In stereochemistry, when an atom is bonded to three or more different groups and the exchange of two groups led to the formation of new stereoisomers then the central atom of the molecule is known as the stereogenic center. Stereoisomers have the same connectivities but are different in the position of the same group in three-dimensional space.
The stereogenic center is also known as the stereocenter. It may have a single or double bond around the central atom. Stereocenters may be tetrahedral or trigonal.
Stereocenters have different chiral configurations. The structures of these molecules are drawn with dashes and wedges to show the configurations. They may have R or S configuration.
The presence of a stereogenic center in a molecule can have significant consequences for its biological activity because stereoisomers have different biological effects. For example, there are two enantiomers of glucose, D-glucose can be used as a supplement in hypoglycemia but L-glucose might be toxic for living organisms as it cannot be phosphorylated by hexokinase during enzymes glycolysis.
Formula to calculate stereoisomers from stereogenic center
1 Stereocenter = 2 Stereoisomers or (maximum stereoisomers)
m = 2n (i)
- m= stereocenters
- 2n = stereoisomers
Identification of stereogenic centers
It is important to be able to identify and distinguish between stereogenic centers in molecules because they have different physical and chemical properties. To figure out whether the central atom of a molecule is the stereogenic center or not, its two atoms are swiped with other. If the interchange of two groups produces two stereoisomers, the central atom is probably a stereocenter. Stereogenic atoms may be sp3 or sp2 hybridized.
Four different groups attached to a central atom
If the central atom is connected to four different groups then the central atom is a stereocenter. Because the exchange of two groups produces new stereoisomers.
For example, two molecules have the same constitution but they differ in the arrangement of the same groups in three-dimensional space. They have different configurations.
Three different groups attached to a central atom
If a central atom is bonded to three different groups and the exchange of two groups produces new stereoisomers then it is a stereocenter.
For example, the molecule shown below has two stereocenters. And swapping of groups around any stereocenter leads to the formation of a new stereoisomer.
What are the main differences between a stereogenic center and a chiral center?
The chiral center is a type of stereocenter. When an atom is bonded to four different groups It is the origin of chirality. All the chiral centers are stereocenters but all stereocenters are not chiral centers. However, chiral centers are tetrahedral stereocenters.
2-Butanol is an example of a chiral center. The red star represents the chiral carbon with sp3 hybridization. It has four different groups attached to the central atom.
The key differences between a chiral and stereogenic center are given below:
The central atom is attached to three or more different groups.
The central atom is attached to four different groups.
Hybridization: Sp3 or Sp2
Single or double bond around the central atom
All bonds are only sigma bonds(single)
May or may not be symmetrical
Trigonal or tetrahedral
Results in stereoisomers
Results in enantiomers
Examples of the stereogenic centers
When a central atom is bonded to the three different groups then they have trigonal stereocenters. They have a double bond around the central atom. Similarly, they have Sp2 hybridization. They result in cis, and trans isomers.
Cis-1,2-dichloroethene is an example of a stereogenic center. It has two stereogenic centers. The exchange of two groups around any stereocenter produces a new stereoisomer. This is the molecule that has a stereocenter but not a chiral center.
Tetrahedral stereogenic center
When a central atom is bonded to four different groups then they have tetrahedral stereocenters. They have single bonds around the central atom. Similarly, they have sp3 hybridization. They result in enantiomers. They can also be classified as chiral centers if they are optically active. A common example of a compound with a tetrahedral stereocenter is given below:
And, molecules having more than one stereocenter and a plane of symmetry are also called meso compounds.
Molecules with multiple stereogenic centers
Glucose is a common example with four stereogenic centers.
2-Bromo,4-heptanol is another example with 2 stereocenters as shown below:
What is a stereogenic center?
If an atom is bonded to three or four different groups and the exchange of two groups produces new stereoisomers then this atom is known as a stereogenic center.
How we can identify a stereogenic center?
We can identify the stereogenic center as
- They have three or four different substituents.
- The exchange of two groups produces new stereoisomers.
- They have different configurations.
Give examples of chiral molecules without the stereogenic center.
The substituted biphenyls and substituted allenes are examples of chiral molecules without stereocenters.
Is the chiral center the same as the stereogenic center?
The chiral center is a type of stereocenter.
What is stereogenic carbon?
A carbon atom bonded to three or four different groups and the exchange of two groups produce a new stereoisomer is called stereogenic carbon.
What is chiral carbon?
A carbon atom that has four different substituents is called chiral carbon.
- 12th edition of Organic Chemistry by T.W Graham Solomons, Craig B. Fryhle, and Scott A. Synder.
- 4th edition of Organic Chemistry As a second language by David Klein.
- Chapter 4 (Colby.edu)