Note : 3D models of conformers for 1,2-dichloroethane are created using Jmol Application version 14.31.32. To use the models interactively, place the mouse over the model, hold the left mouse button, move the mouse to rotate the model and view the model from all angles. Right-click to open the context-menu and explore the features to measure the bond-length, bond-angle, dihedral angle, change style, color of atoms bonds and many more features listed.

Structure 1 (Dichloroethane-eclipse.mol) Structures that can be interconverted by rotation about single bonds are conformers. The energy barrier to the rotation of C-C single bond in 1,2-dichloroethane molecule at room temperature in gas phase is about 12.5 KJ/mol or 3.0 Kcal/mol which is very similar to ethane, 12.1 KJ/mol or 2.9 Kcal/mol. The 3 significant conformations that 1,2-dichloroethane can adopt as the C-C bond is rotated in 60 degree intervals are: eclipsed, gauche and staggered. These conformations have different names. Look at the 3D models of molecules to see why the conformations have these names. In the eclipse conformation, one chlorine atom in the front eclipses the rear second chlorine atom, the front and rear carbons are superimposing each other, dihedral angle between two chlorine atoms is 0 degrees. The dihedral angle is also known as torsional angle. In this conformation, the atoms are very close to each other which results in the electrostatic repulsion between the electronic clouds of two bonded molecules. The dipole-diploe repulsions are strongest. It has the highest potential energy, hence it is the least stable conformation.

Structure 2 (Dichloroethane-gauche.mol) In the gauche conformation the two cholrine atoms are spaced apart with a dihedral angle of 60 degrees. It is more stable than the eclipsed conformation. This conformer exits in considerable quantities in the liquid state and in polar solvents. In polar solvents, the gauche conformer population increases due to decrease in the coulumbic interaction of dipoles in the polar solvent. The gauche conformer with high dipole moment solvates in polar solvent, therefore its population increases. In liquid state both staggered and gauche conformers are almost equally populated.

Structure 3 (Dichloroethane-staggered.mol) In the staggered conformation (anti conformation) the two chlorine atoms appear to be facing opposite directions, dihedral angle is 180 degrees. The staggered conformation is lower in energy than eclipse by 12.1 KJ/mol, this is the value of rotational barrier. In gaseous state at 22 degrees centigrade, 73% of 1,2-diclorobutane exists in staggered form when compared to 67% for n-butane. The higher stability of staggered conformer, also known as anti conformer, is because the two bulky chlorine atoms are furthest from each other. In the staggered conformation there may be some stabilizing interaction between the populated C-H σ bonding orbital on one carbon atom and the empty C-H σ* anti-bonding orbital on the neighbouring carbon. This stabilization is greatest when the two orbitals are parallel, this is present only in the staggered conformation.