Cannizzaro Reaction: Mechanism & Examples

Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

The Cannizzaro reaction is a reaction between two aldehydes forming a carboxylic acid and an alcohol. We will learn how this reaction occurs and the mechanism by which it occurs.

Cannizzaro Reaction Background

Your clothes made from polyester and a nitroglycerin bomb surprisingly have something in common. They both rely on the Cannizzaro reaction to be made. The Cannizzaro reaction is when a non-enolizable aldehyde reacts with itself in a strong base, such as sodium hydroxide (NaOH), to form a carboxylic acid and an alcohol. A non-enolizable aldehyde is one that has no alpha hydrogens available for the aldehyde to form an enol. Remember that an enol is when the carbon next to the aldehyde, the alpha carbon, has a negative charge.

Many aldehydes have a carbon next to them that has a hydrogen attached. This is the alpha hydrogen. This hydrogen can be removed fairly easily, due to resonance stabilization, which is when the charge can be shared between atoms, forming an enol. However, if there are no alpha hydrogens attached, the enol cannot form, and the reaction occurs differently. This is the case for the Cannizzaro reaction.

Which aldehydes have no alpha hydrogens? Two of the most common non-enolizable aldehydes are formaldehyde and benzaldehyde. Formaldehyde is the simplest aldehyde and has no other carbon atoms to become an enol. Benzaldehyde has too many substituents, non-hydrogen bonded molecules, on the alpha carbon, so there are no alpha hydrogens.

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  • 0:02 Cannizzaro Reaction Background
  • 1:28 Cannizzaro Reaction Mechanism
  • 3:00 Cannizzaro Reaction Examples
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Cannizzaro Reaction Mechanism

In a Cannizzaro reaction, there are two distinct steps. In the first step, the OH group is added to the carbonyl. Since there are no alpha hydrogens to be removed in a highly basic environment, the OH group is able to be added to the carbonyl. The OH is very electron rich, so it can act as a nucleophile, meaning electron rich, to be added to the electrophilic, electron deficient, carbon.

Carbon would now have extra electrons, so the electrons from the double bond bump up onto the oxygen, giving the oxygen a negative charge.

The second step can seem kind of confusing because three things happen at once. However, what is really happening is just a hydride transfer. This occurs when the hydrogen from the aldehyde with a new OH group gets transferred to another aldehyde. So the three things that occur all at once are:

  1. Electrons from the carbon-oxygen double bond remove the hydrogen from the oxygen
  2. The electrons from that oxygen-hydrogen bond form a double bond between carbon and oxygen
  3. Since there are now extra electrons on carbon, electrons from the carbon-hydrogen bond transfer to the other carbon, which is now electron deficient, forming a new carbon-hydrogen bond.

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