Note: the dashed wedge indicates the species (molecular entity) is going behind the rest of the structure, whereas the bold wedge indicates it is coming out of the computer screen, towards the viewer.
A chiral molecule refers to a molecule that has at least two mirror image forms that cannot be rotated such that the two mirror forms become identical. These mirror image forms are called stereoisomers. The centre of the part of the molecule that is not super-imposable is said to be the stereocentre. They occur solely when one has four atoms or groups of atoms (e.g., an ethyl [CH2-CH3] group; these atoms/groups of atoms are referred to as substituents) covalently attached (chemically bonded) to a central atom (usually a carbon). Another condition required for a stereocentre to exist is that no two, or more, of the substituents can be identical.
Examples of chiralityEdit
For example, methane, which is composed of a carbon with four hydrogen substituents, is not chiral, as the hydrogen molecules are all identical to each other. Similarly risperidone is achiral (that is, non-chiral) as there are no four-bonded atoms with no symmetry amongst the substituents. Despite this, it is very easy to make risperidone to become chiral, by a simple singular change in the one of the substituents on the piperidine ring. For example, replacing a hydrogen substituent on one of the carbons of the piperidine ring to a methyl group, makes the molecule chiral with the stereocentre being the carbon indicated in this diagram.
The two stereoisomers can be distinguished from each other by means of several different naming systems: there's those based on the optical activity of the molecule, in which + and - is the preferred terminology, which are synonymous with d and l respectively; D and L are based on how the molecule looks the configuration of glyceraldehyde; R and S are based on the Cahn–Ingold–Prelog priority (CIP) rules listed below. For most formal circumstances the R, S system is preferred. Two stereoisomers may also be said to have two different configurations.
Types of stereoisomersEdit
When there are more than one stereocentre, there are more than two stereoisomers, in these cases some additional terminology comes in. For the examples listed earlier in this page both stereoisomers can also be said to be enantiomers of each other.
Two compounds are said to be enantiomers of each other if their configurations at every stereocentre are the exact opposite of each other. For example, for tilidine there are two enantiomers: (1R,2S)- and (1S,2R)- tilidine, respectively, as each of these enantiomers have reverse configurations of each other (that is, R→S, S→R between them).
Diastereomers are those in which not all of the configurations at the stereocentres have been reversed. For example, (1R,2R)-tilidine would be a diastereomer of (1R,2S)-tilidine.
Optical activity refers to if you were to dissolve the enantiomer in water, what would happen if you sent a plane of polarized light through the water - if the light rotates clockwise, it is the (+)-enantiomer, whereas if it rotates counter-clockwise it is the (-)-enantiomer.
These rules basically tell one to find the stereocentres, then assign priorities to the substituents of the stereocentres, based on their atomic number (Z) of the substituent. Then one is told to orient the molecule, in 3D, such that the lowest priority molecule (usually hydrogen when one is dealing with drug molecules) is coming out underneath the stereocentre. From there one is to connect from highest priority substituents to lowest priority: if the direction is clockwise it is (R), if it is counter-clockwise it is (S). See R is short for rectus, which is Latin for right, whereas S is short for sinister, which is Latin for left.
In the above structure it is clear that in dexamfetamine, similarly to many other chiral drug molecules, two of the substituents of the chiral carbon, are, in themselves, carbons. The natural question is, of course, "Then doesn't that violate the rule of stereocentres that every substituent must be unique?" The answer is that we count the entire substituent, including all atoms the joining atom is attached to. Priorities are assigned based on the entirety of the substituent: if two joining atoms are identical, we look to the next atom in the substituent and assign it a priority based on its Z-number.