The word "chirality" refers to the property of "handness". Your right and left hands are very similar, yet they are not identical. They are related to each other as mirror images, and as such they cannot be superimposed on top of each other. Molecules can also be chiral if they contain one or more chiral centers. For the purposes of introductory organic chemistry, a chiral center can be defined as an sp3 hydridized carbon which is bonded to four different groups. A more contemporary term for "chiral center" (or chiral carbon) is stereogenic center (or stereocenter), and the terms will be used interchangeably.

Chirality, resulting from the presence of four different groups around a carbon results from an asymmetry in the molecule. This can be seen by examining the molecules shown below; when two identical groups are on one carbon, there is an internal mirror plane passing through the molecule; when four groups are present, there is no internal mirror plane (no symmetry) and hence the carbon is chiral.

Consider 2-bromobutane:

The carbon which is bound to the bromine (carbon #2) is a stereogenic center; it is sp3 hydridized and it is bonded to four different groups:

  1. a hydrogen,
  2. a bromine,
  3. a methyl group, and
  4. an ethyl group.

All of the other carbons in this molecule are not stereogenic centers, since they are all bonded to at least two hydrogens.

2-Chloro-3-bromobutane has two stereogenic centers:

Carbon #2 in this molecule is sp3 hydridized and is attached to four different groups:

  1. a hydrogen,
  2. a chlorine,
  3. a methyl group, and
  4. a -CHBrCH3 group.

Likewise, carbon #3 is also sp3 hydridized and is attached to four different groups:

  1. a hydrogen,
  2. a bromine,
  3. a methyl group, and
  4. a -CHClCH3 group.

The remaining carbons are both sp3 hydridized, but each is attached to three hydrogens, and neither is a stereogenic center.

Examine the three molecules shown below and attempt to identify any stereogenic centers. You may click on the image to check your answers.




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