Racemization in Organic Chemistry: Definition, Mechanism & Example

Instructor: Korry Barnes

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

How can you go from one, pure enantiomer to two? This lesson covers racemization reactions, their mechanism, and go over some specific examples involving amino acids and an alkyl halide.

A Different Kind of Mixture

Say you are given two bags of jelly beans, one with red ones and one with blue ones. If you were to take both bags, dump them into a large bowl, and then mix them up together, you could say you had a mixture of jelly beans that contain both red and blue ones.

As long as you mix the same amount of red ones with blue ones, you would be accurate in saying that you have a 50:50 mix of jelly beans. This is a little like racemization.

Definition of Racemization

Racemization in organic chemistry occurs when a compound undergoes a reaction and the transformation produces an equal mixture of both possible enantiomers, sometimes called a racemic mixture. When two compounds are classified as enantiomers of one another, it means they are non-superimposable mirror images. For example, consider the two enantiomers of amphetamine:

Enantiomers of amphetamine. They are non-superimposable mirror images of one another

Notice how these two compounds are perfect mirror images of one another, but if we tried to superimpose (stack) them on top of one another they wouldn't match due to a difference in spatial orientation of the two molecules. When racemization occurs we always get a mixture that contains half of one enantiomer and half of the other.

Example of Racemization

It turns out that amino acids (the building blocks of proteins) can actually undergo racemization if they are exposed to basic conditions. The reaction is constantly shuttling back and forth, so at any given time there is a 50:50 mix of both enantiomers, in other words, a racemic mixture.

Amino acids can undergo racemization under basic conditions

Note that when each enantiomer is converting to the other they both go through the same flat, planar intermediate. This is an important part of racemization as we will see in the next section when we study the mechanism.

Reaction Mechanism in which Racemization Occurs

Let's look at a reaction mechanism in which a racemic mixture is produced. In general, substitution reactions involving alkyl halides are great examples of transformations that result in racemization. Consider an alkyl halide that contains a bromine atom as the halogen.

In the first step of the reaction the bromine actually dissociates from the compound, forming what's called a carbocation intermediate (carbon with a positive charge). When this happens, the carbon atom becomes trigonal planar in shape with the key word here being 'planar' (flat). When this intermediate is formed, the stereochemistry of the starting material is destroyed because it's now a perfectly flat species.

When bromine dissociates from the substrate the stereochemistry is destroyed and a flat intermediate results

In the second step of the reaction a water molecule reacts with the carbocation intermediate to form a new carbon-oxygen bond. This is the point in the process when racemization occurs.

Since the carbocation is a flat, planar intermediate, it's statistically and energetically feasible for the water molecule to approach from both sides. Since there's no facial bias for which direction the water molecule can approach from, half of the time it comes in from the top face and half the time it comes in from the bottom face.

Both enantiomers are formed as a result of water approaching from either the top or bottom face of the intermediate

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