Question

Which compound can never be obtained as a product in Cannizaro's reaction followed by acidification? (A) OCc1ccccc1 (B) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{COOH}\) (C) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) (D) O=C(O)c1ccccc1

Short answer

The compound that can never be obtained as a product in Cannizaro's reaction followed by acidification is (B) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{COOH\), as it is a branched carboxylic acid that cannot be formed from the disproportionation of an aldehyde.

Step-by-step solution

Identify the products of Cannizaro's reaction

Cannizaro's reaction takes place between a non-enolizable aldehyde and a base. The products of this reaction are a carboxylic acid and an alcohol.

Analyze each option

We must determine which of the given compounds cannot be formed as a product of Cannizaro's reaction followed by acidification. (A) Benzyl alcohol: This can be obtained from the reaction of benzaldehyde, a non-enolizable aldehyde, with a base. (B) Tert-butyl carboxylic acid: This compound cannot be formed from an aldehyde, as it is a branched carboxylic acid that cannot be formed from the disproportionation of an aldehyde. (C) Ethanol: This can be obtained from the reaction of acetaldehyde, a non-enolizable aldehyde, with a base. (D) Benzoic acid: This can be obtained from the reaction of benzaldehyde, a non-enolizable aldehyde, with a base.

Determine the correct answer

Of the given options, only the compound (B) tert-butyl carboxylic acid cannot be formed from Cannizaro's reaction followed by acidification. Therefore, the correct answer is (B) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{COOH}\).

Key concepts

Non-Enolizable Aldehydes

Non-enolizable aldehydes play a crucial role in Cannizaro's reaction. These are aldehydes that don't have an alpha hydrogen atom next to the carbonyl group. Without this alpha hydrogen, they cannot undergo tautomerization to form enolate ions, which are key in many other aldehyde reactions like the Aldol reaction. This makes them unique.

In Cannizaro's reaction, these non-enolizable aldehydes react with a strong base, like hydroxide ions, to give products in a single-step redox process. The absence of enolizable hydrogen keeps the reaction process simple and straightforward. This is why only specific aldehydes are suitable for this reaction.

• Examples include formaldehyde and benzaldehyde.
• These aldehydes are typically used because their structure is stable without the ability to form enolates.

Remember, the key feature is the absence of the alpha hydrogen, letting the aldehyde directly participate in the Cannizaro's reaction without interference from enolization.

Disproportionation Reaction

The Cannizaro's reaction is a type of disproportionation reaction. This means a single substance transforms into two different compounds through simultaneous oxidation and reduction. In Cannizaro's case, the aldehyde simultaneously gets oxidized and reduced.

What results from this interesting chemical dance? One aldehyde molecule is oxidized to form a carboxylic acid, while another is reduced to form an alcohol. This happens without any external oxidizing or reducing agents, but through internal transformations.

• The process occurs only under strongly basic conditions.
• Both products must be derived from the same aldehyde starting material.

An understanding of this mechanism clarifies why tert-butyl carboxylic acid in the exercise is not a product of Cannizaro's reaction—it does not arise from the simultaneous oxidation-reduction of an aldehyde molecule.

Acidification Process

Following the Cannizaro's reaction, an acidification step is often performed. While Cannizaro’s reaction occurs under basic conditions, the resulting products might need acidification to convert into more stable or desired forms.

For instance, if a carboxylate ion is formed during the reaction, acidification will protonate this ion into its corresponding carboxylic acid. This conversion is crucial because it stabilizes the compounds and makes them more observable or usable for further chemical applications.

• Acidification adjusts the pH and stabilizes the acidic form of the reaction product.
• This step is usually carried out using a dilute acid, like hydrochloric acid.

In the context of the exercise, acidification serves to ensure that the products derived from the reaction are those specified, highlighting why only specific compounds can be achieved through this method.