What is a Stereocenter? - Definition, Identification & Examples

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 what stereocenters are, learn some techniques to identify them, and look at some examples of what are and are not stereocenters.

What is a Stereocenter?

Did you know that milk is full of sugar? Then why isn't it very sweet? Well, this is partly because one of the main sugar molecules in milk is galactose rather than glucose, a main sugar molecule in table sugar.

Despite the huge difference in sweetness between these two sugar molecules, the only structural difference between them is that one stereocenter has the attachments switched.

A stereocenter is an atom, typically carbon, that has four attachments that are different from each other. Since each attachment is unique, if we moved any two attachments into another position, we would form a new molecule.

Recall that simply what is attached isn't what determines a molecule, but also how each attachment is oriented. For example, glucose and galactose are two different sugars, but the only difference is that the fourth carbon on glucose has the OH on the right, while galactose has this same OH on the left.

Glucose and galactose are only different at one stereocenter
Glucose and Galactose

This slight change makes a huge difference in the function of the molecule. That fourth carbon is a stereocenter.

Identifying Stereocenters

There are a few things we need to note when identifying stereocenters. First, one way to draw the orientation of attachments on a molecule is using wedges and dashes. But, don't get confused! Just because wedges and dashes are used doesn't mean it's a stereocenter.

For example, the following compound has wedges and dashes to indicate the orientation of each methyl (CH3) attachment:


Wedges and Dashes


But, this carbon is not a stereocenter because it has two identical methyl groups attached to it.

The second thing we need to watch out for is hidden hydrogen atoms. Most of the time when we draw molecules we leave out carbon atoms, such as with this molecule:


Hydrogen atoms no stereocenter


None of these carbon atoms are a stereocenter, because they are each attached to more than one hydrogen atom, which is the same attachment.

The third thing we need to watch out for when identifying stereocenters is that just because two attachments may look the same at first, does not mean that they are the same. For example, in this molecule this carbon atom is attached to a chlorine, and OH group, and two CH2 groups:


Stereocenter


But, it is still a stereocenter, because the CH2 group on the left is only attached to a single CH3 group while the CH2 on the right is attached to another CH2 and a CH3. This makes the two attachments different because we need to look at the entire group, not simply what is directly attached to the carbon.

Finally, a carbon with a double or triple bond will never be a stereocenter, because that double bond makes two of the attachments the exact same group.

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