How to Identify a Chirality Center (Chiral vs Achiral)

A carbon is a stereogenic center (also called a chirality center or a chiral center) if it has 4 different groups attached to it.

This is an example of a chirality center

These groups can consist of any chains of atoms. But if two groups are the exact same (like two ethyl units or two bromines for example), then the carbon is not a stereogenic center.

In the drawing above, we use two different bond symbols.  is called a wedge, and it indicates that the atom on the bond is coming out of the page.  is called a dash, and it indicates that the atom on the bond is going into the page.

 

This image shows a 3-D representation of a chirality center.

 

Chirality is the property of a molecule that is not superimposable on its mirror image. A molecule might be chiral if it contains one or more stereogenic centers.

 

For a molecule to be chiral, it must contain a chirality center.

 

Example

Is the following molecule chiral? If so, box the stereogenic center.

Picture31

Answer

No, it is not chiral. If we examine the groups coming off the carbon with the chlorine attached, there are two -CH2CH2CH2CH2CH2 groups. Because this molecule does not have 4 different groups, it is not chiral. 

Example:

Is the following molecule chiral? Box all stereogenic centers in the molecule.

Picture2

Answer
Yes, this molecule is chiral. There are 4 different groups coming off the carbon atom with the chlorine attached (making it a chirality center). There is a –Cl, -H, -CH2CHBrCH2CH2CH2, and –CH2CH2CH2CHBrCH2. Note the last two groups are indeed different from one another because of the order that the Br is connected in each group.

Here are the chirality centers:

Picture3

Note that, although there are no clear dashes and wedges on the left chirality center, it still does have four different groups attached to it making it a chirality center.