5.16: Summary
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In this chapter, we’ve looked at some of the causes and consequences of molecular handedness—a topic of particular importance in understanding biological chemistry. The subject can be a bit complex but is so important that it’s worthwhile spending time to become familiar with it.
An object or molecule that is not superimposable on its mirror image is said to be chiral, meaning “handed.” A chiral molecule is one that does not have a plane of symmetry cutting through it so that one half is a mirror image of the other half. The most common cause of chirality in organic molecules is the presence of a tetrahedral, sp3-hybridized carbon atom bonded to four different groups—a so-called chirality center. Chiral compounds can exist as a pair of nonsuperimposable mirror-image stereoisomers called enantiomers. Enantiomers are identical in all physical properties except for the direction in which they rotate plane-polarized light.
The stereochemical configuration of a chirality center can be specified as either R (rectus) or S (sinister) by using the Cahn–Ingold–Prelog rules. First rank the four substituents on the chiral carbon atom, and then orient the molecule so that the lowest-ranked group points directly back. If a curved arrow drawn in the direction of decreasing rank (1 → 2 → 3) for the remaining three groups is clockwise, the chirality center has the R configuration. If the direction is counterclockwise, the chirality center has the S configuration.
Some molecules have more than one chirality center. Enantiomers have opposite configuration at all chirality centers, whereas diastereomers have the same configuration in at least one center but opposite configurations at the others. Epimers are diastereomers that differ in configuration at only one chirality center. A compound with n chirality centers can have a maximum of 2n stereoisomers.
A meso compound contains a chirality center but is achiral overall because it has a plane of symmetry. Racemic mixtures, or racemates, are 50 : 50 mixtures of (+) and (−) enantiomers. Racemates and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point.
A molecule is prochiral if it can be converted from achiral to chiral in a single chemical step. A prochiral sp2-hybridized atom has two faces, described as either Re or Si. An sp3-hybridized atom is a prochirality center if, by changing one of its attached atoms, a chirality center results. The atom whose replacement leads to an R chirality center is pro-R, and the atom whose replacement leads to an S chirality center is pro-S.