Question

Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction. (i) Methanal (ii) 2-Methylpentanal (iii) Benzaldehyde (iv) Benzophenone (v) Cyclohexanone (vi) 1 -Phenylpropanone (vii) Phenylacetaldehyde (viii) Butan-1-ol (ix) 2,2-Dimethylbutanal

Short answer

(i, iii, ix) Cannizzaro; (ii, v, vi, vii) Aldol; (iv, viii), neither.

Step-by-step solution

Understanding Aldol Condensation and Cannizzaro Reaction

Aldol condensation involves enolizable aldehydes or ketones (compounds with alpha hydrogen), while the Cannizzaro reaction occurs with non-enolizable aldehydes (aldehydes without alpha hydrogen). To solve the exercise, we need to determine the presence of alpha hydrogen in each compound and classify them.

Analyze Methanal

Methanal (formaldehyde) does not have any alpha hydrogen atoms because it does not have a carbon adjacent to the carbonyl group. This means it cannot undergo aldol condensation, but it can undergo a Cannizzaro reaction since it is an aldehyde.

Analyze 2-Methylpentanal

2-Methylpentanal has an alpha hydrogen at the alpha (adjacent) position to the carbonyl group. Therefore, it can undergo aldol condensation. It cannot undergo the Cannizzaro reaction because it has alpha hydrogens.

Analyze Benzaldehyde

Benzaldehyde lacks alpha hydrogens, as there is no hydrogen adjacent to the carbonyl carbon. Therefore, it cannot undergo aldol condensation but can undergo the Cannizzaro reaction.

Analyze Benzophenone

Benzophenone is a ketone with two benzene rings attached to the carbonyl, and it lacks alpha hydrogen. Thus, it cannot undergo aldol condensation or the Cannizzaro reaction.

Analyze Cyclohexanone

Cyclohexanone has alpha hydrogens available and is a ketone, enabling it to undergo aldol condensation. However, it cannot undergo a Cannizzaro reaction as it is not an aldehyde and it has alpha hydrogens.

Analyze 1-Phenylpropanone

1-Phenylpropanone has an alpha hydrogen on the carbon adjacent to the carbonyl group, allowing it to participate in aldol condensation. It cannot undergo Cannizzaro reaction because it is a ketone.

Analyze Phenylacetaldehyde

Phenylacetaldehyde contains alpha hydrogens next to its carbonyl group, which allows it to undergo aldol condensation. It cannot undergo the Cannizzaro reaction due to the presence of alpha hydrogens.

Analyze Butan-1-ol

Butan-1-ol is an alcohol, not an aldehyde or ketone. Therefore, it cannot undergo either an aldol condensation or a Cannizzaro reaction.

Analyze 2,2-Dimethylbutanal

2,2-Dimethylbutanal has no alpha hydrogens due to the substitution on the adjacent carbon to the carbonyl group. Thus, it cannot undergo aldol condensation, but it can undergo a Cannizzaro reaction as it is an aldehyde.

Key concepts

Cannizzaro Reaction

The Cannizzaro reaction is a special type of redox reaction where non-enolizable aldehydes transform. This reaction is unique because it involves aldehydes that do not have alpha hydrogens, meaning these aldehydes cannot form enolates.
For instance, methanal and benzaldehyde are good examples of compounds that undergo this reaction. During the Cannizzaro process, one molecule of the aldehyde is reduced to an alcohol, while another is oxidized to a carboxylic acid.
This reaction typically occurs in the presence of a strong base, such as sodium hydroxide ( \( ext{NaOH}\) ), which helps facilitate the electron transfer. Consequently, it's a bifurcated pathway of oxidation and reduction occurring simultaneously in a single reaction test tube.

Alpha Hydrogen

Alpha hydrogen refers to the hydrogen atoms attached to the carbon atom directly adjacent to a carbonyl group. These hydrogens are crucial because they dictate whether a compound can undergo reactions such as aldol condensation.
The reactivity of these alpha hydrogens is due to their acidity. When a strong base is present, it can extract these hydrogens, forming an enolate ion. This enolate ion has a nucleophilic character, allowing it to attack electrophilic carbonyl carbons, facilitating the formation of new carbon-carbon bonds.
In practical terms, compounds with alpha hydrogens, like 2-methylpentanal and cyclohexanone, can effectively participate in aldol condensations due to their ability to create this reactive enolate ion.

Enolizable Aldehydes

Enolizable aldehydes are those that possess one or more alpha hydrogens which can be transformed into an enolate. Such aldehydes can participate in aldol condensation reactions, forming beta-hydroxy aldehydes or ketones.
When exposed to a base, the alpha hydrogen is removed, allowing the aldehyde to form an enol or enolate. This intermediate is highly reactive due to its nucleophilic nature. Aldol condensation typically leads to the formation of a new carbon-carbon bond, expanding and diversifying organic molecules.
Examples of enolizable aldehydes include phenylacetaldehyde and 2-methylpentanal. In these compounds, the presence of alpha hydrogens permits aldol reactions, enabling organic chemists to synthesize complex structures efficiently.

Non-enolizable Aldehydes

Non-enolizable aldehydes lack alpha hydrogens, meaning they cannot form enolates. However, this makes them perfectly suited for Cannizzaro reactions.
Aldehydes like benzaldehyde and 2,2-dimethylbutanal fall into this category. Without any alpha hydrogens, they cannot participate in aldol condensations, but they are highly reactive in the presence of strong bases.
During Cannizzaro reactions, these aldehydes undergo simultaneous oxidation and reduction. As a result, they contribute to generating alcohols and carboxylic acids in a single step, showcasing the unique versatility of these simple yet profound organic reactions.