Alpha Carbon: Acidity & Reactions

Instructor: Laura Foist

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

In this lesson we will learn about the alpha carbon. We will learn what the alpha carbon is and what makes it more acidic than other carbons, and we will see some reactions in which the alpha carbon participates.

Alpha Carbon

We frequently use the word 'alpha' to mean the strongest, best, or first. We can use the same logic for remembering what an alpha carbon is. The alpha carbon is the carbon that is next to a functional group, so we can think of it as the first carbon to come after the functional group. A functional group is just any specifically classified group of atoms. Some common examples of functional groups are alcohols, ketones, aldehydes, halides, alkenes, and amines.

The alpha carbon is the carbon right next to a functional group
Alpha carbon

Alpha Carbon Acidity

One of the ways to determine the acidity of something is to look at the stability of it if the hydrogen is removed. So let's look at the stability of a carbon that is not an alpha carbon with one of its hydrogen atoms removed by a base:

A carbon is typically not very acidic because the resulting carbanion is not very stable
Carbon acidity

You notice that there is a negative charge on that carbon. Carbon doesn't typically like to have a negative charge. It isn't very electronegative, so it doesn't hold a negative charge as well as other atoms do.

Now let's look at an alpha carbon with a hydrogen removed by a base:

The alpha carbon is more acidic due to the stabilization of resonance structure

There is still a negative charge on that carbon, but there is a second structure that it can resonate to. Resonance describes electrons' ability to move within an atom. The reason this helps stabilize the molecule is because the negative charge is never fully on any one atom:

By looking at the hybrid structure the negative charge is only partially on a single atom
Hybrid structure

When looking at the hybrid structure, we see that there is only a partial negative charge on each of the atoms. In the case of the ketone, the major product will be with the negative charge on the oxygen. Since oxygen is very electronegative, it can handle a negative charge. This allows the structure to be stabilized better. So the alpha carbon is more acidic than a typical carbon.

Alpha Carbon Reactions

Due to the acidity of the alpha carbon, it is able to participate in several reactions. The general reaction simply takes the electrons from the negative charge and attacks an electrophile, after which the electrophile gets attached to the carbon.

The negative charge on the carbon can attack an electrophile creating a carbon-carbon bond
General electrophile alpha carbon reaction

So really the important part is simply having an electrophile that can react with the carbon. The electrophile can be a carbonyl, an alkyl halide, or a halogen.

The alkyl halide is the electrophile that the alpha carbon can attack once it has been deprotonated
Alkyl halide reaction

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