Difference Between Configurational and Conformational Isomers

Main Difference – Configurational vs Conformational Isomers

Isomerism is the presence of different structures or spatial arrangements for the same molecular formula. In other words, isomers of a certain compound are composed of the same type of atoms in the same ratio but are different compounds due to the differences in connectivity and the arrangement of these atoms. Configurational and conformation isomerism are two types found in organic compounds. These two types differ from each other due to their rotations. The main difference between configurational and conformational isomers is that configurational isomers cannot be obtained by rotating the molecule around a single bond whereas conformational isomers can be obtained by rotating the molecule around a single bond.

Key Areas Covered

1. What are Configurational Isomers
      – Definition, Explanation of Structure with Examples
2. What are Conformational Isomers
      – Definition, Explanation of Structure with Examples
3. What is the Difference Between Configurational and Conformational Isomers
      – Comparison of Key Differences

Key Terms: Configuration, Configurational Isomers, Conformation, Conformational Isomers, Eclipsed Conformation, Geometrical Isomers, Isomerism, Optical Isomers, Staggered Conformation

What are Configurational Isomers

Configurational isomers are stereoisomers that cannot be converted into one another by rotating the molecule around a single bond. These configurational isomers can be found in two types as geometrical isomers and optical isomers.

Geometrical Isomers

Geometrical isomers are also called cis-trans isomers. This type of isomerism is found mostly in alkenes and rarely in alkanes. Geometrical isomerism describes the presence of two identical groups (that are attached to the vinyl carbon atoms) positioned on the same side or the opposite side of the double bond. If the two identical groups are on the same side, it is called a cis isomer and if the two identical groups are on the opposite sides, it is called a trans isomer.

Figure 1: Cis-trans Isomerism

Here, one isomer cannot be rotated to get the other isomer due to the presence of a double bond. The pi bond prohibits the rotation around it. 

Optical Isomers

Optical isomerism can be found in molecules where chirality is present. Chirality is the presence of chiral carbons that can cause the optical activity of a molecule. A chiral carbon is a carbon atom that has four different groups attached to it. Therefore, the mirror image of this molecule is non-superimposable with the molecule.

Figure 2: Optical Isomerism

The above image shows two optical isomers. These isomers are able to rotate plane polarized light into opposite directions. The R isomer can rotate plane polarized light into the opposite direction that the s isomer can rotate the light. The letter R indicates the clockwise direction whereas the S indicates the counterclockwise direction.

What are Conformational Isomers

Conformational isomers are stereoisomers that can be converted into one another by rotating the molecule at a single bond. These molecules are called conformers. The conformation of a molecule is given in either staggered conformation or eclipsed conformation. The conformation of a molecule is the orientation or the arrangement of the atoms of a molecule when looked through the single bond that can be used for rotating the molecule.

The conformations of molecules are related to their potential energies. The staggered conformation has a minimized strain between the atoms. Therefore, it minimizes the potential energy in that molecule. The eclipsed conformation has the maximum strain between atoms. Therefore, the eclipsed conformation has the highest potential energy. The angle between the atoms in these conformations is called the dihedral angle. For staggered conformation, the dihedral angle is 60^o whereas the dihedral angle for eclipsed conformation is 0^o.

Figure 3: Two Main Conformations of Ethane

Moreover, there are two other conformations named as gauche and anti. When the molecule has a substituent, these conformers can be seen. The gauche conformation has a dihedral angle of 60^o between the substituents. The anti conformation has an 180^o dihedral angle.

Figure 4: Gauche, Anti and Eclipsed Conformations of Butane

The above image shows the gauche, anti and eclipsed conformations of butane. Here, the angle between two methyl groups is the dihedral angle.

Difference Between Configurational and Conformational Isomers

Definition

Configurational Isomers: Configurational isomers are stereoisomers that cannot be converted into one another by rotating the molecule around a single bond.

Conformational Isomers: Conformational isomers are stereoisomers that can be converted into one another by rotating the molecule at a single bond.

Types of Isomers

Configurational Isomers: There are two types of configurational isomers as geometrical isomers and optical isomers.

Conformational Isomers: There are four types of conformational isomers as eclipsed conformation, staggered conformation, gauche conformation and anti conformation.

Rotation of the Molecule

Configurational Isomers: The rotation of the molecule around a single bond does not give its isomer in configurational isomers.

Conformational Isomers: The rotation of the molecule around a single bond can give multiple isomers in conformational isomers.

Conclusion

Configurational and conformational isomers are two different types of isomers. The main difference between configurational and conformational isomers is that configurational isomers cannot be obtained by rotating the molecule around a single bond whereas conformational isomers can be obtained by rotating the molecule around a single bond.

References:

1. “Definitions: Examples of Conformational Isomers.” Definitions: Conformational Isomers (Examples), Available here. Accessed 12 Sept. 2017.
2. “5.2: Conformational Isomers.” Chemistry LibreTexts, Libretexts, 13 May 2017, Available here. Accessed 12 Sept. 2017.
3. “Conformational isomerism.” Wikipedia, Wikimedia Foundation, 13 Aug. 2017, Available here. Accessed 12 Sept. 2017.

Image Courtesy:

1. “Cis-trans example” By JaGa – Self-made using BKChem and Inkscape (CC BY-SA 3.0) via Commons Wikimedia
2. “Limonene struttura” By User:Paginazero – Own work (Public Domain) via Commons Wikimedia
3. “Escalonada e eclipsada” By Pauloquimico – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “Conformers” By Odie5533 – wp-en (Public Domain) via Commons Wikimedia