Problem 11 - Advanced Organic Chemistry. Part A. Structure and Mechanisms

Question

The Cannizzaro reaction is a disproportionation that takes place in strongly basic solution and converts aromatic aldehydes to the corresponding benzyl alcohol and sodium benzoate. $$ 2 \mathrm{ArCH}=\mathrm{O}+\mathrm{NaOH} \longrightarrow \mathrm{ArCH}_{2} \mathrm{OH}+\mathrm{ArCO}_{2} \mathrm{Na} $$ Several mechanisms, all of which involve a hydride transfer as a key step, have been postulated. On the basis of the following information, formulate one or more mechanisms that would be consistent with all the data provided. Indicate the significance of each observation with respect to the mechanism(s) you postulate. 1\. When the reaction is carried out in \(\mathrm{D}_{2} \mathrm{O}\), the benzyl alcohol contains no deuterium in the methylene group. 2\. When the reaction is carried out in \(\mathrm{H}_{2}{\underline{\phantom{xx}}}^{18} \mathrm{O}\), both the benzyl alcohol and sodium benzoate contain \({ }^{18} \mathrm{O}\). 3\. The overall reaction rate is given by the expression $$ \text { Rate }=k_{\mathrm{obs}}[\mathrm{Ph} \mathrm{CH}=\mathrm{O}]^{2}\left[{ }^{-} \mathrm{OH}\right] $$ 4\. The rates of substituted benzaldehydes are correlated by a Hammet LFER with \(\rho=+3.76\) 5\. The solvent isotope effect \(k_{\mathrm{D} 2 \mathrm{O}} / k_{\mathrm{H} 2 O}=1.90\)

Step-by-Step Solution

Answer

The mechanism involves aldehyde nucleophilic attack, hydride transfer, and oxygen exchange with the solvent, with the rate-determining step involving two aldehydes.

Step 1: Understand the Reaction

The Cannizzaro reaction involves the disproportionation of aromatic aldehydes in a strong base to yield benzyl alcohol and sodium benzoate. This means an aldehyde molecule is reduced to alcohol, while another one is oxidized to a carboxylic acid salt.

Step 2: Observation 1: Deuterium in D2O

The first observation tells us that when the reaction is conducted in D\(_2\)O, there is no deuterium in the methylene group of the benzyl alcohol. This suggests that hydride transfer does not involve exchange with the solvent hydrogen atoms.

Step 3: Observation 2: Oxygen-18 in H2O18

The second observation indicates that the oxygen from water \(\text{H}_2{}^{18}\text{O}\) integrates into both the benzyl alcohol and the sodium benzoate products. This suggests that the aldehydes get an oxygen from the solvent, implying that the carbonyl oxygen is replaced during the reaction.

Step 4: Observation 3: Reaction Rate Expression

The rate expression \(\text{Rate} = k_{\mathrm{obs}}[\mathrm{PhCH}=\mathrm{O}]^{2}[\mathrm{OH}^-]\) shows that the reaction is second-order in the aldehyde and first-order in hydroxide. This suggests a mechanism in which either two aldehydes or two steps involving aldehydes are involved in the rate-determining step.

Step 5: Observation 4: Hammet LFER

The Hammet correlation with a positive \(\rho=+3.76\) indicates that the reaction is sensitive to electron-withdrawing groups, suggesting the build-up of negative charge on the aromatic ring in the transition state.

Step 6: Observation 5: Solvent Isotope Effect

The solvent isotope effect \(k_{\mathrm{D} 2 \mathrm{O}} / k_{\mathrm{H} 2 \mathrm{O}}=1.90\) indicates that the reaction slows down in D\(_2\)O. This suggests that there is a proton transfer involved in the rate-determining step of the reaction.

Step 7: Proposed Mechanism

Based on these observations, one proposed mechanism is: (1) a nucleophilic attack by hydroxide on the carbon of the aldehyde to form a tetrahedral intermediate, (2) hydride transfer from one aldehyde molecule to another, facilitated by oxygen exchange with the solvent, forming the alcohol and acid salt. The rate-determining step involves either two aldehyde molecules participating concurrently or a transition state where the charge build-up occurs, involving proton transfer, consistent with the observations.

Key Concepts

Disproportionation

The Cannizzaro reaction is a classic example of a disproportionation process, which is the simultaneous oxidation and reduction of two identical molecules. In this specific reaction, aromatic aldehydes undergo disproportionation in a strongly basic environment to yield benzyl alcohol and a carboxylic acid. Here's how it works:
The aldehyde acts as both an electron donor and an electron acceptor. One aldehyde molecule gets reduced to benzyl alcohol, while the other is oxidized to sodium benzoate. The term "disproportionation" highlights that both the oxidation and reduction processes occur within the same molecular setting, transforming the reactant into two different products.
This type of reaction requires all of the following:

• A high pH environment, typically provided by a strong base such as sodium hydroxide (NaOH).
• Inert reaction conditions that prevent side reactions.
• Careful management of temperature to maintain reaction efficiency and yield.

Understanding disproportionation is crucial for predicting and manipulating similar redox reactions in varied chemical processes.

Hydride Transfer

The hydride transfer in the Cannizzaro reaction is the key transformation step. It involves the transfer of a hydride ion (H⁻) from one aldehyde molecule to another. This transfer is what leads to the formation of the two different products.
What makes this reaction unique is the involvement of the solvent molecules. From the given observations:

• Observation 1 shows no deuterium incorporation when the reaction is in D₂O, suggesting hydride transfer doesn't involve solvent hydrogen atoms.

This implies that the hydride transfer is an internal process between the aldehyde molecules themselves. It's this direct hydride transfer that reduces one molecule to alcohol and oxidizes the other to a carboxylate ion.
Understanding hydride transfer's role in reactions like the Cannizzaro is crucial as it points to a single, important transformation step rather than a multistep sequence, streamlining the mechanism and clarifying solvent involvement.

Reaction Mechanism

Developing a reaction mechanism for the Cannizzaro reaction involves combining all the observations and performing logical deductions. The second-order nature in aldehyde concentration suggests that two aldehyde molecules are involved in the rate-determining step, as seen in:

• Observation 3, where the reaction's rate law shows dependence on the square of the aldehyde concentration:

Rate = k_{\text{obs}}{[\mathrm{PhCH}=\mathrm{O}]^2}[\mathrm{OH}^-].
A logical mechanism involves several phases:

• Initially, a nucleophilic hydroxide ion attacks the carbonyl group of the aldehyde, leading to the formation of a tetrahedral intermediate.
• This intermediate facilitates the hydride transfer to another aldehyde molecule.
• The transition state of this reaction might exhibit a significant charge build-up, supported by the Hammet correlation with a positive \(\rho=+3.76\), indicating electron withdrawal.

This mechanism showcases the crucial roles of both the intermediate formation and the hydride transfer, coming together to effectively produce the final reaction products.

Hammet Correlation

The Hammet correlation is a way to characterize the electronic nature of a reaction mechanism involving substituted benzaldehydes. It correlates the reaction rates of these reactants with their electronic properties.
In the Cannizzaro reaction, a positive \(\rho\) value of +3.76 denotes a substantial sensitivity to electron-withdrawing groups. Here's what this means:

• Such a high \(\rho\) value indicates considerable negative charge development on the aromatic ring in the transition state.
• The reaction proceeds faster when the aromatic ring has substituents that withdraw electrons, stabilizing the intermediate.

The Hammet correlation acts as a powerful tool for speculating about transition states and predicting the reaction pathways influenced by substituent electronic effects. Tailoring reactions based on this information can enhance yields and streamline reaction pathways, making this a key insight for organic chemists working with related chemical transformations.