Aldehydes that have no αhydrogen undergo an intermolecular...

Aldehydes that have no $\alpha$ hydrogen undergo an intermolecular oxidation-reduction called the Cannizzaro reaction when they are treated with concentrated base. An example is the following reaction of benzaldehyde: CAN'T COPY THE FIGURE (a) When the reaction is carried out in $D_{2} O$, the benzyl alcohol that is isolated contains no deuterium bound to carbon. It is $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OD}$. What does this suggest about the mechanism for the reaction? (b) When (CH $_{3}$ ) $\mathrm{CHCHO}$ and $\mathrm{Ba}(\mathrm{OH})_{2} / \mathrm{H}_{2} \mathrm{O}$ are heated in a sealed tube, the reaction produces only $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{OH}$ and $\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCO}_{2} \mathrm{l}_{2} \mathrm{Ba} .$ Provide an explanation for the formation of these products. \right.

Aldehydes that have no $\alpha$ hydrogen undergo an intermolecular oxidation-reduction called the Cannizzaro reaction when they are treated with concentrated base. An example is the following reaction of benzaldehyde:
CAN'T COPY THE FIGURE
(a) When the reaction is carried out in $D_{2} O$, the benzyl alcohol that is isolated contains no deuterium bound to carbon. It is $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OD}$. What does this suggest about the mechanism for the reaction?
(b) When (CH $_{3}$ ) $\mathrm{CHCHO}$ and $\mathrm{Ba}(\mathrm{OH})_{2} / \mathrm{H}_{2} \mathrm{O}$ are heated in a sealed tube, the reaction produces only $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{OH}$ and $\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCO}_{2} \mathrm{l}_{2} \mathrm{Ba} .$ Provide an explanation for the formation of these products. \right.

Key Concepts

Substituent Effects and Regioselectivity in Aldehyde Reactions

Variations in aldehyde structure, including the presence of substituents that alter steric and electronic environments, can influence the course of the Cannizzaro reaction. Such effects determine which molecule of the aldehyde is more predisposed to act as the hydride donor or acceptor, thereby controlling the selectivity in the formation of the alcohol versus the carboxylate product.

Use of Isotopic Labeling (Deuterium Labeling) to Probe Mechanism

Incorporating deuterium, such as by using D2O as the solvent, serves as a diagnostic tool to elucidate reaction pathways. The absence of deuterium at the carbon center of the formed alcohol indicates that the hydride (or hydrogen) that reduces the carbonyl is transferred intramolecularly from another aldehyde molecule, rather than being abstracted directly from the solvent.

Cannizzaro Reaction

A reaction in which aldehydes that lack alpha-hydrogens undergo simultaneous oxidation and reduction in the presence of a strong base. One molecule of the aldehyde is oxidized to a carboxylate and another is reduced to an alcohol through a mechanism that involves deprotonation, formation of an alkoxide, and subsequent hydride transfer between molecules.

Hydride Transfer Mechanism

At the heart of the Cannizzaro reaction is the transfer of a hydride ion from one aldehyde molecule to another. This mechanism involves the nucleophilic attack of a hydroxide ion on the aldehyde to form an alkoxide intermediate, which then donates a hydride ion to a second aldehyde molecule, leading to the formation of an alcohol and a carboxylate, respectively.

Transcript

Please give Ace some feedback