Question

1. Make a model of each compound, draw it in its most symmetric conformation, and determine whether it is capable of showing optical activity.

1. 1-bromo-1-chloroethane
2. 1-bromo-2-chloroethane
3. 1,2-dichloropropane
4. Cis-1,3-dibromocyclohexane
5. Trans-1,3-dibromocyclohexane
6. Trans-1,4-dibromocyclohexane.

2.Star (*) each asymmetric carbon atom in part (1), label each as (R) or (S) and compare your result from part (1) with the prediction you would make based on the asymmetric carbons.

Short answer

Drawing Newman projections is the clearest way to determine symmetry of conformations.

Predictions of optical activity based on asymmetric centres give the same answers as predictions based on the most symmetric conformation.

Step-by-step solution

Step-1. Explanation of part 1. (a):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (a), 1-bromo-1-chloroethane is optically active and chiral center has “R” configuration and is capable of showing optical activity.

Step-2. Explanation of part 1. (b):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (b), 1-bromo-2-chloroethane is not optically active due to presence of plane of symmetry and does not have any chiral center and thus, is not capable of showing optical activity.

Step-3. Explanation of part 1. (c):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (c), cis-1,3-dibromocyclohexane is optically active and chiral center has “R” configuration and is capable of showing optical activity.

Step-4. Explanation of part 1. (d):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (d), 1-bromo-1-chloroethane is not optically active due to presence of plane of symmetry though it has two asymmetric carbons. It does not show any optical activity.

Step-5. Explanation of part 1. (e):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (e), trans-1,3-dibromocyclohexane is optically active and both chiral centers have “R” configuration and is capable of showing optical activity.

Step-6. Explanation of part 1. (f):

The single bonds in organic molecules are free to rotate due to “end to end” nature of their orbital overlap. Different arrangements resulting from sigma bond rotation are referred to as conformations. A Newman projection interprets the conformation of a chemical bond from front to back with the front atom represented by the dot and the back atom as a circle. The structure appears as viewed along the bond between these two atoms and the bonds from them to other groups are drawn as projections in the plane of paper.

In part (f), trans-1,4-dibromocyclohexane is not optically active due to presence of plane of symmetry through carbon-1 and carbon-4 and structure has no asymmetric carbons and as a result, will not show any optical activity.

Step-7. Explanation of part 2:

Asymmetric carbons have been star marked wherever possible in structures of part 1 and configurations have been assigned also. Predictions of optical activity based on asymmetric centers give the same answers as predictions based on the most symmetric conformation.