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When the precise arrangement of substituents at a stereogenic center is known, the absolute configuration of the molecule is known. This is usually accomplished by solving the x-ray crystal structure of a molecule, a method that is not always readily available.
The arrangement of atoms in an optically active molecule, based on chemical interconversion from or to a known compound, is a relative configuration. Relative, because there is no way of knowing just by looking at a structure whether the assignment of (+) or (-) is correlated to a particular isomer, R or S.
If the name of a compound includes both the sign of rotation and the designation R or S then the absolute configuration of that compound is known.
Let's see how chemists can determine the relative configurations of optically active compounds by chemically interconverting them.
The reaction of an alcohol with TsCl is known to occur with retention of configuration, that is the group priority of the stereogenic center has not been altered. The reaction of the tosylate with nitrile occurs with inversion (by an SN2 mechanism), as a result the group priority at the stereogenic center has been altered.