Alexander Crum Brown and the Structural Chemical Formula

Chemistry Before Structural Formulae

To understand what Crum Brown actually did, it helps to picture the confusion of organic chemistry in the 1850s. Chemists could not even reliably agree on atomic weights: water might be written as HO or as H₂O depending on which of several rival conventions a given chemist happened to follow, and competing systems of formulae multiplied confusion rather than resolving it. Two chemists working on the same substance might, in effect, be speaking different dialects.

The turning point was the Karlsruhe Congress of September 1860 — the first international meeting of chemists, organised by Kekulé, Wurtz and Weltzien — where the Italian chemist Stanislao Cannizzaro persuaded the assembled community to adopt a consistent set of atomic weights based on Avogadro's hypothesis, with hydrogen at roughly 1, carbon at roughly 12, and oxygen at roughly 16. This standardisation is easy to overlook in retrospect, but it was the essential precondition that made structural thinking finally tractable: without a shared, stable notion of relative atomic weight, there was no stable foundation on which to build a picture of how atoms connect.

Two further ingredients were already in place by the time Crum Brown began his own thinking. First, valency: in 1852 the English chemist Edward Frankland proposed that each element possesses a fixed "combining power" — a set number of other atoms it can hold in combination. Second, the older type theory of Gerhardt and Laurent, which classified organic compounds by analogy to simple "types," such as water and ammonia, into which more complex substances could loosely be fitted.

Then, in 1858, came the decisive theoretical breakthrough, reached independently by August Kekulé and the Scottish chemist Archibald Scott Couper: carbon is tetravalent, and — crucially — its atoms can link to one another, forming chains, which finally explained the almost limitless multiplicity of organic compounds built from just a handful of common elements. This is the theory that made structural chemistry possible in principle. What was still missing, on the eve of Crum Brown's own work, was a clear and workable way of actually drawing it.

The 1864 Innovation

Here is the crucial distinction to get exactly right, and this article states it as plainly as possible. Kekulé and Couper supplied the theory of how atoms link together. What chemistry still lacked was a clear, systematic way of drawing that theory. Kekulé's own diagrams were the unwieldy "sausage" formulae — elongated, linked shapes that other chemists soon found impractical and abandoned. Couper used dotted lines between atomic symbols, which was closer to the modern look but still relied on older notational conventions. It was Crum Brown who produced the clean, schematic picture that we would actually recognise today as a chemical structure diagram.

He first drew these diagrams in his 1861 Edinburgh MD thesis, On the Theory of Chemical Combination — on page 12 he depicted, for the first time, molecules of ammonia and ethanol with valency lines connecting the atoms. Because the thesis itself was unpublished, however, the notation did not become known to the wider chemical world at that point. It only reached a public readership through his 1864 paper, On the Theory of Isomeric Compounds, in the Transactions of the Royal Society of Edinburgh (vol. 23, no. 3, pp. 707–719), later reprinted as "On the theory of isomeric compounds" in the Journal of the Chemical Society 18 (1865): 230–245. This three-year gap between private invention and public disclosure is a genuine and important detail of the story, not a pedantic footnote, and this article treats the 1861 and 1864 dates as two distinct milestones rather than collapsing them into a single year.

In a footnote on page 708 of the 1864 paper, Crum Brown explained his system in his own words:

"I may here shortly explain the graphic notation which I employ to express constitutional formulæ, and by which… I do not mean to indicate the physical, but merely the chemical position of the atoms. An atom is represented by its usual symbol, surrounded by a circle with as many lines proceeding from it as the atom contains equivalents… When equivalents mutually saturate one another, the two lines representing the equivalents are made continuations of one another."

In plainer terms: draw each atom's chemical letter inside a circle; give that circle as many lines as the atom's valence requires; and join two atoms into a bond by making their respective lines continuous with one another. He was insistent — and this is a point of real historical importance — that his formulae showed the chemical, not the physical, arrangement of atoms: he was not claiming to depict the true three-dimensional geometry of a molecule, only its pattern of connections. He used dotted, or broken, lines at first, later moving to solid ones, and introduced two parallel lines to represent a double bond, applying this to unsaturated compounds such as ethylene, whose carbon-carbon double bond he was among the first to represent graphically. He illustrated the whole system with molecules including water, ammonia, ethanol, and the isomeric acids fumaric and maleic.

The notation spread because it simply worked better than the alternatives. Just a few years later, Edward Frankland adopted it throughout his influential 1866 Lecture Notes for Chemical Students, simplifying it — most notably by dropping the circles — and coining the term "bond" for the linkages themselves. Frankland did this so effectively that the resulting system became known, somewhat unfairly to its actual originator, as "Frankland's notation." Once the circles were dropped, Crum Brown's letter-and-line diagram is, for practical purposes, the modern structural formula. As the historian Christopher Ritter writes in "An Early History of Alexander Crum Brown's Graphical Formulas" (in Tools and Modes of Representation in the Laboratory Sciences, ed. Ursula Klein, 2001): "After 1861 the core of this project came to involve the graphical formulas of Alexander Crum Brown, which became 'Frankland's notation,' which became modern structural notation."

Why Structural Formulae Matter

Before structural formulae existed, a chemist writing "C₂H₆O" knew only the tally of atoms in a substance — two carbons, six hydrogens, one oxygen — but nothing whatsoever about how those atoms were arranged relative to one another. And yet that single formula describes two utterly different substances: ethanol (the alcohol found in wine and spirits, CH₃CH₂OH) and dimethyl ether (a gas, CH₃OCH₃). Same atoms, different architecture, wildly different physical and chemical properties. These are what chemists call isomers, and the phenomenon was genuinely baffling until structural formulae supplied the answer: it is not merely which atoms a molecule contains but how they are connected that determines what the substance is and how it behaves.

That single insight lit the fuse for the explosion of organic chemistry across the second half of the nineteenth century. Once chemists had a reliable way to draw connectivity, they could begin to predict reactivity from structure, design deliberate multi-step syntheses toward a target molecule, and understand isomerism not as a puzzling anomaly but as an expected and explicable consequence of structural variation. The structural formula became — and remains today — the fundamental visual language of chemistry: every organic reaction mechanism, every pharmaceutical patent, every diagram of a protein or a strand of DNA is written in a notation descended, however many simplifications removed, from Crum Brown's circles and lines. It is arguably the single most important and universally recognised notation system in all of science, more widely legible across languages and specialisms than almost any other technical diagram in use.

Crum Brown's Other Contributions

Remarkably, the structural formula was not Crum Brown's only major scientific achievement, and a full account of his career has to make room for at least two more genuinely significant contributions.

The sense of balance. In work published in 1874, Crum Brown independently explained how the semicircular canals of the inner ear detect rotation — the physiological basis of our sense of balance. He arrived at this insight at essentially the same time as, and quite independently of, Ernst Mach and Josef Breuer, and the combined finding is sometimes referred to as the "Mach–Breuer–Brown" theory. In one respect his contribution actually went further than the others: he worked out how the canals on both sides of the head act together, so that the two horizontal canals respond to opposite directions of motion, letting even a blindfolded person distinguish leftward rotation from rightward rotation. He built a working model of the mechanism using two counter-rotating wheels, and for this achievement he received the Keith Prize of the Royal Society of Edinburgh for 1873–75. In the interests of honest attribution, it should be added that Breuer and Mach are usually given fuller credit for this discovery in the wider history of physiology, though Crum Brown's independent, near-simultaneous work is well documented and was recognised in his own lifetime.

Structure and drug action. In 1867–68, working with the pharmacologist Thomas R. Fraser, Crum Brown published a pioneering study establishing that a substance's physiological action depends directly on its chemical constitution. Their stated method "consists in performing upon a substance a chemical operation which shall introduce a known change into its constitution, and then examining and comparing the physiological action of the substance before and after the change." Applying this method, they showed that converting several well-known alkaloids — including brucine, codeine, morphine, strychnine, atropine and coniine — into quaternised (quaternary ammonium) salts gave them all a uniform, curare-like action, paralysing skeletal muscle regardless of what each alkaloid did in its original form. As the pharmacologist W. C. Bowman later observed, this work "seems to have been the first valid structure–action relationship to be established in any pharmacological field." It is now recognised as the origin of the structure–activity relationship (SAR), a founding principle that still underpins modern rational drug design.

Mathematics, knots and crystals. A lifelong lover of physical models and topology, Crum Brown knitted three-dimensional interlocking mathematical surfaces by hand and, through his brother-in- law, the physicist Peter Guthrie Tait, influenced the early development of knot theory. Decades before X-ray crystallography made such things routine, he built a model of common salt, sodium chloride, from knitting needles and coloured wool, correctly showing each atom surrounded by six atoms of the other kind — a strikingly intuitive anticipation of a crystal structure that would not be confirmed instrumentally for years.

Archibald Scott Couper — The Tragic Scottish Precursor

Crum Brown at Edinburgh

Crum Brown flourished within Edinburgh's remarkable scientific tradition — the city of Joseph Black, Thomas Graham, and his own predecessor Lyon Playfair. He was elected a Fellow of the Royal Society of Edinburgh in 1864, serving on its Council for forty-four years, and a Fellow of the Royal Society of London in 1879; he served as President of the Chemical Society, a forerunner of the modern Royal Society of Chemistry, from 1891 to 1893.

By all accounts he was a beloved, if somewhat chaotic, teacher, affectionately known to generations of students as "Crummie," his lectures frequently dissolving into cheerful rowdiness. His student Arthur Conan Doyle — later the creator of Sherlock Holmes — recalled "kindly Crum Brown" carefully sheltering behind a screen before an experiment that usually failed to explode, emerging afterward with a mild "Really, gentlemen!" His laboratory assistant James Dewar went on to pioneer the liquefaction of gases and invent the vacuum flask; other students who passed through his classroom included David Orme Masson and Prafulla Chandra Ray, later celebrated as "the father of Indian chemistry." During the First World War, the elderly professor was reportedly often seen knitting socks for soldiers while riding the Edinburgh trams.

He is commemorated today by Alexander Crum Brown Road at the University's King's Buildings campus and by the Crum Brown Chair of Chemistry, established in 1967. He is buried in Edinburgh's Dean Cemetery, not far from many of the other figures who shaped the city's scientific golden age.

Modern Legacy

Every day, across the world, chemists, students and drug designers draw molecules using skeletal, line-bond formulae — lines standing in for bonds, letters standing in for the atoms that are neither carbon nor hydrogen. This everyday convention is the direct descendant of the Kekulé–Couper–Crum Brown tradition, with Crum Brown's specific schematic clarity sitting at its heart. Three-dimensional molecular modelling and modern computational chemistry all begin, conceptually, from the two-dimensional structural formula. Every pharmaceutical on the market today was developed, at some stage, by chemists modifying and comparing structural formulae on paper or on screen.

The double helix of DNA, described by James Watson and Francis Crick in 1953, is drawn in exactly this notational tradition. So are the countless diagrams of proteins found in every biochemistry textbook, the blueprints of new materials, and the constructs of synthetic biology. All of them are expressed in a visual grammar that traces back, through more than a century and a half of incremental simplification and refinement, to a young Edinburgh doctor's thesis, quietly submitted in 1861 and made public only three years later, in 1864.

Timeline

Myths & Facts

Did You Know?

• Every structural diagram of a molecule in every chemistry textbook in the world — every single line connecting atoms — traces back to a notation set out in Edinburgh by Alexander Crum Brown, invented in his 1861 MD thesis and published in 1864.
• His inspiration was his uncle, the chemist Walter Crum FRS — not, as often claimed, the politician-chemist Lyon Playfair, who was actually his predecessor in the Edinburgh chair.
• He independently worked out how the semicircular canals of the inner ear sense rotation — a second major discovery, in a completely different field of science.
• Before structural formulae, the formula C₂H₆O could mean either ethanol (alcohol you can drink) or dimethyl ether (a gas) — the same atoms, utterly different substances. Structural formulae solved exactly this puzzle.
• Archibald Scott Couper of Kirkintilloch proposed similar structural ideas in 1858 — before Crum Brown's notation — but a publication delay cost him priority over Kekulé, and a breakdown ended his career at just 28.
• Crum Brown held the Edinburgh Chair of Chemistry for 39 years, taught Arthur Conan Doyle, knitted socks for soldiers on Edinburgh's trams in his old age, and lies in Dean Cemetery — a prophet barely honoured in his own city.

Honest Caveats

• The "1861 versus 1864" dating is a genuine subtlety, not editorial pedantry: the notation was invented in the unpublished 1861 thesis and published in 1864. Some secondary sources loosely say "1864" for both events; this article deliberately keeps them distinct.
• Crum Brown's 1864 paper's page range is cited by the most authoritative sources as Trans. R. Soc. Edin. 23: 707–719; some sources give 707–720.
• The semicircular-canal priority is genuinely shared. Breuer and Mach usually receive fuller historical credit than Crum Brown, though his independence and his specific insight into paired canal action are well attested.
• The exact date Crum Brown switched from dotted to solid connecting lines in his diagrams is drawn from secondary sources and could not be pinned to a primary document.
• The claim that his father was "minister of Biggar" could not be verified and appears to be an error; reliable sources place him at Broughton Place, Edinburgh.

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