[Chemistry Class Notes] on Conformation Pdf for Exam

Conformation can be defined as the shape adopted by a molecule caused by the rotation around one or more single bonds. For instance, in the case of alkanes, there is a distribution of electrons around the internuclear axis of the C-C bond. This further permits free rotation around C-C single bonding which leads to different spatial arrangements of the carbon atoms. The process explained above is called conformation. Moreover, alkanes thereby also have an infinite number of conformations by rotation around single C-C bonds. The Conformational Isomers are also ranked differently. The ranking of conformational isomers is contingent upon the energy levels. 

What are Conformational Isomers?

Conformational isomers are commonly referred to as stereoisomers as they can be converted into one another by rotation around the sigma single bond. For example, in the case of ethane, the different spatial arrangements of hydrogen atoms attached to one carbon atom concerning the hydrogen atoms attached to the other carbon atom are observed, and these are called conformational isomers or conformers. 

As mentioned above, the ranking of conformational isomers depends on the energy levels. Considering the energy levels to be lowest to highest, the ranking of conformational isomers are: anti, gauche, eclipsed, and fully eclipsed.  

Types of Conformational Isomers

Even though there are infinite spatial arrangements created by conformational isomers, there are two broad categories classified into two different cases as Eclipse conformation and Staggered Conformation. 

In the case of Eclipse conformation, hydrogen atoms are attached to two carbon areas that are as near to each other as possible. A torsion angle is created between two substituents such as X and Y with adjacent atoms such as A and B. This torsion angle between X-A-B-Y is of 0o. This is how conformed are formed by eclipsed conformation.

On the other hand, in the case of Staggered conformation, hydrogen atoms are attached to two carbons that are as far as possible from each other. Moreover, in terms of stability, staggered conformation is relatively more stable and the repulsive forces are minimum, energy is minimum due to large separations between the electron clouds of C-H bonds. 

Representation of Eclipsed and Staggered Conformation

There are two popular ways of representation of eclipsed and staggered conformation. These representations are known as Sawhorse projections and Newman Projections. Let’s define sawhorse projections first.

In Sawhorse projections, carbon atoms are bonded as if they are in a long straight line. In this straight line, the lower end of the line designates the front carbon atom whereas the rear carbon atom is designated by the upper end. Another key characteristic of the Sawhorse projection is that the C-H bonds are inclined at an angle of 120° to each other.

The second way of representation of eclipsed and staggered conformation is known as Newman’s projections. A new man projection is also helpful in the stereochemistry of alkanes. For example, in the case of ethane, out of the two carbon atoms present, the closer one is marked as a dot while the rear carbon atom is represented as a circle. The front carbon is called proximal and the back carbon is called the distal. 

A specific dihedral angle is illustrated in this type of representation between the proximal and distal atoms. The lines are again inclined at an angle of 120° to each other in such a way that the three lines, from three hydrogen atoms attaching to one carbon atom, seem to either bulge out of the circle or diverging from the dotted lines.