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Stereoisomers have different spatial orientations despite the same molecular formula. Enantiomers are mirror images that can have different biological properties. Chiral centers are atoms with stereoisomers. Cis-trans isomers have double bonds with hydrogen substitutions on both sides. A new German naming system uses atomic numbers to assign priority. Structural isomers have atoms in different orders and are not stereoisomers.
Stereoisomers are compounds that have atoms in a different spatial and three-dimensional orientation, even if the atoms are connected in the same order and have the same molecular formula. The classic example of these is that of a mirror image between compounds, such as a right and left hand. Compounds that differ in this way are known as enantiomers. They can be differentiated in how they rotate polarized light. A compound can have different atoms which have stereoisomers and these are known as chiral centers.
Differences in stereoisomerism can profoundly affect the biological properties of compounds. For example, there were enantiomers of the drug thalidomide that caused many birth defects in the mid-1900s. Only one of the enantiomers caused the birth defects. With most chiral compounds, only one form is found in nature.
Enantiomers are also known as optical isomers. Traditionally, compounds chemically related to a known dextrous compound were known as D compounds. Their enantiomers were known as L compounds. This nomenclature is still applied to amino acids and carbohydrates. Most natural amino acids are in the L form, while those in carbohydrates, such as the biologically active form of glucose, are in the D form.
The nomenclature of these stereoisomers has been updated to negate the need for a reference compound. Each chiral position in a molecule is assigned an R or an S, which comes from the Latin rectus for right, or sinister for left. There is a code for prioritizing atoms attached to the chiral center.
There is another type of stereoisomer which is not a mirror image. These have double bonds with a hydrogen substitution on both sides of the bond. These compounds are known as cis-trans isomers. This terminology also comes from the Latin.
Imagine two carbon atoms connected by a double bond. There are two chlorine atoms at either end of the molecule, along with two hydrogen atoms. Both chlorine atoms can be on the same side of the double bond, or they can be on opposite sides. Atoms on the same side are cis, since on this side. If I’m on the other side, I’m trans, sideways.
There is a new additional stereoisomer naming system due to some ambiguity. This system is based on German terminology and uses the atomic number of substituents to assign priority. If the atoms with the higher atomic number are on the same side, the molecule is Z for zusammen, or together. If they are on different sides, the compound is E for entgegen, or opposite. This is not always interchangeable with cis and trans.
It is worth noting that compounds can have the same molecular formula, but not be stereoisomers. This is the case for structural isomers, which have their atoms in different orders. For example, there are multiple forms of bromobutane. CH3CH2CH2CH2Br and CH2BrCH2CH2CH3 share the same atoms, but are not stereoisomers.
