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

The Cannizzaro reaction involves the base-induced disproportionation of non-enolizable aldehydes to produce an alcohol and a carboxylate. For benzaldehyde (Ph-CHO), in the presence of a strong base like OH-, one part is oxidized to PhCO_2^- (a carboxylate) and another is reduced to Ph-CH_2OH (an alcohol).

Step 2: Identifying Reaction Mechanism Steps

The general mechanism of the Cannizzaro reaction involves the following steps: 1) attack of OH- at the carbonyl carbon, 2) hydride transfer from one aldehyde molecule to another, 3) deprotonation of the resulting alcohol, and 4) abstraction of a proton from carboxylic acid equilibrium.

Step 3: Determining the Rate-Determining Step

The rate-determining step (slowest step) is usually the step that requires the largest energy barrier to overcome. In the Cannizzaro reaction, the key step that requires significant rearrangement of electrons is the transfer of a hydride ion from one molecule to another; this is a concerted process involving a rearrangement of covalent bonding.

Step 4: Evaluating Given Options

Given the options: (a) is the hydroxide attack, (b) is the hydride transfer, (c) abstraction of proton from carboxylic acid involves simple deprotonation which is typically faster, and (d) is further deprotonation of the alcohol which is also relatively fast compared to hydride transfers. Considering the energy barriers involved, option (b), the transfer of hydride ion, is typically the slowest step.

Key Concepts

mechanism of Cannizzaro reaction

The Cannizzaro reaction is an intriguing process that allows for the transformation of aldehydes into an alcohol and a carboxylate. This reaction occurs under basic conditions, specifically utilizing a strong base such as hydroxide ion ( OH^- ). The magic of this reaction lies in its stepwise mechanism, which involves a couple of finely orchestrated steps. Each step leads to a fascinating conversion of molecules, showcasing the transformative power of chemistry. Initially, the OH^- ion attacks the carbonyl carbon of a non-enolizable aldehyde. This creates a charged intermediate known as a "tetrahedral" intermediate. This step is crucial for setting the stage for the subsequent transformations. Following this, a remarkable event occurs: the transfer of a hydride ion. In this step, a hydride ion ( H^- ) shifts from one aldehyde molecule to another, leading to the reduction of one molecule to alcohol and the oxidation of the other to a carboxylate. Lastly, there may be further exchanges involving the deprotonation of the alcohol and the possible deprotonation of the acid product, completing the transformation process.

hydride transfer

The core of the Cannizzaro reaction mechanism is the hydride transfer step, a fascinating moment where chemistry truly comes alive. A hydride ion, represented as H^{−} , is a unique species. Its significance stems from its identity as a hydrogen atom carrying an extra electron, essentially a negatively charged hydrogen. During the reaction, after the initial formation of the tetrahedral intermediate, this hydride ion is transferred from one aldehyde molecule to another. This may seem trivial, but it's a chemically demanding process. The hydride transfer is crucial as it allows the simultaneous oxidation and reduction of the reactant molecules. One aldehyde donates a hydrogen ion to another, leading to the creation of an alcohol and a carboxylate ion. This step necessitates a careful rearrangement of electrons and bonds, resulting in the conservation of the carbon skeleton's integrity while altering the location and oxidation states of hydrogens. It requires significant energy and precision, making it a fundamental part of the reaction's complexity and importance.

rate-determining step

In the world of chemical reactions, the rate-determining step is the "bottleneck." It's the slowest step that governs the overall speed of the reaction. Understanding this step is crucial in reactions like the Cannizzaro reaction, where several transformations occur in sequence. For the Cannizzaro reaction, the rate-determining step is the aforementioned hydride transfer. This step involves a delicate and energetically demanding rearrangement of chemical bonds and electrons.

• The hydride transfer involves breaking and forming covalent bonds.
• It requires overcoming a significant energy barrier to rearrange the electrons and bond configurations.
• The transition state, where old bonds are partially broken and new ones are forming, represents the maximum energy point.

These factors combine to make the hydride transfer the slowest step in the pathway, dictating the reaction's overall pace. By knowing this, chemists can design catalysts or modify conditions to potentially speed up this step, thus enhancing the reaction efficiency.