Stanislao Cannizzaro

Early research

Cannizzaro’s chemical interests centred on natural products and on reactions of aromatic compounds. In 1853 he discovered that when benzaldehyde is treated with concentrated base, both benzoic acid and benzyl alcohol are produced—a phenomenon known today as the Cannizzaro reaction. Despite the fact that Cannizzaro struggled through much of his career with inadequate laboratory facilities, his published research was important and influential. In 1861 he returned to his native Palermo, where he taught for 10 years, making the local university the centre of chemical education and research in Italy. Among his students in Palermo was Wilhelm Körner, a German chemist who made his subsequent career in Italy and whose “absolute” method of determining the structure of aromatic derivatives solved a problem that had bedeviled organic chemists for many years. Cannizzaro’s last move, in 1871, was to the University of Rome, where he spent the rest of his long and distinguished career.

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Atomic weights and Avogadro

Cannizzaro’s historical significance is most closely associated with a long letter he wrote on March 12, 1858, to his friend Sebastiano de Luca, professor of chemistry at Pisa, and subsequently published as “Sunto di un corso di filosofia chimica fatto nella R. Università de Genova” (“Sketch of a Course in Chemical Philosophy at the Royal University of Genoa”). To make clear the significance of this pamphlet, it is necessary to describe something of the state of chemical theory at the time.

The English scientist John Dalton published his atomic theory in 1808, and certain of his central ideas were soon thereafter adopted by most chemists. However, uncertainty persisted for half a century about how atomic theory was to be configured and applied to concrete situations. Lacking a way to directly weigh particles as small as atoms and molecules, and having no means to unambiguously determine the formulas of compounds, chemists in different countries developed several different incompatible atomistic systems. A paper that suggested a way out of this difficult situation was published as early as 1811 by the Italian physicist Amedeo Avogadro, who used vapour densities to infer the relative weights of atoms and molecules and suggested that elementary gases must consist of molecules with more than one atom.

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Despite their apparent promise, Avogadro’s ideas were distressingly abstract and burdened by some anomalies, which delayed their adoption by chemists. An additional barrier to acceptance was the fact that many chemists were reluctant to adopt physical methods (such as vapour-density determinations) to solve their problems. By mid-century, however, some leading figures had begun to view the chaotic multiplicity of competing systems of atomic weights and molecular formulas as intolerable. Moreover, purely chemical evidence began to mount that suggested Avogadro’s approach might be right after all. During the 1850s, younger chemists, such as Alexander Williamson in England, Charles Gerhardt and Adolphe Wurtz in France, and August Kekule in Germany, began to advocate reforming theoretical chemistry to make it consistent with Avogadrian theory.

A single system of atomic formulas

In his 1858 pamphlet, Cannizzaro showed that a complete return to the ideas of Avogadro could be used to construct a consistent and robust theoretical structure that fit nearly all of the available empirical evidence. The few remaining anomalies, he argued, could easily be understood as minor (and legitimate) exceptions to general rules. For instance, he pointed to evidence that suggested that not all elementary gases consist of two atoms per molecule—some were monoatomic, most were diatomic, and a few were even more complex. Another point of contention had been the formulas for compounds of the alkali metals (such as sodium) and the alkaline-earth metals (such as calcium), which, in view of their striking chemical analogies, most chemists had wanted to assign to the same formula type. Cannizzaro argued that placing these metals in different categories had the beneficial result of eliminating certain anomalies when using their physical properties to deduce atomic weights.

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Cannizzaro’s striking summary from this careful and perceptive analysis was that “the conclusions drawn from [Avogadro’s theory] are invariably in accordance with all physical and chemical laws hitherto discovered.” This meant (to Cannizzaro, at least) that it was possible and desirable to construct a single “true” atomistic system that should immediately replace the chaos of competing conventional systems of the 1850s. Unfortunately, Cannizzaro’s pamphlet was published initially only in Italian and had little immediate impact.

The real breakthrough came with an international chemical congress held in the German town of Karlsruhe in September 1860, at which most of the leading European chemists were present. The Karlsruhe Congress had been arranged by Kekule, Wurtz, and a few others who shared Cannizzaro’s sense of the direction chemistry should go. Speaking in French (as everyone there did), Cannizzaro’s eloquence and logic made an indelible impression on the assembled body. Moreover, his friend Angelo Pavesi (a professor at Pavia) distributed Cannizzaro’s pamphlet to attendees at the end of the meeting; more than one chemist later wrote of the decisive impression the reading of this document provided. Cannizzaro thus played a crucial role in winning the battle for reform. The system advocated by him, and soon thereafter adopted by most leading chemists, is substantially identical to what is still used today.