Question

Construct a qualitative potential energy diagram for rotation about the C-C bond of 1,2-dibromo ethane. Which conformation would you expect to be most stable? Label the anti and gauche conformations of 1,2-dibromo ethane.

Short answer

An infinite number of spatial arrangement of group or atoms possible, in a molecule due to the free rotation of constituent group or atoms around carbon-carbon single bond is calledconformation.

Conformations can be classifies into two types- Eclipsed and staggered,

For example- in ethane eclipsed is when the hydrogen atoms present on singly bonded carbon atoms are as near as possible to each other. The dihedral angle in eclipsed conformation is least i.e., $0°$.

Staggered conformation is when hydrogen atoms attached to the singly bonded carbon atoms in consideration are as far apart as possible. The dihedral angle is$60°$ . Staggard conformations are more stable because, due to large separation between the electron clouds of C-H bonds, the repulsion is minimum and hence energy is minimum.

The conformation can be represented both by Sawhorse and Newman’s Projection formula.

It is to be noted that, when two bulky groups are placed at a dihedral angle of ${\mathbf{180}}^{\mathbf{\circ }}$in a staggered conformation, the conformation is called as an anti conformation, as both the bulky groups are anti periplaner to each other.

While when two bulky groups are positioned at a dihedral angle of more than ${\mathbf{0}}^{\mathbf{\circ }}$ but less than ${\mathbf{120}}^{\mathbf{\circ }}$, the staggered conformation then is called as Gauche conformation.

Step-by-step solution

Conformation

An infinite number of spatial arrangement of group or atoms possible, in a molecule due to the free rotation of constituent group or atoms around carbon-carbon single bond is calledconformation.

Conformations can be classifies into two types- Eclipsed and staggered,

For example- in ethane eclipsed is when the hydrogen atoms present on singly bonded carbon atoms are as near as possible to each other. The dihedral angle in eclipsed conformation is least i.e., $0°$.

Staggered conformation is when hydrogen atoms attached to the singly bonded carbon atoms in consideration are as far apart as possible. The dihedral angle is$60°$ . Staggard conformations are more stable because, due to large separation between the electron clouds of C-H bonds, the repulsion is minimum and hence energy is minimum.

The conformation can be represented both by Sawhorse and Newman’s Projection formula.

It is to be noted that, when two bulky groups are placed at a dihedral angle of ${\mathbf{180}}^{\mathbf{\circ }}$in a staggered conformation, the conformation is called as an anti conformation, as both the bulky groups are anti periplaner to each other.

While when two bulky groups are positioned at a dihedral angle of more than ${\mathbf{0}}^{\mathbf{\circ }}$ but less than ${\mathbf{120}}^{\mathbf{\circ }}$, the staggered conformation then is called as Gauche conformation.

Drawing the conformations obtained on rotation about the C-C bond of 1,2-dibromo ethane

First of all you will draw all the possible conformations (eclipsed and staggered) of 1,2-dibromo ethane . And name them as E1,S1,E2,S2,E3,S3.