Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. Geometric isomerism is sometimes considered synonymous with stereoisomerism, but is often used to describe just those stereoisomers that are not mirror-images of each other.
A stereoisomer generally occurs when a double or triple bond is present, because the pi pond involved prevents that bond from being "twisted" the same way that a single bond can be. One of the best examples is C2H2Cl2. Consider the two examples below:
H H H Cl \ / \ / C = C C = C / \ / \ Cl Cl Cl H
These are stereoisomeres because you cannot rotate the two carbon atoms relative to each other, due to the rigidity caused by the pi bond between them. Therefore one will not fit on top of the other.
Stereoisomers always require that an additional nomenclature prefix be added to the IUPAC name in order to indicate their spatial orientation, for example, cis (Latin, meaning on this side) and trans (Latin, meaning across) in the 2-butene case.
Stereoisomers are also observed in certain disubstituted (and higher substituted) cyclic compounds.
As in the case of the 2-butene stereoisomers, disubstituted cycloalkane stereoisomers may be designated by nomenclature prefixes such as cis and trans.
Stereoisomers are molecules that are identical in atomic constitution and bonding, but differ in the three-dimensional arrangement of the atoms.
Stereoisomers include not only the mirror image enantiomers, but also geometric (cis/trans) isomers and diastereoisomers (isomers of drugs with more than one chiral center that are not mirror images of one another).
To develop a single stereoisomer from a mixture that has already been studied non-clinically, an abbreviated, appropriate pharmacology/toxicology evaluation could be conducted to allow the existing knowledge of the racemate available to the sponsor to be applied to the pure stereoisomer.
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