

In the Cannizzaro reaction given below, \(2 \mathrm{Ph}-\mathrm{CHO} \stackrel{\mathrm{OH}^{-}}{\longrightarrow} \mathrm{Ph}-\mathrm{CH}_{2} \mathrm{OH}+\mathrm{PhCO}_{2}^{-}\) the slowest step is (a) the attack of \(-\mathrm{OH}\) at the carbonyl group (b) the transfer of hydride ion to the carbonyl group (c) the abstraction of proton from the carboxylic acid (d) the deprotonation of \(\mathrm{Ph}-\mathrm{CH}_{2} \mathrm{OH}\).

Step-by-Step Solution

Answer

(b) the transfer of hydride ion to the carbonyl group.

Step 1: Understanding the Cannizzaro Reaction

In a Cannizzaro reaction, non-enolizable aldehydes undergo self-oxidation and reduction in the presence of a strong base (like OH^-). In this reaction, one aldehyde molecule is reduced to an alcohol, and another is oxidized to a carboxylate ion.

Step 2: Identify Key Reaction Steps

The key reaction steps for the Cannizzaro reaction are: (1) the nucleophilic attack of OH^- on the carbonyl carbon of the aldehyde, (2) the transfer of a hydride ion from one aldehyde molecule to another, and (3) the deprotonation of the resulting alcohol.

Step 3: Analyzing the Slowest Step

In the Cannizzaro reaction, the slowest step is typically the transfer of the hydride ion between two aldehyde molecules. Hydride ion transfer is generally slower compared to the initial nucleophilic attack by OH^-.

Key Concepts

Aldehyde Chemistry

Aldehydes are an important class of organic compounds with the general formula \(RCHO\), where \(R\) is an organic group and \(CHO\) is the aldehyde functional group. These compounds are known for their reactivity, mainly due to the presence of the polar carbonyl group (\(C=O\)).
The carbon in the carbonyl group is partially positive, making it susceptible to attacks by nucleophiles.

• The carbonyl group in aldehydes plays a crucial role in various organic reactions, including nucleophilic addition and oxidation-reduction reactions.
• Aldehydes can participate in unique reactions like the Cannizzaro Reaction, which is typical for non-enolizable aldehydes.

The Cannizzaro Reaction is a noteworthy example of aldehyde chemistry, illustrating both oxidation and reduction in a single process without the formation of enolate ions.

Hydride Ion Transfer

Hydride ion transfer is a distinctive and fascinating step in many organic reactions, particularly in the Cannizzaro Reaction. A hydride ion (\(H^-\)) is essentially a hydrogen atom with an additional electron.
This step involves the transfer of a hydride ion from one molecule to another, and often, this process has a significant impact on the speed of the reaction. The hydride ion transfer is the rate-determining step in the Cannizzaro Reaction.

• This step is typically slower because it involves a precise movement and interaction between molecules.
• Effective hydride transfer requires the proper alignment and energy status of the participating molecules.

The slowness of this step makes it a crucial factor in the overall reaction rate.

Non-enolizable Aldehydes

Non-enolizable aldehydes are aldehydes lacking a hydrogen atom on the carbon adjacent to the carbonyl group (also known as the \(\alpha\)-carbon).
This feature means that these aldehydes cannot form enolates, a common species in many other reactions involving aldehydes or ketones.

• The absence of this \(\alpha\)-hydrogen eliminates the possibility of tautomerization to enols.
• Without enol formation, the reaction pathway of non-enolizable aldehydes differs, leading to unique reactions such as the Cannizzaro Reaction.

The inability of non-enolizable aldehydes to undergo keto-enol tautomerism makes them ideal candidates for reactions like the Cannizzaro, where the typical keto-enol equilibrium is bypassed.

Nucleophilic Attack

A nucleophilic attack is a primary method by which chemical reactions occur, especially in the realm of carbonyl chemistry. This involves a nucleophile, which is an electron-rich species, attacking an electrophile, such as a carbon atom in a carbonyl group.
In the Cannizzaro Reaction, the \(OH^-\) ion acts as a nucleophile and attacks the electron-deficient carbon atom of the carbonyl group in the aldehyde.

• This step sets the stage for further transformation, as the nucleophilic attack forms a tetrahedral intermediate.
• The initial nucleophilic attack is typically faster than the hydride transfer step, mainly due to the strong driving force of electron-pair sharing.

The ability of nucleophiles to effectively attack electrophilic centers is essential for many organic transformation processes, including the Cannizzaro Reaction.