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Absolute and Relative Configurations

The precise arrangement of substituents at a stereogenic center is known as the absolute configuration of the molecule.
This is usually accomplished by solving the x-ray crystal structure of a molecule, a method that is not always readily available, or by inference based on chemical reactions of specific stereochemistry involving a compound whose absolute configuration is known.

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, as a result the group priority at the stereogenic center has been altered. The absolute configuration of the parent is known while only the relative configurations of the tosylate and the nitrile are known. The mechanistic aspects behind this will be discussed in more detail in the next chapter.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary