T

Trans & Cis

Trans and Cis isomerism is actually called Geometric Isomerism, a form of stereoisomerism. This form of stereoisomerism arises from some restricted movement or rotation in the molecule. Pi bonds in the compound cause this restricted rotation. Thus, causing the surrounding Sigma bonds to be stuck in the same place. This restriction causes certain sigma bonds to be either in front of the primary carbons or behind the primary carbons. The key to determining whether a molecule is Trans or Cis is if the sigma bonds are on the same side as each other. Trans would be on the opposite side, one molecule in front and one in back, or Cis, which would be both in front or back.

For example:

See how the R groups are on the same side making it a Cis-isomer and having them on the opposite sides makes the molecule a Trans-isomer?

Trans-isomers are usually more stable than Cis-isomers due to the steric repulsion cause by having both of the R groups adjacent to each other. This usually is a dead give away for which molecule is more stable. Think of the R groups as same charged magnets. They will always push away from each other and never quite be “happy” unless they are near a magnet of the opposite charge, like the Hydrogen groups below.