What is the Difference Between Aldol Condensation and Cannizzaro Reaction

The main difference between aldol condensation and Cannizzaro reaction is that aldol condensation is a coupling reaction, whereas Cannizzaro reaction is a redox reaction.

Aldol condensation and Cannizzaro reaction are important synthesis reactions in organic chemistry. Aldol condensation includes the combination of two different molecules; thus, we name it as a coupling reaction. However, in Cannizzaro reaction, oxidation and reduction occur; thus, we name it as a redox reaction.

Key Areas Covered

1. What is Aldol Condensation 
     – Definition, Characteristics 
2. What is Cannizzaro Reaction
     – Definition, Characteristics 
3. Difference Between Aldol Condensation and Cannizzaro Reaction
    – Comparison of Key Differences

What is Aldol Condensation

Aldol condensation is an organic chemical reaction in which an enol or enolate ion reacts with a carbonyl compound, giving a conjugated enone. Furthermore, this reaction has two major steps: aldol reaction and condensation reaction. In the aldol reaction, an enol or an enolate ion reacts with a carbonyl compound to give a beta-hydroxyaldehyde or beta-hydroxyketone. Moreover, in the next step, dehydration occurs, giving a conjugated enone. The reaction is as follows:

Figure 01: Aldol Condensation Reaction

The aldol condensation reaction is very important in organic synthesis reactions because it provides a pathway to form C-C bond easily. The name aldol originates from the combination of aldehyde + alcohol because, in its usual form, the aldol condensation reaction involves the nucleophilic addition of a ketone enolate to an aldehyde. It gives a beta-hydroxy ketone or “aldol”.

What is Cannizzaro Reaction?

Cannizzaro reaction is an organic redox reaction in which disproportionation of aldehydes give carboxylic acids and alcohols. The reaction requires a strong base as the catalyst. In this redox reaction, a hydride transfers from one substrate to another. There, one aldehyde molecule undergoes oxidation, yielding an acid, while the other aldehyde molecule undergoes reduction, forming an alcohol. However, this reaction sometimes produces unwanted byproducts.

Figure 02: An Example for Cannizzaro Reaction

There are three major steps in Cannizzaro reaction:

1. A nucleophile (i.e. OH^– ion) attacks the carbonyl group of an aldehyde, which leads to the disproportionation reaction. Further, this step forms an anion that carries two negative charges.
2. The produced anion can act as a hydride reducing agent. Here, this anion is unstable; thus, it releases a hydride anion. The hydride anion attacks the other aldehyde molecule, forming a carboxylate anion. The second aldehyde molecule then converts into an alkoxide anion.
3. In the final step, a water molecule gives a proton to the alkoxide anion and forms the final alcohol product. At the same time, carboxylate ion forms a carboxylic acid.

Difference Between Aldol Condensation and Cannizzaro Reaction

Definition

Aldol condensation is an organic chemical reaction in which an enol or enolate ion reacts with a carbonyl compound giving a conjugated enone while Cannizzaro reaction is an organic redox reaction in which disproportionation of aldehydes give carboxylic acids and alcohols.

Type of Reaction

The key difference between aldol condensation and Cannizzaro reaction is that the former is a coupling reaction, whereas the later is a redox reaction. 

Importance

Aldol condensation is useful in the synthesis of organic compounds because it easily forms C-C bonds while Cannizzaro reaction is important because we can obtain both an alcohol and a carboxylic acid using the same reaction.

Conclusion

Overall, aldol condensation and Cannizzaro reaction are important organic synthesis reactions. The main difference between aldol condensation and Cannizzaro reaction is that aldol condensation is a coupling reaction, whereas Cannizzaro reaction is a redox reaction.

Reference:

1. Helmenstine, Anne Marie. “Cannizzaro Reaction in Organic Chemistry.” ThoughtCo, Mar. 6, 2019, Available here.

Image Courtesy:

1. “Condensationaldolique” By I, Pansanel (CC BY-SA 3.0) via Commons Wikimedia
2. “Cannizzaro reaction-benzaldehyde” By Krishnavedala – Own work (Public Domain) via Commons Wikimedia