Question

What is regioselectivity and stereoselectivity?

Answer 1

Hint: Many chemical reactions involve the transformations of a reactant to multiple isomeric products. The preference for one product over another is known as selectivity. Regioselectivity and stereoselectivity are related to the formation of a selective isomer over another.

Complete answer:
The addition of a molecule to the reactant species having more than one attacking site can result in the formation of more than one product. The products can be isomeric with each other. But experimentally it was found that in such reactions, the formation of one product dominates over the another. This preferential formation of products is known as selectivity.
The reaction in which the formation of one constitutional isomer over another is known as regioselective reaction and this property of bonding at a particular site of a molecule to yield one of the isomers in a larger amount is known as regioselectivity.
Markovnikov additions are common examples of regioselective reactions. In addition to HBr to but-1-ene, we would see a preference for the formation of a secondary carbocation over primary carbocation, though it would seem either orientation is possible.

The reaction is said to be stereoselective when a reactant yields more than one stereoisomer product under the same reaction conditions and one of the products formed is in the larger amount. It is concerned with what conformation the products will be after the reaction.
A basic example of this is ${\text{S}}_{\text{N}}\text{2}$ reactions as given below.

Note:
A reaction in which the stereochemistry of the reactants decides the outcome of the reaction and produces a specific product is known as a stereospecific reaction. In simple words, stereoselectivity means preferential product formation over the others while stereospecificity means specific stereoisomers of reactant forms specific products.