Question

In Cannizzaro reaction given below \(2 \mathrm{Ph} \mathrm{CHO} \stackrel{\stackrel{\ominus}{\mathrm{OH}}}{\longrightarrow} \mathrm{PhCH}_{2} \mathrm{OH}+\mathrm{PhCO}_{2}^{\Theta}\) the slowest step is : (1) the transfer of hydride to the carbonyl group (2) the abstraction of proton from the carboxylic group (3) the deprotonation of \(\mathrm{PhCH}_{2} \mathrm{OH}\) (4) the attack of : \(\stackrel{\Theta}{\mathrm{OH}}\) at the carboxyl group

Short answer

The slowest step is the transfer of hydride to the carbonyl group (Option 1).

Step-by-step solution

- Understand the Reaction

The Cannizzaro reaction involves the reaction of an aldehyde with no α-hydrogens under basic conditions to give a primary alcohol and a carboxylate salt. The reaction in this scenario involves benzaldehyde reacting with hydroxide ion to yield benzyl alcohol and benzoate ion.

- Identify Potential Slow Steps

In the given reaction, the potential steps are: (1) hydride transfer from the benzaldehyde to another molecule of benzaldehyde, (2) proton abstraction from the carboxylic group, (3) deprotonation of benzyl alcohol, and (4) attack of hydroxide ion on the carboxyl group.

- Analyze Each Step

Evaluate the mechanisms of each step to determine which is likely to be the slowest. Typically, hydride transfer involves a high activation energy, making it slower compared to proton abstraction or nucleophilic attack steps.

- Conclude the Slowest Step

Based on the analysis, hydride transfer usually involves higher energy constraints and therefore is the rate-determining step in such organic reactions.

Key concepts

Organic Reaction Mechanism

Understanding organic reaction mechanisms is essential for mastering organic chemistry. Mechanisms detail the step-by-step processes of how reactants transform into products. For the Cannizzaro reaction, it's crucial to know that this reaction involves an aldehyde reacting under basic conditions to form both an alcohol and a carboxylate. The key steps include nucleophilic attack, hydride transfer, and proton transfers.

• Nucleophilic Attack: Here, the hydroxide ion attacks the carbonyl carbon of the aldehyde, forming a tetrahedral intermediate.
• Hydride Transfer: One molecule of aldehyde donates a hydride ion to another molecule of the aldehyde, leading to the formation of an alcohol and a carboxylate.

These steps require understanding the movement of electrons, formation of intermediates, and the energy changes throughout the process.

Hydride Transfer

Hydride transfer is crucial in the Cannizzaro reaction. In chemical terms, this step involves the movement of a hydride ion (H⁻) from one molecule to another. This is generally the slowest step in the reaction mechanism due to the high activation energy needed.

• Hydride Ion: A hydride ion is a negatively charged ion of hydrogen, denoted as H⁻. Unlike a proton (H⁺), which is a positive ion, a hydride carries an extra electron.
• Rate-Determining Step: The hydride transfer involves breaking a C-H bond and forming a new C-H bond, which is energetically costly and slow. This step's rate largely determines the overall reaction rate.

Hydride transfer often occurs with the aid of a base, which helps stabilize the transition state and facilitate the transfer.

Aldehyde Reaction

Aldehydes are reactive compounds that play a key role in many organic reactions, including the Cannizzaro reaction. An aldehyde is an organic compound containing a formyl group, written as R-CHO.

• Formyl Group: The functional group in an aldehyde consists of a carbonyl center (C=O) bonded to a hydrogen atom and an R group (which can be a hydrogen or a hydrocarbon chain).
• Example - Benzaldehyde: In the Cannizzaro reaction example provided, benzaldehyde (PhCHO) reacts. Here, the “Ph” stands for a phenyl group (C₆H₅-), illustrating an aromatic ring attached to the carbonyl group.
• Reactivity: Aldehydes are highly reactive towards nucleophiles due to the partial positive charge on the carbonyl carbon. This makes them prone to undergo nucleophilic addition reactions, such as those in the Cannizzaro reaction.

Understanding the behavior of aldehydes in reactions is foundational for predicting and explaining organic mechanisms like the Cannizzaro reaction.