Stereogenic Center vs Chiral Center

Main Difference – Stereogenic Center vs Chiral Center

Stereogenic center and chiral center are two terms used to explain stereoisomerism in organic chemistry. These two terms are often considered as the same but there is a difference between stereogenic center and chiral center.  All chiral centers are stereogenic centers but all stereogenic centers are not chiral centers. It is important to know the difference between stereogenic center and chiral center since it is the presence of these centers that give rise to a stereoisomer.

Key Areas Covered

1. What is a Stereogenic Center
      – Definition, Characteristics
2. What is a Chiral Center
      – Definition, Characteristics
3. Stereogenic Center vs Chiral Center
      3.1.  Simillarities Between Stereogenic Center and Chiral Center
              – Outline of Common Features
      3.2. Difference Between Stereogenic and Chiral Center
              – Comparison of Key Differences

Key Terms: Chirality, Chiral Center, Isomers, Hybridization, Stereoisomerism, Stereogenic Center

What is a Stereogenic Center

A stereogenic center is any atom that gives different isomers when atoms or groups attached to it are exchanged. These isomers are called stereoisomers because the structures of the molecules are the same but their spatial arrangements are different from each other. In other words, stereoisomers differ from each other only in their three-dimensional arrangement.

The atom which is the stereogenic center can be either sp² or sp³ hybridized. This means that it may have either double bonds or single bonds. Achiral molecules can sometimes also have stereogenic centers. Therefore, all the chiral centers are stereogenic centers. But a stereogenic center is not essentially a chiral center. A good example for this is cis–trans geometry.

Figure 1: trans-2-butene

Above image shows a trans–2-butene molecule. It has no chiral center. But it has a stereogenic center. Among the carbon atoms, both 2 and 3 carbon atoms are stereogenic centers because the two groups attached to the carbon atoms are –H and –CH₃ and these groups can be exchanged in order to obtain a new molecule, which is the stereoisomer of this molecule. The new molecule is the cis–2-butene.

What is a Chiral Center

A chiral center is a carbon atom directly connected to four different groups. A molecule with a chiral center cannot be superimposed with its mirror image. Due to this reason, the molecule and the mirror image are considered as two different molecules. The carbon atom is always sp³ hybridized so that it can attach to four groups.

Therefore, a molecule with a chiral center can always give rise to a stereoisomer and a chiral center is always a stereogenic center. A single molecule can have more than one chiral center.

Figure 2: A Molecule having a Chiral Center

The above image shows a carbon atom attached to a hydrogen atom (H) along with three other groups, which are different from each other. (X, Y, Z). There, the carbon atom is a chiral center. It is sp³ hybridized and is attached to four different groups. Its mirror image is non-superimposable with the molecule.  

Stereogenic Center vs Chiral Center

Similarities Between Stereogenic Center and Chiral Center

• The presence of a stereogenic center or a chiral center gives rise to a stereoisomer.

• Chiral centers are always stereogenic centers, but all stereogenic centers are not chiral centers.

Difference Between Stereogenic and Chiral Center

Definition

Stereogenic Center: A stereogenic center is any atom that gives different isomers when atoms or groups attached to it are exchanged.

Chiral Center: A chiral center is an atom directly connected to four different groups.

Hybridization

Stereogenic Center: The carbon atom can be either sp² or sp³ hybridized in the stereogenic center.

Chiral Center: The carbon atom is always sp³ hybridized in the chiral center.

Number of Groups

Stereogenic Center: The stereogenic center can be attached to either three or four groups.

Chiral Center: The chiral center always bears four groups.

Bonds

Stereogenic Center: Stereogenic center can have either single bonds or double bonds.

Chiral Center: Chiral centers have only single bonds. 

Chirality

Stereogenic Center: Stereogenic centers can be found in either chiral molecules or achiral molecules.

Chiral Center: Chiral centers are found only in chiral molecules.

Exchange of Groups

Stereogenic Center: Some molecules with stereogenic centers give stereoisomers when the attached groups are exchanged. (ex: cis-trans isomers)

 Chiral Center: Molecules having chiral centers does not show a difference when attached groups are exchanged.

Conclusion

All chiral centers are stereogenic centers but all stereogenic centers are not chiral centers. There are many distinct characteristics that can be used to identify the difference between stereogenic center and chiral center.

References:

1.Helmenstine, Anne Marie. “What Is a Chiral Center in Chemistry?” ThoughtCo. N.p., n.d. Web. Available here. 20 June 2017. 
2.Stereochemistry.” The Stereogenic Center. N.p., n.d. Web. Available here. 20 June 2017.

Image Courtesy:

1.”Trans-2-butene” By JaGa – Self-made using BKChem and Inkscape (CC BY-SA 3.0) via Commons Wikimedia
2. “Chiral center” By Morivert – created by Morivert (Public Domain) via Commons Wikimedia