Diastereoselectivity: Definition, Mechanism & Example

Instructor: Laura Foist

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

Often in reactions there are two possible products, which are called diastereomers. In this lesson we will learn how some reactions are diastereoselective.

The Double Staircase

Imagine a house with a double staircase leading up into it. A group of people need to get into the house, and can take either stair case. Most of the time both stair cases are used, but sometimes something happens causing one stair case to be preferred over another.

In chemical reaction when one stair case (or diastereomer) is preferred over another then this is a diastereoselective reaction. There are several different methods by which a reaction can occur diastereoselectively.

Hydrogen On Same Side

Looking at our staircase let's say that it was raining outside and you were sharing an umbrella with your friend. In this case, even though you could choose opposite staircases, you will probably choose the same staircases, because you two are bound together with the umbrella. This is similar to the diastereoselective method by which hydrogen atoms are put onto the same side of a bond.

The preferred product is circled in red
Hydrogen reaction

We can add hydrogen to a double bond, reducing it into a single bond. By using a metal catalyst with the hydrogen atoms coated on it the hydrogen atoms can be added to the double bond. Yet, since the hydrogen atoms are coated on the metal surface, they aren't free to rotate around the molecule to be added to either side of the molecule. So, they need to be added to the same side of the molecule. This means that although, theoretically, there are two possible diastereomers, only one is typically created in the reaction.

Hydrogen is attached to each other, so it needs to attack the molecule on the same side
3D reaction of hydrogen

Bromine on Opposite Sides

Now, let's look back at the stair case. Let's say that one side was completely blocked by someone else, and they were not budging anywhere. You would probably take the opposite staircase. This is the case with bromine getting added to opposite sides of a molecule.

Bromine can either be added to the same side or to opposite sides, but opposite sides is preferred
Two diastereomers for bromine

When bromine reacts, the first bromine forms a bond with both carbon atoms, completely blocking this side of the molecule from attack. So the second bromine needs to attack from the opposite side. This means that the diastereomer with the bromines on two opposite sides is primarily selected.

We can see that the first bromine blocks the top from attack, so the second bromine must be added to the bottom
Bromine mechanism

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