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Portrait of Chirality

Science · Chemistry

Chirality

1848 (Pasteur's separation); 1893 (named by Lord Kelvin)

Chemistry — stereochemistry

Chirality is the property of a molecule that cannot be superimposed on its mirror image, exactly as a left hand cannot be laid onto a right. The usual cause is a carbon atom carrying four different groups. The two mirror forms, called enantiomers, share every ordinary physical property — melting point, density, spectra — and differ only in the direction they rotate plane-polarised light and in how they interact with other handed things. That last exception matters enormously, because living systems are themselves handed: all the amino acids in our proteins are left-handed, all our sugars right-handed. So one enantiomer may smell of spearmint and its twin of caraway, or one may be a sedative and its twin a teratogen. Louis Pasteur separated the first pair by hand in 1848; Lord Kelvin gave the property its name in 1893, from the Greek for hand.

How to spot them

  • ▸A molecule that cannot be superimposed on its mirror image
  • ▸Named from the Greek word for 'hand'
  • ▸Caused by a carbon bonded to four different groups
  • ▸Its two forms rotate plane-polarised light in opposite directions
  • ▸The reason one smell is spearmint and its mirror image is caraway

Key facts

Lord Kelvin gave the property its name in 1893, from the Greek word for 'hand' (1893)

Delivered in his Boyle Lecture at Oxford and published in 1894; the idea is more often associated with his Baltimore Lectures.

Louis Pasteur first separated the two mirror forms in 1848, picking tartrate crystals apart under a lens with tweezers (1848)

He noticed that crystals of sodium ammonium tartrate came in two mirror-image shapes; separated, their solutions rotated polarised light in opposite directions.

A carbon atom bonded to four different groups produces it

Van 't Hoff and Le Bel independently proposed the tetrahedral carbon in 1874 to explain it.

Its two forms rotate plane-polarised light by equal amounts in opposite directions, and are otherwise physically identical

A 50:50 mixture, called racemic, rotates light not at all.

One form of carvone smells of spearmint, its mirror image of caraway

Limonene does the same trick: orange one way round, turpentine or lemon the other.

The 1957 sedative thalidomide is the standard cautionary tale about it (1957)

One enantiomer sedates, the other causes severe birth defects — though the two interconvert in the body, so selling a single form would not in fact have prevented the disaster.

Knowles, Noyori and Sharpless shared the 2001 Nobel Prize in Chemistry for catalysing reactions to give just one of the two forms (2001)

Knowles's hydrogenation gave the first industrial single-form synthesis, of the Parkinson's drug L-DOPA.

Every amino acid in terrestrial proteins has the same handedness — the left-handed form

And every sugar in our biochemistry the right-handed one. Why life picked a side is still unexplained.

Key ideas

  • The two forms only behave differently in a handed environment. A receptor or enzyme is itself made of handed building blocks, so it binds one enantiomer snugly and the other badly — the same reason a right glove fits only one hand.
  • Naming is by the Cahn–Ingold–Prelog rules, which rank the four groups by atomic number and label the arrangement R or S; the older D/L system, and the (+)/(−) labels for the direction of optical rotation, are separate schemes that do not always agree.

Quick-fire facts

  • •Lord Kelvin's definition: a figure is chiral if its mirror image cannot be brought to coincide with itself.
  • •Pasteur's 1848 work was done when he was 25; it founded stereochemistry.
  • •Jacobus van 't Hoff, who with Joseph Le Bel proposed the tetrahedral carbon that explains it, won the first Nobel Prize in Chemistry in 1901.
  • •'Racemic' comes from the Latin racemus, a bunch of grapes, via racemic acid found in wine casks.
  • •Thalidomide was marketed in West Germany as Contergan from 1957 and withdrawn in 1961; around 10,000 babies were born with malformations.
  • •Frances Oldham Kelsey of the US Food and Drug Administration refused to approve it, and was given a presidential award in 1962.
  • •Barry Sharpless won a second Nobel Prize in 2022, for click chemistry — one of only a handful of double laureates.
  • •Many modern drugs are 'chiral switches': esomeprazole is the single active form of omeprazole, levocetirizine of cetirizine. Ibuprofen is still sold as a mixture, because the body converts the inactive form into the active one.
  • •Common misconception: the thalidomide tragedy is often said to prove drugs should always be sold as single enantiomers. Thalidomide's two forms interconvert in the body within hours, so in that particular case it would have made no difference.

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