Optical isomerism.

HookThe drug that proved handedness can kill

ModelChirality — the four-different-groups test

A carbon atom is chiral (an asymmetric carbon, or stereocentre) when it is bonded to four different groups. That is the entire test, and examiners want it word for word: four different groups. When it is satisfied, the molecule and its mirror image cannot be superimposed — rotate one however you like and it will never sit exactly on top of the other. The everyday version is your hands: a left hand is the mirror image of a right, but no amount of turning makes a left glove fit a right hand.

Because the difference is three-dimensional, you must draw it in three dimensions. The convention is wedge and dash: two bonds drawn as normal lines in the plane of the paper, one bold wedge coming towards you and one hatched dash going behind. To show the two isomers you draw one structure, then its reflection in an imaginary mirror placed alongside it, so that the wedge on one becomes the dash on the other. A single asterisk on the chiral carbon is the accepted shorthand for 'optical isomerism occurs here'.

Spotting the centre in an unfamiliar molecule is a pure AO2 skill. Scan every carbon; the moment one carries four groups that are all distinct — counting whole branches, not just the first atom — you have found it. If any two of the four groups are the same, that carbon is achiral and no optical isomerism arises there.

ModelOptical activity — bending light two ways

Ordinary light waves oscillate in every plane. Pass light through a polariser and only one plane survives — this is plane-polarised light. The single measurable difference between two enantiomers is what they do to it: one rotates the plane of polarisation clockwise, the other rotates it by the same angle anticlockwise. The instrument that measures the rotation is a polarimeter, and a substance that rotates the plane at all is described as optically active.

The form that rotates the plane clockwise (to the right) is dextrorotatory, labelled \((+)\); the form that rotates it anticlockwise is laevorotatory, labelled \((-)\). The magnitudes are identical because the molecules are perfect mirror images — if one gives \(+13.5^{\circ}\) under fixed conditions, its partner gives exactly \(-13.5^{\circ}\).

Everything else about a pair of enantiomers is the same: identical melting point, identical boiling point, identical density, identical solubility, and identical reactions with ordinary (achiral) reagents. They diverge only in two situations — the direction they twist polarised light, and how they react with other chiral things. Your smell and taste receptors are chiral, which is why the two enantiomers of carvone are different scents entirely: one is the spearmint in chewing gum, its mirror image is the caraway in rye bread. The two forms of limonene split the same way, one smelling of oranges and the other of lemons, from a single molecular formula.

MechanismRacemic mixtures — why the lab version is optically dead

A racemic mixture (a racemate) is a 50:50 mixture of the two enantiomers. Because it contains equal numbers of left- and right-handed molecules, every clockwise rotation is cancelled by an equal anticlockwise one, and the sample shows no net optical activity at all. This is a favourite trap: an individual molecule can be chiral while the bulk sample is optically inactive, simply because both hands are present in equal measure.

What matters for the exam is why so many lab reactions hand you a racemate. Whenever a new chiral centre is created by nucleophilic addition to a carbonyl, the carbonyl carbon starts out flat — trigonal planar, with the \(\text{C=O}\) and its two other groups in one plane. A nucleophile can attack that flat carbon from either face with exactly equal probability. Attack from above gives one enantiomer; attack from below gives the mirror image; the two happen equally often, so a 50:50 racemate results. You will meet the concrete case in the next section, where cyanide ion adds to an aldehyde to make a 2-hydroxynitrile, but the reasoning is general: a planar intermediate attacked from both sides yields equal amounts of each enantiomer.

Nature does the opposite. Enzymes are themselves chiral, so they build and react with only one handedness — which is why the sugars and amino acids in living things are single enantiomers, not racemates, and why a lab synthesis that gives a useless 50:50 mixture is such a headache for the pharmaceutical chemist.

CaseOne hand heals, the other harms

The thalidomide tragedy turned optical isomerism from a textbook curiosity into a regulatory obsession. The \((R)\)-form is the effective sedative; the \((S)\)-form is the teratogen that damages developing limbs. The cruel twist, established later, is that the two forms racemise in the body under physiological conditions: administer the pure \((R)\)-enantiomer and the body will manufacture some \((S)\) regardless. So handedness alone was not a clean fix here — but the episode forced regulators worldwide to demand that the two enantiomers of any new drug be studied separately.

That demand reshaped the industry. Naproxen, a common anti-inflammatory, is sold as a single enantiomer because its mirror image is toxic to the liver. Ibuprofen, by contrast, is still sold as a racemate: only the \((S)\)-form is active, but the body converts the inactive \((R)\)-form into \((S)\), so the cheaper 50:50 mixture works well enough. Deciding between these routes is a multimillion-pound question, because making a single enantiomer cleanly — asymmetric synthesis, using chiral catalysts or enzymes — is far harder and costlier than making a racemate and separating it.

This is why optical isomerism sits on the A-level spec at all: it is the point where the shape of a single carbon centre reaches out and decides whether a molecule is a medicine or a poison.

VocabularyKey terms the mark scheme pays for

TrapsMisconceptions that cost marks

ExamWhat examiners want