Question

Label the molecules as cis or trans.

Answer 1

Cyclohexane has axial and equitorial bonds. axialbonds are parallel to the axis of the ring and equitorial bonds are perpendicular to the axis of the ring and lie along the equator of the chair form Equitorial and axial bonds of chair form of the cyclohexane are shown below. Equitorial bonds Axial bonds The axial positions are located above and below the ring whereas equitorial hydrogens are located in the plane of the ring. In the cyclohexane there are three up and three down carbons and they alternate around the ring. These can be shown asshown below Up carbon Down carbon The axial substituent is down on a down carbon, hence theequtorial substituent must be up. The axial substituent is up on an up carbon, hence the equitorial position must be down. A cis isomer has two substituents on the same side, either both on up bonds or both on down bonds A trans isomer has two substituents on opposite sides, one on up and one down.

In the structures A, B CI group lie below the plane and Br group lie above the plane. So, the molecules are trans. Br Br Cl Cl In the structures C, D Cl group lie below the plane and Br group lie below the plane. So, the molecules are cis. Br CI Cl Br