Alpha Beta Unsaturated Ketone: Formation & Reduction

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary focal point of this lesson will be on a specific kind of organic compound known as an alpha beta unsaturated ketone. Our main points of discussion will induce their structure, formation, and reduction reactions.

Not Just Fats are Unsaturated

You're probably aware that there are different types of fats right? Saturated fats get a lot of press for being bad for your health and causing serious issues like blocking your arteries. Unsaturated fats on the other hand, tend to be not quite as bad for us, due to the fact that our bodies can break them down more than saturated fats.

Did you know that organic compounds can be unsaturated as well? When we say that an organic compound is unsaturated, it essentially means that the molecule has lost hydrogen atoms and gained one or more double bonds. Unsaturated organic compounds hold a lot of synthetic utility to chemists due to their unique reactivity and structural properties. Today we are going to be discussing a specific type of unsaturated compound, called an alpha beta unsaturated ketone. We will be studying these compounds in the context of their structure, formation, and reduction reactions.

Structure of Alpha Beta Unsaturated Ketones

Let's get started by taking a brief look at the structure of alpha beta unsaturated ketones so we'll know what we're looking at. An organic compound that can be classified as an alpha beta unsaturated ketone has two important features: it has a carbon-oxygen double bond (the ketone functional group, generally called a carbonyl) and it has a carbon-carbon double bond directly adjacent to the carbonyl group. Consider as an example the compound known as chalcone.

Chalcone is an example of an alpha beta unsaturated ketone

Notice how chalcone contains a carbonyl group (carbon-oxygen double bond) and also note the presence of the carbon-carbon double bond directly adjacent to that group. The carbon-carbon double bond, called an alkene, is actually what makes the compound 'unsaturated.' Any time we exchange hydrogen atoms for multiple bonds on an organic molecule, this is essentially known as introducing unsaturation to the system. The carbon atoms directly adjacent to the carbonyl group are labeled alpha and beta respectively, which is why we call it an alpha beta unsaturated ketone!

Formation of Alpha Beta Unsaturated Ketones

Now that we know what an alpha beta unsaturated ketone looks like, let's talk about how they can be made synthetically in the lab setting. The most common method of synthesizing these types of compounds is through an organic reaction called an aldol condensation reaction. In this reaction, a ketone is reacted with an aldehyde in the presence of a base. The result of an aldol reaction joins the two compounds together (which is why it's called a condensation) and forms a new carbon-carbon double bond between the aldehyde and the ketone.

If we keep with our chalcone molecule as our model system, we can envision how it could be made via an aldol condensation. Benzaldehyde and acetophenone react in the presence of a base to condense with one another and form chalcone as the final alpha beta unsaturated ketone product.

When benzaldehyde and acetophenone react with one another in the presence of a base, an aldol condensation takes place with forms chalcone, an alpha beta unsaturated ketone

Reduction Reactions of Alpha Beta Unsaturated Ketones

Alpha beta unsaturated ketones can undergo reduction reactions in which the double bonds can be transformed into single bonds. The good news about this reaction is that we can selectively reduce the carbon-oxygen double bond while leaving the carbon-carbon double bond intact simply by strategically choosing the right reducing conditions. For example, the ketone in carvone (a naturally occurring alpha beta unsaturated ketone) can be selectively reduced to an alcohol in the presence of a special reducing agent known as sodium borohydride (NaBH4). Notice how the alkene (carbon-carbon double bond) is not affected by the reaction.

The ketone in carvone can be selectively reduced to an alcohol by the action of sodium borohydride, while leaving the alkene bond intact

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