<P> Tautomers are structural isomers that spontaneously interconvert with each other, even when pure . They have different chemical properties and, as a consequence, distinct reactions characteristic to each form are observed . If the interconversion reaction is fast enough, tautomers cannot be isolated from each other . An example is when they differ by the position of a proton, such as in keto / enol tautomerism, where the proton is alternately on the carbon or oxygen . </P> <P> A simple example of isomerism is given by propanol: It has the formula C H O (or C H OH) and occurs as two isomers: propan - 1 - ol (n - propyl alcohol; I) and propan - 2 - ol (isopropyl alcohol; II) </P> <P> Note that the position of the oxygen atom differs between the two: It is attached to an end carbon in the first isomer, and to the center carbon in the second . </P> <P> There is, however, another isomer of C H O that has significantly different properties: methoxyethane (ethyl - methyl - ether; III). Unlike the isomers of propanol, methoxyethane has an oxygen connected to two carbons rather than to one carbon and one hydrogen . Methoxyethane is an ether, not an alcohol, because it lacks a hydroxyl group, and it has chemical properties more similar to other ethers than to either of the above alcohol isomers . </P>

Isomers only occur when a molecule or ion has one or more double bonds