Regioselectivity

The term regioselectivity refers to which region, or part of the molecule, is involved in the reaction. Regioselectivity comes into play when doing elimination reactions (E1 and E2) as well as for addition reactions to multiple bonds (double and triple bonds).

Imagine that you are trying to perform an elimination reaction of 2-butanol. Following the pick up of a proton (H⁺) to produce the sec-butyloxonium ion, a secondary carbocation will be formed when the water comes off. This 2^o carbocation will then lose a proton from either carbon-1 (C-1) or carbon-3 (C-3). Which carbon loses the proton is based on the regioselectivity of the reaction. For example, if the proton comes off of C-1, then 1-butene would be produced.  On the other hand, if the proton came off of C-3, then 2-butene would be formed. In fact, Zaitsev's Rule used to predict the more stable product is based on regioselectivity criteria.

Another example involving regioselectivity could be an addition reaction.  The best example of an addition reaction involving regioselectivity would be hydrohalogenation or hydration reactions.  When you perform a hydrohalogenation reaction, you apply Markovnikov's Rule which states that, "the H goes to the carbon that has the most hydrogens" because the other carbon forms the more stable carbocation.  Hence, the region of the molecule involved in the reaction is determined by regioselectivity. Likewise, for a hydration reaction, the -H goes to the carbon that has the most hydrogens, and the -OH goes to the other carbon. This is regioselectivity.

Therefore, whenever you are concerned with which part of the molecule is involved in a reaction, then regioselectivity is the term being referred to.

For determination of which stereoisomer (such as for the creation of a cis- or trans- geometric isomer) is formed, the term would be stereoselectivity.