Jacobus Henricus van 't Hoff

Jacobus Henricus van 't Hoff: The Shape of the Invisible

In the autumn of 1874 a Dutch student three months short of his doctorate published a pamphlet arguing that the four bonds of a carbon atom point toward the corners of a tetrahedron. Nobody had seen a molecule; nobody would for decades. Van 't Hoff had reasoned the shape out from the way certain substances twist a beam of polarised light. He was twenty-two, and he was right. One of Germany's most eminent chemists responded by accusing him of climbing onto Pegasus.

Rotterdam, Delft, and Byron

He was born on August 30, 1852, in Rotterdam, the third of seven children of a physician, Jacobus Henricus van 't Hoff Sr., and Alida Kolff. Science interested him early, but so did Byron, whose poetry and philosophy he read seriously as a schoolboy — a detail worth keeping, because the quality that made his career was closer to poetic than to laboratory patience.

He took a chemical technologist's degree at Delft between 1869 and 1871, finishing in two years rather than the customary three, then studied at Leiden, at Bonn under August Kekulé, and in Paris under Adolphe Wurtz, before completing his doctorate at Utrecht in 1874 under Eduard Mulder.

The Pamphlet

The problem he attacked was old and stubborn. Certain compounds rotated polarised light, and certain compounds with identical formulas behaved differently from one another. Nothing in flat, two-dimensional structural chemistry explained why.

Van 't Hoff's answer was to give molecules a third dimension. If carbon's four bonds radiate toward the vertices of a regular tetrahedron, then a carbon atom carrying four different groups can be arranged in two ways that are mirror images and cannot be superimposed — and those two arrangements would rotate light in opposite directions. Optical activity and isomerism fell out of a single geometric assumption. The Dutch pamphlet appeared in late 1874, the French version, *La chimie dans l'espace*, in May 1875. The French chemist Joseph Le Bel reached the same conclusion independently and at the same time.

To persuade a profession that could not see what he was describing, van 't Hoff posted three-dimensional paper models of tetrahedral molecules to the leading chemists of the day.

Kolbe's Pegasus

The reception was hostile, and the most quoted attack came from Hermann Kolbe, who mocked the young man for having "mounted Pegasus" in order to proclaim "how the atoms appeared to him." It was meant as annihilation. Since the atoms did in fact appear to him, and appeared correctly, the insult has aged into an accidental compliment.

The theory waited about six years for respectability. Around 1880 Johannes Wislicenus and Viktor Meyer came out in support, and stereochemistry became a field rather than a scandal.

Chemical Dynamics

Having settled the shape of molecules, he turned to the question of what they do, and here his contribution was less a single insight than the wholesale import of one discipline into another. *Études de dynamique chimique* (1884) laid out graphical methods for determining the order of a reaction and applied the laws of thermodynamics to chemical equilibrium. It replaced the vague notion of chemical affinity — the mysterious tendency of substances to combine — with a quantity that could be calculated from thermodynamic data.

The relation between an equilibrium constant and temperature that emerged is still taught as the van 't Hoff equation. In 1889 he supplied the physical reasoning that underpinned Svante Arrhenius's equation for reaction rates.

Solutions Behaving Like Gases

In 1886 he demonstrated the analogy that most impressed his contemporaries: highly dilute solutions obey mathematical laws closely resembling those governing gases, with osmotic pressure playing the part that gas pressure plays in a container. A dissolved substance, in other words, could be treated as though it were a gas occupying the volume of the solvent.

The consequence was a whole discipline. With Wilhelm Ostwald he founded the *Zeitschrift für physikalische Chemie* in 1887, giving physical chemistry a journal and therefore an identity, and he worked with Arrhenius on the dissociation of electrolytes. Chemistry, which had been a science of substances, became a science of measurable quantities.

Berlin, and the Salt

He lectured first at the Veterinary College in Utrecht, then held a chair at the University of Amsterdam for close to two decades as professor of chemistry, mineralogy and geology, eventually chairing the department. In 1896 he moved to Berlin, taking a position at the Prussian Academy of Sciences constructed to give him research time rather than teaching. There he applied his thermodynamics of solutions to the oceanic salt deposits at Stassfurt, work of considerable value to Prussian industry.

In 1901 the Swedish Academy awarded him the very first Nobel Prize in Chemistry, "in recognition of the extraordinary services he has rendered by the discovery of the laws of chemical dynamics and osmotic pressure in solutions." He regarded it as the summit of his career.

Why Jacobus Is Called a Genius

The faculty in question is spatial imagination disciplined by evidence, and the 1874 pamphlet is as clean an example as the history of science offers. Van 't Hoff took a measurement — the rotation of polarised light — and inferred from it the geometry of an object no instrument of his era could resolve. He was not describing something observed; he was arguing that reality must have a particular shape for the numbers to come out as they did. It is the reason molecules have been drawn in three dimensions ever since.

The second achievement is different in kind and arguably larger. Importing thermodynamics wholesale into chemistry, and showing that dissolved substances obey gas-like laws, converted a descriptive science into a quantitative one. Very few people contribute foundationally to two fields; van 't Hoff founded stereochemistry at twenty-two and physical chemistry in his thirties.

The qualifications are real. Le Bel reached the tetrahedral carbon independently in the same year, which suggests the idea was ripe rather than unreachable — the Royal Society split the Davy Medal between them in 1893 for that reason. The proposal was a leap without direct evidence, and the profession rejected it for six years; had the geometry been otherwise, the pamphlet would read today as a young man's speculation. And his Nobel Prize was awarded not for the tetrahedron, his most famous idea, but for the solutions work. Kolbe's charge of riding Pegasus was wrong about the destination, not about the method.

Legacy

He married Johanna Francina Mees in 1878; they had four children. Honours accumulated steadily — the Legion of Honour in 1894, the Pour le Mérite in 1895, foreign membership of the Royal Society in 1897, honorary doctorates from Harvard and Yale in 1901, Manchester in 1903, Heidelberg in 1908, and the Helmholtz Medal in the year of his death. He died of tuberculosis at Steglitz, outside Berlin, on March 1, 1911, aged fifty-eight.

His name survives in working vocabulary rather than in monuments: the van 't Hoff equation, the van 't Hoff factor, the Le Bel–van 't Hoff rule. An asteroid was named for him in 2021. Every structural formula drawn with a wedge and a dashed line descends from a pamphlet that a great chemist thought was written by a man who had lost his mind.