Chirality and Optical Isomers

A molecule is described as chiral if it cannot be superimposed on its own mirror image, in the same way that your left hand cannot be superimposed on your right hand. In organic chemistry, chirality almost always arises from a carbon atom bonded to four different groups. This carbon is called a chiral centre (sometimes called an asymmetric carbon), and it is usually marked with an asterisk, \( C^{*} \), when you draw a structure.

If even two of the four groups attached to a carbon are identical, the molecule has a plane of symmetry and is not chiral. Spotting the chiral centre is the first step in any question on chirality and optical isomers, so always check each carbon in a structure for four distinct substituents before deciding whether optical isomerism is possible.

When a molecule contains one chiral centre, it exists as two distinct three-dimensional arrangements called enantiomers. These two forms have exactly the same molecular formula and the same connectivity, but the groups around the chiral carbon are arranged as non-superimposable mirror images of each other, much like your two hands.

Because enantiomers have identical bonds, they also share almost all physical and chemical properties, such as melting point, boiling point and solubility. The one property that differs is how each enantiomer interacts with plane-polarised light, and how each interacts with other chiral molecules, which matters greatly in biological systems such as amino acids.

Enantiomers are described as optically active because each one rotates the plane of plane-polarised light. One enantiomer, labelled dextrorotatory or \( (+) \), rotates the light clockwise by an angle \( \alpha \), while the other, labelled laevorotatory or \( (-) \), rotates the light by the same angle in the anticlockwise direction. This is measured using a polarimeter.

The amount of rotation depends on the concentration of the solution and the length of the sample tube, so chemists use the specific rotation to compare compounds fairly: \( [\alpha] = \dfrac{\alpha}{c\,l} \), where \( \alpha \) is the observed rotation, \( c \) is the concentration and \( l \) is the path length.

To distinguish the two enantiomers in writing, chemists use the R and S naming system. Each of the four groups on the chiral carbon is ranked by priority using atomic number (the same Cahn–Ingold–Prelog rules used elsewhere in organic nomenclature). Viewing the molecule with the lowest-priority group pointing away, if the remaining three groups decrease in priority clockwise the centre is labelled R (from the Latin rectus), and if they decrease anticlockwise it is labelled S (from sinister).

Note that R and S refer to the actual spatial arrangement, while \( (+) \) and \( (-) \) refer to the direction of rotation measured experimentally; the two labelling systems are not directly linked, and an R isomer is not automatically the \( (+) \) form.

A mixture containing equal amounts of both enantiomers is called a racemic mixture, or racemate. Because the rotation caused by each enantiomer is equal and opposite, the two effects cancel out and the mixture shows no overall optical activity, even though it is made entirely of optically active molecules.

Racemic mixtures are produced whenever a reaction creates a new chiral centre from a flat, planar intermediate, since there is an equal chance of the attacking group approaching from either face. This happens in nucleophilic addition to the carbonyl group in aldehydes and ketones, and it is also the reason that adding a Grignard reagent to an unsymmetrical ketone, as covered in increasing carbon chain length using Grignard reagents, gives a racemic alcohol product rather than a single enantiomer.

Chirality is not just a laboratory curiosity, it is central to biology and medicine. Naturally occurring amino acids, discussed alongside amines and amides in amines, amides and amino acids, almost all exist as a single enantiomer in living systems, which is why the body can respond very differently to the two forms of a chiral drug molecule.

When answering exam questions, work through a structure systematically: locate every carbon with four different substituents, mark it with an asterisk, and state how many chiral centres are present. A molecule with \( n \) chiral centres can have up to \( 2^{n} \) stereoisomers, although this exact rule only applies when the centres are truly independent of one another.

Consider 2-hydroxypropanoic acid (lactic acid), \( CH_3CH(OH)COOH \). The central carbon is bonded to a methyl group, a hydroxyl group, a hydrogen atom and a carboxylic acid group, four different groups, so it is a chiral centre. This compound therefore exists as two enantiomers, which rotate plane-polarised light equally but in opposite directions, and a sample prepared without any preference for either form would be a racemic mixture with no net optical activity.

Related lessons

• Uk A Level Chemistry