Question

Determine the point groups for a. Ethane (staggered conformation) b. Ethane (eclipsed conformation) c. Chloroethane (staggered conformation) d. 1,2-Dichloroethane (staggered anti conformation)

Answer 1

Answer and Explanation:

Every molecule has a set of symmetry operations that helps in determining its overall symmetry. This set of operations determines the point group of a molecule.

(a) Ethane (staggered conformation)

Ethane in the staggered conformation has:

• two $C_{3}$ axis (the C-C line)
• 3 perpendicular $C_{2}$ rotational axis bisecting the C-C line, in the plane of the two C's and H's on the opposite sides of the two C's
• no σ$_{h}$ plane
• but has 3σ$_{d}$ planes containing two carbons and anti two hydrogens

hence the point group of ethane in staggered conformation is  $D_{3d}$

(b) Ethane (eclipsed conformation)

Ethane in the eclipsed conformation has:

• two $C_{3}$ axis (the C-C line)
• three perpendicular $C_{2}$ axis (in the plane of the two C's and H's on the same side of the two C's)
• mirror planes include σ$_{h}$ and 3σ$_{d}$

Hence the point group of ethane in eclipsed conformation is $D_{3h}$

(c) chloroethane (staggered conformation)

Chloroethane in the staggered conformation has:

• no rotational axis

• σ plane containing two carbons and chlorine and the anti hydrogen of Cl

Hence the point group of staggered conformation in chloroethane is $C_{s}$

(c) 1,2-Dichloroethane (staggered anti conformation)

1,2-Dichloroethane in the staggered anti conformation has:

• $C_{2}$ axis perpendicular to the C-C bond and perpendicular to the plane of Cl-C-C-Cl
• contain a σ$_{h}$ plane through Cl-C-C-Cl, and an inversion centre

Hence the point group in the staggered anti conformation of 1,2-Dichloroethane is $C_{2h}$