[Chemistry Class Notes] on Conformers Pdf for Exam

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

1. 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.

2. 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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