In structural isomers, sometimes referred to as constitutional isomers, atoms and functional groups are joined in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group. This group includes chain isomerism for which the hydrocarbon chains have variable branching quantities; positioning of isomerism, which refers to the position of a functional group on a chain; and isomerism of the functional group, in which a functional group is divided into several.
In skeletal isomers, the main carbon chain is different between the two isomers. This type of isomerism is more identifiable in the isomers of secondary and tertiary alcohol.
Tautomers are structural isomers that spontaneously interconnect with each other, even when they are pure. They have different chemical properties and, consequently, distinct characteristic reactions of each form are observed. If the interconversion reaction is fast enough, tautomers can not be isolated from one another. An example is when they differ from the position of a proton, as in the keto / enol tautomerism, where the proton is alternatively on carbon or oxygen.
Example : propanols and methoxyethane
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and is presented as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)
Note that the position of the oxygen atom differs between the two: it is attached to a final carbon in the first isomer and to the central carbon in the second.
There is, however, another isomer of C3H8O which has significantly different properties: methoxyethane (ethyl methyl ether; III). Unlike propanol isomers, methoxyethane has an oxygen connected to two carbon atoms rather than a carbon and a hydrogen. Methoxyethane is an ether, not an alcohol, because it lacks a hydroxyl group, and has chemical properties more similar to other ethers than to one of the aforementioned alcohol isomers.