In this article named as “conformational isomers example”, different types of examples on conformational isomers with detailed explanations are discussed briefly.
The examples of conformational isomers are-
- Ethane
- Propane
- Butane
- 2,3-Dimethylbutane
- 1,2-Difluroethane
- Hydrogen peroxide
- Ethylamine
- Ethylene glycol
- Hydrazine
- 2,3-dibromobutane
- 1,2-dichloroethane
What are conformational isomers?
Conformational isomers is a very significant term in stereochemistry. It is defined as the interconversion between two forms of same molecule by rotating of the single or sigma bonds.
The study of the energy diagram of different conformations is called conformational analysis. It helps to explain the relative stability of different conformational isomers of a molecule. It also determines the product selectivity, mechanisms and the rate of any reaction. Conformational isomers are classified in the following categories. They are-
- Eclipse conformation
- Staggered conformation
The above two conformation types can be represented by the following conformation-
- Sawhorse projection
- Newman projection
Ethane
In ancient time it was thought that that rotation about carbon-carbon single bond is free. But from the entropy calculation it was decided that this rotation is not totally free. There is an energy barrier of 3kcal/mol for ethane molecule.
Among all the conformers of ethane, staggered form is the most stable due to minimize the repulsion between hydrogen atoms and eclipsed form is the least stable due to the maximum repulsion between hydrogen atoms.
Dihedral angle (the angle between two planes containing C1 – H and C1 – C2 bond and another plane contains C1 – C2 and C2 – H bond) is 600 for staggered conformation and 00 foe eclipsed conformation.
Image Credit: Wikimedia Commons.
Propane
Propane is the next simplest compound in the alkane family after ethane. Like ethane, it has also two conformational isomers, staggered and eclipsed. The rotational energy barrier between the carbon-carbon single bond is slightly higher than ethane, 3.35 kcal/mol.
In eclipsed form (dihedral angle 00) hydrogen and the methyl group is proximal to each other. Thus, this conformer is relatively unstable than the staggered conformer or anti conformer.
Butane
Image Credit: Wikimedia Commons.
In the above image the different conformers of butane are shown with their respective dihedral angles. The conformations of butane with 600 and 3000 dihedral have mirror image relationship with each other. Same is applicable for the conformers having dihedral angle 1200 and 2400. All the conformers are introduced with different names-
| Dihedral Angle | Name of the conformation |
| 00 | Syn periplanar |
| 600 | Synclinal or gauche |
| 1200 | Anticlinal |
| 1800 | Antiperiplanar |
| 2400 | Anticlinal |
| 3000 | Synclinal or gauche |
In the syn-periplanar and anti-peri planar conformation, the dihedral angle is 00 and 1800 respectively.
2,3-Dimethylbutane
Image Credit: Wikimedia Commons.
In the above image, conformers are shown with the difference in dihedral angle 600. Number ‘1’ denoted conformers are known as anti-periplanar with the dihedral angle 1800 and this is the most stable conformer among all of the above due to having minimum repulsion between two large methyl groups.
1,2 Difluroethane
The energy barrier of carbon-carbon single bond rotation is 3.4-3.6 kcal/mol. Like the previous examples, anti form of 1,2 Difluroethane is not the most stable conformer. In this molecule, gauche form is the most stable form due to a stabilizing interaction between two fluorine atoms.
Though there is a dipole-dipole repulsion between the two F atoms but there is also a favourable interaction between σ C-H and σ* C-F bonds. (σ and σ* represents bonding and antibonding molecular orbital respectively). This specific interaction is called “gauche effect”.
Image Credit: Wikimedia Commons.
Hydrogen peroxide
Like the example of 1,2-Difluroethane, gauche conformation of hydrogen peroxide (H2O2) is stable rather than anti conformer. In gauche conformer two lone pairs of oxygen are is 600 angle with each other. There is a stabilizing interaction working between the two lone pairs.
Thus, gauche form of hydrogen peroxide is more stable than anti conformer.
Ethylamine
The energy required for a carbon-carbon single bond rotation is lesser than the carbon-nitrogen single bond rotation due to vander waals repulsions as a consequence of shorter carbon-nitrogen bond with respect to carbon-carbon sigma bond.
Like the previous example, gauche interaction stabilizes the gauche conformer with comparing to anti conformer because lone pairs have relatively small size than hydrogen or any other substituent. Thus, this gauche interaction becomes favorable.
Ethelene glycol
One of the significant example of gauche interaction is ethelyne glycol. In this molecule gauche conformation is stabilized due to the intramolecular hydrogen bonding.
When two OH groups are in 600-650 angle with each other, two OH groups of ethylene glycol participate in H-bonding with each other. This reduces the system energy relatively. Thus, gauche conformer is more stable than anti conformer. Because no H-bonding is not possible in anti conformer due to having 1800 angle between two OH groups (shown below).
Hydrazine
Hydrazine is an conformational isomers example of carbon-hetero atom bond. Between staggered and eclipsed conformation, staggered conformation is most stable. Very precisely gauche conformation is the most stable among all the staggered representation.
The two lone pairs of two nitrogen atoms are participating in gauche interaction. This favorable interaction between two lone pairs having angle 600 with each other, stabilizes the gauche staggered conformation over the anti conformer.
2,3-dibromobutane
This compound is also appeared in different conformations. Among all the conformations, anti conformer is the most stable due to having minimum repulsion.
But different anti conformations are available for 2,3 dibromobutane. In which anti conformer two Br groups, two methyl and two hydrogen atoms are in 1800 angle with other is the most stable. Because Br and methyl both are large atom and group. So, in eclipsed or staggered conformer the molecule faces huge repulsion. Thus, eclipses conformers have very much huge energy due to having the repulsion of two Me and two Br groups.
1,2-dichloroethane
This is almost a similar type example of 2,3-dibromo butane. Due to having dipole moment and steric repulsion of two chlorine atoms, anti form is more stable than the gauche form.
To minimize the energy and dipolar repulsion. 1,2 dichloroethane presents in anti conformer.
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