Chiral molecules have two forms (at each point of asymmetry), which differ in their optical characteristics: The levorotatory form (the (−)-form) will rotate counter-clockwise on the plane of polarization of a beam of light, whereas the dextrorotatory form (the (+)-form) will rotate clockwise on the plane of polarization of a beam of light. The two forms, which are non-superposable when rotated in 3-dimensional space, are said to be enantiomers. The notation is not to be confused with  and  naming of molecules which refers to the similarity in structure to -glyceraldehyde and -glyceraldehyde. Also, (R)- and (S)- refer to the chemical structure of the molecule based on Cahn–Ingold–Prelog priority rules of naming rather than rotation of light. R/S notation is the primary notation used for +/- now because D and L notation are used primarily for sugars and amino acids.
Racemization occurs when one pure form of an enantiomer is converted into equal proportion of both enantiomers, forming a racemate. When there are both equal numbers of dextrorotating and levorotating molecules, the net optical rotation of a racemate is zero. Enantiomers should also be distinguished from diastereomers which are a type of stereoisomer that have different molecular structures around a stereocenter and are not mirror images.
Partial to complete racemization of stereochemistry in solutions are a result of SN1 mechanisms. However, when complete inversion of stereochemistry configuration occurs in a substitution reaction, an SN2 reaction is responsible.