Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.
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.
An error occurred trying to load this video.
Try refreshing the page, or contact customer support.
You must cCreate an account to continue watching
Register to view this lesson
As a member, you'll also get unlimited access to over 84,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.
Get unlimited access to over 84,000 lessons.Try it now
Already registered? Log in here for accessBack
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:
- Electrons from the carbon-oxygen double bond remove the hydrogen from the oxygen
- The electrons from that oxygen-hydrogen bond form a double bond between carbon and oxygen
- 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.
A Cannizzaro reaction doesn't necessarily occur in this order. In fact, there is no real 'order' in which this occurs because it happens all at the same time. Instead, it may be helpful to think about electrons being momentarily shared between all of the atoms.
Cannizzaro Reaction Examples
Let's first look at the simple non-enolizable aldehyde, formaldehyde:
Scientists have marked one of the hydrogens on formaldehyde in order to figure out how this reaction occurs. By following one of the hydrogens on the first formaldehyde (which is in red), we can see that the hydrogen transfers from one formaldehyde to another. This has allowed us to determine the mechanism of the Cannizzaro reaction. We now know that a hydrogen transfer occurs between the formaldehydes instead of taking a hydrogen from the OH group.
Now let's look at benzaldehyde:
By adding an actual 'R' group, which is a defined group of molecules with specific attributes such as a carbon chain, to the aldehyde, we can see that both products have the original 'R' group (in this case benzene). So, we end up with a benzoic acid and benzyl alcohol.
The Cannizzaro reaction is an important reaction for producing both alcohols and carboxylic acids from a single reaction. In order for it to occur, we need a non-enolizable aldehyde, which is an aldehyde that has no alpha hydrogen atoms, and a basic environment.
In the Cannizzaro reaction, there are two steps. First, the base (OH group) is added to the carbonyl, putting a negative charge on the oxygen. Second, there are three electron movements which occur simultaneously during a hydride transfer when the hydrogen from the first aldehyde with a new OH group gets transferred to another aldehyde. This then forms an alcohol and a carboxylic acid.
To unlock this lesson you must be a Study.com Member.
Create your account
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack
Cannizzaro Reaction: Mechanism & Examples
Related Study Materials
Explore our library of over 84,000 lessons
- College Courses
- High School Courses
- Other Courses
- Create a Goal
- Create custom courses
- Get your questions answered