Stereoisomers and Chirality

An object or molecule that cannot be superimposed on its mirror image is chiral (from the Greek for hand). In organic chemistry the usual cause is a chiral carbon (also called a stereocentre or asymmetric carbon): a tetrahedral carbon bonded to four different groups. Take butan-2-ol, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. Carbon 2 carries –H, –OH, –$\mathrm{C}{\mathrm{H}}_3$ and –$\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$: four different groups, so it is chiral. The four groups can be arranged around the tetrahedron in two ways that are mirror images, and no rotation turns one into the other. On paper this is drawn with a wedge (bond coming toward you) and a dash (bond going away). Swapping any two groups on a chiral carbon converts one enantiomer into the other. Compare propan-2-ol, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_3$: carbon 2 has two identical $\mathrm{C}{\mathrm{H}}_3$ groups, so its mirror image can be rotated to match it exactly, and the molecule is achiral. The same test explains biology: every amino acid in your proteins except glycine has a chiral carbon (glycine has two H atoms on its central carbon).

The two mirror-image forms are enantiomers. Because every bond length, angle and intermolecular force is the same, they have identical boiling points, melting points, densities and solubilities, and they react identically with achiral reagents. They differ in only two ways. First, they rotate plane-polarized light (light whose waves vibrate in one plane, produced by a polarizing filter) by equal angles in opposite directions. This is optical activity, measured with a polarimeter: the enantiomer that turns the plane clockwise is dextrorotatory, written (+); the other is levorotatory (−). Second, they interact differently with other chiral molecules, such as enzymes and receptors. A 50:50 mixture of the two, called a racemic mixture, shows no rotation because the effects cancel. Ordinary laboratory syntheses that create a chiral carbon from achiral starting materials give racemic mixtures, since both arrangements form equally easily. Making one enantiomer selectively needs a chiral catalyst, work recognized by the 2001 Nobel Prize in Chemistry.

Proteins are built only from L-amino acids, so enzymes and receptors are chiral, like a right-handed glove. One enantiomer fits a binding site with three or more contact points lined up; its mirror image can match at most two at once. Carvone shows this with your nose: (R)-carvone smells of spearmint, while (S)-carvone smells of caraway seed (the flavour of rye bread). Ibuprofen is sold as a racemic mixture, but only the (S) enantiomer blocks the enzyme that makes inflammatory prostaglandins; luckily the body slowly converts much of the (R) form into (S). Because of examples like these, many newer drugs are sold as a single enantiomer, such as escitalopram (an antidepressant) and esomeprazole (for stomach acid).