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The possibilities for different isomers continue to multiply as more stereocenters are added to a molecule. In general, the number of stereoisomers of a molecule can be determined by calculating 2 n, where n = the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms.
The naturally occurring plant form of alpha-tocopherol is RRR-α-tocopherol whereas the synthetic form (all-racemic vitamin E, or dl-tocopherol) is equal parts of the stereoisomers RRR, RRS, RSS, SSS, RSR, SRS, SRR, and SSR with progressively decreasing biological equivalency, so that 1.36 mg of dl-tocopherol is considered equivalent to 1.0 mg ...
A racemic mixture is denoted by the prefix (±)-or dl-(for sugars the prefix dl-may be used), indicating an equal (1:1) mixture of dextro and levo isomers. Also the prefix rac- (or racem- ) or the symbols RS and SR (all in italic letters) are used.
An example of a molecule that does not have a mirror plane or an inversion and yet would be considered achiral is 1,1-difluoro-2,2-dichlorocyclohexane (or 1,1-difluoro-3,3-dichlorocyclohexane). This may exist in many conformers ( conformational isomers ), but none of them has a mirror plane.
In this approach: identify the chiral center, label the four atoms directly attached to the stereogenic center in question, assign priorities according to the sequence rule ( from 1 to 4), rotate the molecule until the lowest priority (number 4) substituent is away from the observer/viewer, draw a curve from number 1 to number 2 to number 3 ...
A molecule may contain any number of stereocenters and any number of double bonds, and each usually gives rise to two possible isomers. A molecule with an integer n describing the number of stereocenters will usually have 2 n stereoisomers, and 2 n−1 diastereomers each having an associated pair of enantiomers.
Absolute configuration showing the determination of the R and S descriptors. In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or group) that is chiral, and its resultant stereochemical description. [1]
Glyceraldehyde has one chiral center and therefore exists as two different enantiomers with opposite optical rotation: . In the D/L nomenclature, either D from Latin Dexter meaning "right", or L from Latin Laevo meaning "left"