Chirality (chemistry) - Stereogenic Centers

Stereogenic Centers

In general, chiral molecules have point chirality at a single stereogenic atom, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism).

Normally when an atom has four different substituents, it is chiral. However in rare cases, two of the ligands differ from each other by being mirror images of each other. When this happens, the mirror image of the molecule is identical to the original, and the molecule is achiral. This is called pseudochirality.

A molecule can have multiple stereogenic centers without being chiral overall if there is a symmetry between the two (or more) stereocenters themselves. Such a molecule is called a meso compound.

It is also possible for a molecule to be chiral without having actual point chirality. Common examples include 1,1'-bi-2-naphthol (BINOL), 1,3-dichloro-allene, and BINAP, which have axial chirality, (E)-cyclooctene, which has planar chirality, and certain calixarenes and fullerenes, which have inherent chirality.

A form of point chirality can also occur if a molecule contains a tetrahedral subunit which cannot easily rearrange, for instance 1-bromo-1-chloro-1-fluoroadamantane and methylethylphenyltetrahedrane.

It is important to keep in mind that molecules have considerable flexibility and thus, depending on the medium, may adopt a variety of different conformations. These various conformations are themselves almost always chiral. When assessing chirality, a time-averaged structure is considered and for routine compounds, one should refer to the most symmetric possible conformation.

When the optical rotation for an enantiomer is too low for practical measurement, it is said to exhibit cryptochirality.

Even isotopic differences must be considered when examining chirality. Replacing one of the two 1H atoms at the CH₂ position of benzyl alcohol with a deuterium (2H) makes that carbon a stereocenter. The resulting benzyl-α-d alcohol exists as two distinct enantiomers, which can be assigned by the usual stereochemical naming conventions. The S enantiomer has _D = +0.715°.