Structural and Spatial Isomers

An isomer is one of two or more molecules that have the same molecular formula but different arrangements of atoms and bonds. Isomerism is often not possible in small molecules, such as water and carbon dioxide, but it becomes possible in larger molecules because the number of possible isomers increases greatly with the number of available atoms. There are two main types of isomerism. A structural isomer is one of two or more molecules that have the same molecular formula but different bond structures. Butane and isobutane, for example, both have the molecular formula C₄H₁₀. The arrangements of atoms and the shapes of the molecules, however, are different. Butane is a chain of four carbon atoms, but isobutane is a chain of three carbon atoms with a carbon atom attached to the central carbon atom. Although they have the same molecular formulas, butane and isobutane have different shapes and different chemical properties. Another example of structural isomers is 1-butyne and 1,3-butadiene. Both of these compounds have the chemical formula C₄H₆, but their atoms are arranged differently. Two of 1-butyne's carbon atoms are connected by a triple-bond, but 1,3-butadiene has two double bonds. A spatial isomer, or stereoisomer, is one of two or more molecules that have the same molecular formula and bond structure but different arrangements of atoms. Spatial isomerism can occur in various ways. Consider the molecule bromoiodomethane (CH₂BrI). In this molecule, two hydrogen atoms, one bromine atom, and one iodine atom are bonded to a central carbon. A mirror image shows that rotating the molecule results in the original molecule. The mirror image can be rotated 180 degrees and superimposed on its mirror image, so there is no isomerism in this case.
When a carbon atom has four different atoms attached to it, this situation changes. Consider fluorochlorobromomethane (CHBrClF), a compound with one bromine, one chlorine, one fluorine, and one hydrogen atom bonded to a central carbon. These four atoms can be attached to the carbon in two different ways, forming two enantiomers. An enantiomer has a mirror image that is not superimposable on itself. The molecules look similar but cannot be superimposed on each other regardless of how they are rotated. Another way spatial isomerism can occur is around double bonds. Molecules generally can rotate freely around single bonds, but this is not possible around a double bond because it has a fixed geometry. When carbon atoms have single bonds, they can rotate freely. Two molecules are cis−trans isomers if they have the same molecular formula and the same bond structure but the atoms differ in position around a rigid, or fixed, part of the molecule. Both of the isomers have exactly the same atoms joined up in exactly the same order. That means the van der Waals dispersion forces between the molecules will be identical in both cases. The difference between the two is that the cis isomer is a polar molecule and has its functional groups on the same side of its carbon chain, whereas the trans isomer is nonpolar and has its functional groups on opposite sides of the carbon chain. Consider the compound 1,2-dichloroethene. In a molecule of cis-1,2-dichloroethene, the chlorine atoms are on the same side of the carbon-carbon double bond. In trans-1,2-dichloroethene, the chlorine atoms are on opposite sides on the double bond.