Final answer:
The active forms of labetalol, (S,R)- and (R,R)-labetalol, are diastereomers because they have different configurations at one of their chiral centers and are not mirror images of each other.
Step-by-step explanation:
The active forms of the drug labetalol, (S,R)- and (R,R)-labetalol, can be described as diastereomers. Diastereomers are stereoisomers (compounds with the same molecular formula and sequence of bonded atoms) that have different configurations at one or more (but not all) of the corresponding chiral centers and are not mirror images of each other. The prefixes (S) and (R) define the absolute configuration of the chiral centers within the molecule. When two compounds differ in configuration at only one chiral center, they are diastereomers, as is the case with (S,R)- and (R,R)-labetalol; they share one chiral center with the same configuration and differ at the other.
In enantiomers, the molecules are mirror images of each other and non-superimposable, such as D- and L-alanine. This is not the case for (S,R)- and (R,R)-labetalol, which are not mirror images of each other. Conformational isomers, on the other hand, differ only by rotation around a single bond, which is not relevant for the configuration of labetalol. Therefore, (S,R)- and (R,R)-labetalol are not enantiomers or conformational isomers.