[1][2] It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base will abstract from an organic molecule, or where on a substituted benzene ring a further substituent will be added.
Various examples of regioselectivity have been formulated as rules for certain classes of compounds under certain conditions, many of which are named.
Among the first introduced to chemistry students are Markovnikov's rule for the addition of protic acids to alkenes, and the Fürst-Plattner rule for the addition of nucleophiles to derivatives of cyclohexene, especially epoxide derivatives.
In this reaction, an oxygen is regioselectively inserted near an adjacent carbonyl group.
In ketones, this insertion is directed toward the carbon which is more highly substituted (i.e. according to Markovnikov's rule).