Geometric Isomers vs Structural 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. 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. Enantiomers are stereoisomers, whose molecules are nonsuperposable 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.
Geometric isomers are a type of stereoisomers. This type of isomers results, when molecules have a restricted rotation, basically, due to a double bond. When there is a single carbon – carbon bond, the rotation is possible. Therefore, however we draw the atoms, their arrangement will be the same. But when there is a carbon – carbon double bond, we can draw two arrangements of atoms in a molecule. The resulting isomers are known as cis, trans isomers or E-Z isomers. In cis isomer, same types of atoms are in the same side of the molecule. But in trans isomer, same types of atoms are in the opposite side of the molecule. For example, the cis and trans structures for 1,2-dichloroethane are as follows.
For a molecule to have geometric isomers, it is not only sufficient to have a double bond only. The two atoms or groups attached to one end of the double bond should be different. For example, following molecule doesn’t have geometric isomers, both atoms at the left hand end are hydrogens. Because of that, if we draw it in either cis or trans, both molecules are the same.
But it doesn’t matter if the all four attached groups or atoms are different. In that occasion, we can name them as E or Z.
These are also known as constitutional isomers. 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.
Since their connectivities are different, two molecules have different physical and chemical properties. Structural isomers can be formed by hydrocarbons where they have minimum four carbon atoms. There are three types of structural isomers as skeletal, positional and functional group isomers. In the skeletal isomerism, as given in the above example, the skeletal is rearranged to give different isomers. In position isomers, a functional group or another group changes position. In functional group isomers, though they have the same formula, molecules differ to having different functional groups.
What is the difference between Geometric Isomers and Structural Isomers?
• Geometric isomers are stereoisomers. Therefore, there connectivities are also same compared to structural isomers, where isomers differ due to atoms’ connectivities. In geometric isomers, they differ due to the three dimensional arrangement in the space.
• Often for a molecule there are two geometric isomers as cis, trans or E, Z, but for a molecule there can be a large number of structural isomers.
• Geometric isomerism is basically shown by molecule with carbon-carbon double bonds. Structural isomerism is shown by alkanes, alkenes, alkynes and aromatic compounds, as well.