The field of Stereochemistry involves the study of bond distances and dihedral angles along with the basic principles, conformations, and configurations. It includes the methods of writing structures in two-dimensional and three-dimensional projections. It involves the atom group configurations in a molecule and the energy associated with these configurations. Atom groups usually rotate around different carbon-carbon axes, and the various shapes obtained are called conformations. The compounds that have the same molecular formula are known as isomers. When a group of atoms that make up the molecules of different isomers is bonded together then a constitutional isomer is formed.
Conformational Isomers
It involves rotation about sigma bonds and does not have any difference in the connectivity or geometry of the bonding. Two of the same molecules that only differ in terms of the angle of about one or more sigma bonds can be categorized as conformational isomers or conformers. If the group is large enough to significantly affect the rotation energy, it tends to prefer certain spatial arrangements. The spatial structures of the groups give rise to conformers.
Types of Conformational Isomers
Eclipse Conformation
The carbons are aligned in a way such that the hydrogen atoms are lined up with one another. It creates a steric hindrance between them. The hydrogen atoms attached to the two carbon atoms are as close to each other as possible in the eclipsed conformation. It is termed to be a little unstable due to the closeness of hydrogen atoms.
Staggered Conformation
Here the hydrogen atoms attached to the two carbon atoms are as far away from each other as possible. In the staggered conformation, the atoms are all equally spaced from each other, and these conformations are more stable than the eclipsed conformation and are more favoured. The reason for it being more stable is that the hydrogen atoms are far away from each other. The spacing creates minimum repulsive force and minimum energy between the electron clouds of C-H bonds.
Conformational Isomers of Ethane
Ethane, organic compound, is a colourless and odourless gas at room temperature. It consists of seven sigma bonds and six carbon-hydrogen bonds. The six carbon-hydrogen bonds protrude two carbons at 120° angles. Its lowest energy conformation is called the 'staggered' conformation. In this conformation, all of the C-H bonds on the front carbon are positioned at 60° relative to the back carbon's C-H bonds. Along with the 60° positioning, the distance between bonds is also maximized.
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Furthermore, upon 60° clockwise rotation of the front CH3 group, the molecules now attained the highest energy conformation of ethane, which is the 'eclipsed' conformation. In this conformation, the hydrogens on the front carbon are as close as possible to the back carbon's hydrogens. The energy produced by the eclipsed conformation is 3 kcal/mol higher compared to the staggered conformation.
Conformational Isomers of Butane
The alkane called Butane has C-C bonds. It is a little different than that of ethane. In butane, when the molecule is rotated at the C-C bond axis, different conformational isomerism is obtained. Butane has two substituents, which is the methyl group attached to the two end carbon atoms. The methyl group is more extensive than hydrogen atoms. If the front methyl group is rotated by 60°, then we attain the gauche or staggered conformation of butane. If we rotate the methyl group by 120°, then the gauche turns into what is known as the eclipsed conformation of butane.
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FAQs on Conformers
1. What are conformers in chemistry?
Conformers, or conformational isomers, are different spatial arrangements of the same molecule that can be interconverted by rotation around one or more single (sigma) bonds. These arrangements are not different compounds because they interconvert rapidly at room temperature and do not involve breaking or making chemical bonds. They represent various momentary shapes a molecule can adopt.
2. How are the eclipsed and staggered conformers of ethane different?
The eclipsed and staggered conformers of ethane represent the highest and lowest energy states, respectively. Their key differences are:
- Arrangement: In the staggered conformation, the hydrogen atoms on the back carbon are positioned exactly in between the hydrogen atoms of the front carbon (dihedral angle of 60°). In the eclipsed conformation, the hydrogens on both carbons are aligned directly behind one another (dihedral angle of 0°).
- Stability: The staggered conformation is more stable because the distance between the electron clouds of the C-H bonds is maximised, minimising repulsive forces.
- Energy: The eclipsed conformation has higher energy due to torsional strain caused by the repulsion between the aligned C-H bonds.
3. How does a Newman projection help in visualising conformers?
A Newman projection is a way of visualising a molecule by looking down the axis of a specific carbon-carbon bond. The front carbon is represented by a dot, and the back carbon is represented by a circle. This view makes it easy to see the dihedral angle between substituents on the front and back carbons, which clearly distinguishes between staggered, eclipsed, or gauche conformations and helps in analysing their relative stability.
4. What are the main conformers of butane, and how do they differ from those of ethane?
Unlike ethane, butane (CH₃-CH₂-CH₂-CH₃) has bulkier methyl (-CH₃) groups, which leads to more types of conformers when rotating around the central C2-C3 bond. The main conformers are:
- Anti: The most stable form, where the two methyl groups are 180° apart, minimising all repulsion.
- Gauche: A type of staggered conformation where the methyl groups are 60° apart. It is less stable than anti due to steric hindrance between the methyl groups.
- Eclipsed: The least stable forms where groups are aligned. The highest energy form is when the two methyl groups are fully eclipsed (0° apart).
The key difference from ethane is the introduction of steric hindrance, in addition to torsional strain.
5. Why is the staggered conformation generally more stable than the eclipsed conformation?
The staggered conformation is more stable primarily because it minimises torsional strain. Torsional strain is the repulsion that occurs between the electron clouds of bonds on adjacent atoms. In the eclipsed conformation, these bonds are aligned, maximising this repulsion and raising the molecule's potential energy. In the staggered form, the bonds are positioned as far apart as possible, leading to the lowest energy and greatest stability.
6. What is the key difference between conformers and constitutional (structural) isomers?
The fundamental difference lies in their atomic connectivity and interconversion. Conformers have the same molecular formula and the same connectivity; they are just different 3D shapes of the same molecule that interconvert by rotation around single bonds. Constitutional isomers have the same molecular formula but different connectivity, meaning the atoms are bonded in a different order. Converting between constitutional isomers requires breaking and reforming chemical bonds.
7. Can conformers be isolated as separate compounds at room temperature? Why or why not?
No, individual conformers generally cannot be isolated at room temperature. This is because the energy barrier required for rotation around a single bond is very low (e.g., about 12 kJ/mol for ethane). The thermal energy available at room temperature is sufficient for the molecules to rotate freely and rapidly interconvert between all possible conformations, existing as a dynamic equilibrium of these forms.
