Difference Between

Cis vs Trans Isomers
 

Isomers are different compounds with the same molecular formula. There are various types of isomers. Isomers can be mainly divided into two groups as constitutional isomers and stereoisomers. Constitutional isomers are isomers where the connectivity of atoms differs in molecules. Butane is the simplest alkane to show constitutional isomerism. Butane has two constitutional isomers, butane itself and isobutene.

In stereoisomers, atoms are connected in the same sequence, unlike constitutional isomers. Stereoisomers differ only in the arrangement of their atoms in space. Stereoisomers can be of two types, enantiomers and diastereomers. Diastereomers are stereoisomers whose molecules are not mirror images of each other. The cis trans isomers of 1,2-dichloroethene are diastereomers. Enantiomers are stereoisomers whose molecules are non-superposable mirror images of each other. Enantiomers occur only with chiral molecules. A chiral molecule is defined as one that is not identical with its mirror image. Therefore, the chiral molecule and its mirror image are enantiomers of each other. For example, 2-butanol molecule is chiral, and it and its mirror images are enantiomers.

As stated above cis trans isomerism or, in other words, E-Z isomerism is a form of stereoisomerism. Cis trans naming system applied for naming simple compounds, whereas E-Z system is used for the more complicated molecules. When a molecule has a restricted rotation in some place, cis and trans isomers can exist. That is when there are carbon – carbon double bonds, there is a restricted rotation, thus cis Trans isomers can be present. In a molecule like 1,2 dibromoethene, two bromine groups can be on the same side of the double bond or on the opposite side of the double bond. This doesn’t matter if the bond is a single bond, because then the atoms can rotate. However, in this case, two molecules aren’t the same. Though Cis and trans isomers have the same molecular formula and molecular weight, their physical properties are different.

Cis Isomers

The molecule in which two of the same atoms are on the same side of the double bond is known as the cis isomer. The cis isomer has the higher boiling point compared to the trans isomer. The reason for this is the stronger intermolecular forces in cis isomers. For example in 1,2-dichloroethene, when the molecule is cis, two more electronegative chlorine atoms are on one side of the molecule. Because of this, that side of the molecule will have a slight negative charge while the other side will have a slight positive charge. Therefore, the molecule becomes a polar and dipole-dipole interactions may occur between molecules. These extra intermolecular forces in cis isomer give it a higher boiling point compared to trans isomers.

Trans Isomers

Molecule with two of the same atoms in the opposite side of the double bond is known as the trans isomer. Trans isomers will have lower boiling points because although there is a charge separation, the overall molecule becomes non polar. But trans isomers have a higher melting point. trans isomers have a straighter shape, and they pack well. So a higher energy is needed to melt the molecule which gives it a higher melting point.