Final answer:
An addition reaction creating a new asymmetric center in a molecule with an existing one leads to the formation of stereoisomers, which include both enantiomers and diastereomers, due to multiple chiral centers.
Step-by-step explanation:
When an addition reaction creates an asymmetric center in a compound that already has an asymmetric center, stereoisomers are formed, specifically a mixture of diastereomers and enantiomers. An asymmetric center, also known as a chiral center, is a carbon with four different groups attached. If the original molecule has one asymmetric center and another one is created during the reaction, two new chiral centers may give rise to multiple stereoisomers.
The stereoselectivity of electrophilic addition reactions can lead to the formation of either (R) or (S) configurations at a new chiral center, resulting in a racemic mixture when reactants and catalysts are achiral. However, due to the original asymmetric center, the resulting compounds are not simply mirror images (enantiomers) but may also be non-superimposable, non-mirror images of each other, known as diastereomers.
For example, in the addition of bromine to an alkene with an existing asymmetric center, the bromine can attack from either side of the planar carbocation intermediate formed during the reaction, leading to the formation of both (R) and (S) configurations at the new chiral center. This results in a combination of enantiomers and diastereomers.