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 \(\mathrm{Ph}-\mathrm{CH}_{2} \mathrm{OH}\).

Short answer

The slowest step is the transfer of hydride ion to the carbonyl group (b).

Step-by-step solution

Understanding the Cannizzaro Reaction

The given reaction is the Cannizzaro reaction, which involves the disproportionation of an aldehyde without a hydrogen atom on the alpha carbon. It leads to the formation of a primary alcohol and a carboxylate anion from two aldehyde molecules.

Analyze the Reaction Mechanism

In the Cannizzaro reaction mechanism, the first step involves the nucleophilic attack by the hydroxide ion \( \mathrm{OH}^- \) on the carbonyl carbon of one benzaldehyde molecule. This is followed by a hydride transfer to the carbonyl group of the second molecule.

Identify the Slowest Step

The slowest, and thus rate-determining, step in reactions typically involves the movement or transfer of hydride ions, which is often a challenging step because it requires breaking a strong bond. In this reaction, the transfer of a hydride ion to the carbonyl group is the slowest step.

Key concepts

Disproportionation

The term "disproportionation" refers to a type of redox reaction wherein a single compound undergoes simultaneous oxidation and reduction. In the context of the Cannizzaro reaction, this phenomenon is evident. Here, two molecules of an aldehyde decompose into two different products: a carboxylate ion and an alcohol. Both processes occur simultaneously. One aldehyde molecule undergoes oxidation to form a carboxylate, while the other is reduced to form an alcohol.

This dual path of redox transformation is central to the Cannizzaro reaction.

• The compound being transformed (aldehyde) has to have no hydrogen atoms on the alpha carbon.
  
• Disproportionation is an essential step in understanding how the Cannizzaro reaction leads to the distinct products from identical starting materials.
  
• The process facilitates the division of tasks within a single substrate, showcasing the unique redox balance.
  

Aldehydes

Aldehydes are organic compounds characterized by the presence of a carbonyl group ( C=O) bonded to at least one hydrogen atom. In the Cannizzaro reaction, we specifically look at aldehydes that lack an alpha hydrogen. This feature is crucial, as it prohibits enol formation, which can potentially undergo other reaction pathways, like aldol condensation.

As such, aldehydes in the Cannizzaro reaction are perfectly poised for disproportionation without diverting into unwanted side reactions.

• Common examples include formaldehyde and benzaldehyde.
• The absence of an alpha hydrogen makes these aldehydes ideal candidates for the Cannizzaro reaction.
• They play a central role as the starting materials and directly influence the types of products formed post-reaction.

Reaction Mechanism

The Cannizzaro reaction mechanism is structured around specific interactions between molecules, predominantly shaped by nucleophilic activities. Initially, the nucleophilic hydroxide ion ( OH^- ) attacks the carbonyl carbon of the aldehyde.
This forms an alkoxide intermediate. Following the formation of this intermediate, a hydride ion transfer takes place, marking the shift in balance between the two aldehyde molecules.

• The nucleophilic attack is facilitated by the electron-deficient nature of the carbonyl carbon, making it susceptible to OH^- < attack.
• The hydride transfer from one molecule to the other is a critical step, ensuring the reduction of one aldehyde to an alcohol, while the other is oxidized to a carboxylate ion.
• The interplay of electron-rich and electron-deficient centers drives this chemical process forward.

Hydride Transfer

In the Cannizzaro reaction, the slowest step is the transfer of a hydride ion ( H^-) from one aldehyde molecule to another. This step is pivotal as it determines the reaction rate. The hydride transfer involves a critical redistribution of electrons that's necessary for the redox change via the Cannizzaro route.

Understanding the importance of hydride transfer elucidates why this step is rate-limiting:

• Hydride ions are not freely floating entities; they are usually strongly bonded within organic compounds.
• Transferring a hydride often demands breaking and reforming robust carbon-hydrogen bonds, a process that inherently requires significant energy.
• Successfully meeting the energy requirements for this bond maneuver ensures that one aldehyde can reduce, while the other oxidizes, synthesizing both the alcohol and carboxylate ion under basic conditions.