What is regioselectivity?
Regioselectivity is the preference for one orientation over another in the arrangement of a reaction product.
Regiochemisry in alkene addition reactions refers to which part the reagent adds to which part of the double bond. This is also called the "orientation of addition."
Regioselectivity is the preference for one orientation over another in the arrangement of a reaction product, such as in an addition reaction. This is exemplified in Markovnikov's rule, which states that, "in an electrophile addition to an alkene, the electrophile adds in such a way as to generate the most stable intermediate" (1).
For example, where Markvnikov's rule is applicable, we would see preference for the formation of a tertiary carbocation over a secondary or primary carbocation, though it would seem either orientation is possible.
Here is an example for the addition of HBr to but-2-ene (1):
We might predict that we could have either the above product or
but we only observe the first through experimentation. This is predicted by Maokovnikov's rule; there is a preference for the bromine atom to bond to the more highly substituted carbon atom, giving the more stable orientation. Because there is a preference for one of these two possible orientations, the addition of HBr (and other hydrogen halides) is regioselective.
Similarly, in elimination reactions, for example, where we might see rearrangements in the products, this is always to achieve a more stable arrangement, indicating there is a preference for one orientation over the other. It is usually the case where the more stable product in an elimination is that in which the pi bond has a higher degree of substitution (bonded to more substituents), by Zaitsev's rule.
- Wade, L. G.; Simek, J. W. In Organic Chemistry; Pearson: Glenview, IL, 2013; pp 362–364.