Stereoisomers are the isomers that have different geometrical arrangement of bonds in space but the same composition. Conformation is a property in which rotation of groups about a bond takes place due to which different spatial arrangements arise. Conformation isomers are a type of stereoisomers in which we can convert one isomers to another one, just by rotating it around a single bond. Now let us understand it with the help of an example:
Take the structure of butane and draw its Newman projection. We will get the structure as given below:
It has two substituents i.e. methyl groups attached to the two end carbon atoms. Methyl group is comparatively larger than hydrogen atoms. Everything on the front side is eclipsing the structure in the backside when looked at from any one carbon atom to the other. One methyl group is just in front of the other methyl group, and the other hydrogen atoms are kept just at the back of other hydrogen atoms. So this structure is completely eclipsing i.e. they are fully eclipsed. This conformational isomer of butane is known as Syn conformation. Now if we rotate the front methyl group by \(60^\circ \) i.e. the dihedral angle is now \(60^\circ \). Dihedral angle is the relative angle between the two methyl groups. In the first conformation, the dihedral angle is \(0^\circ \) so it is eclipsed. In the second one difference in the angle between methyl and methyl group is \(60^\circ \), it is known as gauche conformation. If we make the dihedral angle as \(120^\circ \) then this form of isomer is also known as eclipsed conformation. But this eclipsed form of isomer is different from the previous eclipsed or syn conformation because in this the methyl group is eclipsing the hydrogen atom and not the methyl group as earlier so this is also called eclipsed but it is different from the previous one. Again if we rotate the isomer by 60°then the dihedral angle is 180° that means the two methyl groups are exactly opposite to each other. So this conformation is known as staggered conformation or anti conformation. Rotating it by 60° more we get a gauche conformation which is just the opposite of the 2nd conformation. If the dihedral angle is made to 300° then we will get another conformation. But if we rotate it again by 60° then we reach the initial stage only. So there is a total of 5 different conformations of butane.
Which of the following will be the most stable conformation?
Observe the last diagram in the figure. This is anti conformation. In this, the dihedral angle is \(180^\circ \) and the methyl groups are just opposite to each other, due to which the repulsive force within the molecule is less and the geometrical arrangement is stable.
Stability of conformation of isomers:
Anti > Gauche > Eclipsed(not fully) > Eclipsed or syn(fully eclipsed)
We can also have the sawhorse projection.
Watch the video to know more about Conformational Stereoisomerism in detail.