Stanislao Cannizzaro

How to use this course

Stanislao Cannizzaro is remembered for two things that sound unrelated. Organic chemists know the Cannizzaro reaction, in which an aldehyde in strong alkali turns into an alcohol and an acid. Historians of science know the Sunto of 1858, a teaching outline that settled the atomic weights and, through the Karlsruhe Congress of 1860, made the periodic table possible. Between the two lies a life in revolution, exile and the politics of a new Italian state.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, worked arguments and exercises with folded solutions. Following the usage of the time, “vapour density” means the density of a gas relative to hydrogen gas at the same temperature and pressure, so that the molecular weight on the scale H = 1 is twice the vapour density.

Timeline

Palermo, Pisa and 1848

Stanislao Cannizzaro was born in Palermo on 13 July 1826, the youngest of ten children. His father, Mariano Cannizzaro, was a magistrate who in that year headed the police of Sicily for the Bourbon Kingdom of the Two Sicilies; his mother was Anna Di Benedetto. His father died in 1836. In the cholera epidemic of 1837 two of his brothers died, and the boy himself caught the disease and survived.

In 1841, aged 15, he entered the University of Palermo. Medicine was the only scientific course on offer. He was drawn to physiology under Michele Foderà, and through experiments on the nervous system he came to see that physiology needed chemistry. At the congress of Italian scientists in Naples in 1845 he met Raffaele Piria, the leading Italian chemist of the day and the discoverer of salicylic acid. Piria took him to Pisa as a laboratory assistant for the academic years 1845–46 and 1846–47. There he learned careful organic analysis and met two lifelong friends, Cesare Bertagnini and Sebastiano De Luca.

Revolution

Back in Sicily in 1847, Cannizzaro joined the conspiracy that broke into the revolution of January 1848 against Bourbon rule. He served as an artillery officer, was elected deputy for Francavilla, and became the youngest member and then a secretary of the Sicilian parliament. After Messina fell in September 1848 he was sent to Taormina to raise local forces. When the war resumed in 1849 the revolution collapsed. On 23 April 1849 he was among those sentenced to death, and in May he escaped by sea to Marseille.

Exile: Paris and Alessandria, 1849–1855

From Marseille he went to Lyon and then, in October 1849, to Paris. He found a place in the laboratory of Michel-Eugène Chevreul at the Jardin des Plantes and attended the lectures of the physicist Henri-Victor Regnault at the Collège de France. Regnault’s precise measurements of the specific heats of the elements would later be one of the two pillars of the Sunto (Lesson 06). With François Cloëz, Cannizzaro prepared cyanamide, \(\mathrm{NH_2CN}\), by the action of cyanogen chloride on ammonia. It was published in 1851, his first research paper.

Exiles from the south found refuge in the Kingdom of Sardinia, the one Italian state with a constitution. In November 1851 Cannizzaro became professor of chemistry and physics at the Collegio Nazionale in Alessandria, in Piedmont. The laboratory was poor, but there, in 1853, he made the discovery that carries his name: benzaldehyde treated with alcoholic potassium hydroxide gives equal amounts of benzyl alcohol and potassium benzoate (Lesson 08). The reaction gave the first convenient route to benzyl alcohol, and he went on to study its derivatives.

Genoa and the Sunto, 1855–1860

In 1855 the Piedmontese minister of education, Giovanni Lanza, appointed him to the chair of chemistry at the University of Genoa. Around this time he married Henrietta Withers, the daughter of an English clergyman, in Florence; they had a son and a daughter. Genoa had almost no research laboratory, and for two years he concentrated on teaching. Teaching forced a problem on him. He had to present chemistry to students, and the chemistry of the 1850s did not hang together.

Chemists used several incompatible systems of atomic weights and formulas (Lesson 04). Cannizzaro decided to build his course on a single principle: Avogadro’s hypothesis of 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. He traced how the history of chemistry since John Dalton and Joseph Louis Gay-Lussac had led back to it, and showed that, applied consistently and combined with the law of Pierre Dulong and Alexis Petit on specific heats, it fixed every atomic weight.

He wrote the outline as a letter to his friend Sebastiano De Luca, then professor at Pisa. It appeared in 1858 in the Pisan journal Il Nuovo Cimento as Sunto di un corso di filosofia chimica fatto nella Reale Università di Genova, “sketch of a course of chemical philosophy”, and was reprinted as a pamphlet in 1859. At first it was read by few. Its moment came two years later at Karlsruhe (Lesson 09).

The muddle of formulas before 1858

For the general reader. Chemical analysis tells you the proportions in which elements combine, but not how many atoms of each are in a molecule. Water is one part hydrogen to eight parts oxygen by mass. That fits HO with oxygen weighing 8, or H₂O with oxygen weighing 16, equally well. Without an extra principle the choice is a convention, and by the 1850s every school had its own.

Rival systems

• Equivalents. Many chemists, following Leopold Gmelin, avoided atoms and used equivalent weights: H = 1, O = 8, C = 6. Water was HO and acetic acid C\(_4\)H\(_4\)O\(_4\).
• Berzelius. Jöns Jacob Berzelius used gas volumes for the gaseous elements, giving O = 16 and water H\(_2\)O, but his weights for several metals, silver among them, were twice the modern values.
• Gerhardt. Charles Gerhardt and Auguste Laurent wrote organic formulas on a two-volume basis, close to modern ones (H\(_2\)O, C\(_2\)H\(_4\)O\(_2\) for acetic acid). But Gerhardt wrote all metal oxides like water, M\(_2\)O, which halved the atomic weights of most metals.

Avogadro had proposed the way out in 1811: equal volumes contain equal numbers of molecules, and elementary molecules can split. Two difficulties kept chemists from using it. Berzelius’s electrochemical theory made it hard to imagine two identical atoms bonding into H\(_2\). And Jean-Baptiste Dumas’s vapour densities for mercury, phosphorus, arsenic and sulphur implied, by Avogadro’s reasoning, elementary molecules of one, four, four and six atoms, which seemed to show the hypothesis gave nonsense. Cannizzaro’s achievement was to show that these were not objections at all.

The law of atoms

For the general reader. Cannizzaro’s rule is simple. Take every gas or vapour that contains a given element. Weigh a fixed volume of each, which by Avogadro is weighing the same number of molecules. Work out how much of the element is in each. The amounts are always whole multiples of one smallest amount, and that smallest amount is the weight of one atom.

The method

Cannizzaro took as unit the weight of half a molecule of hydrogen, that is, one hydrogen atom. Since equal volumes hold equal numbers of molecules, the weight of a molecule of any gas on this scale is

Analysis gives the mass fraction \(w_E\) of element E in the compound, so one molecule contains the mass \(M w_E\) of E.

The law is an inference: if molecules are made of atoms and Avogadro is right, the masses of E per molecule must be integer multiples of the atomic mass. The smallest observed value is an upper bound; it equals the atomic weight once enough compounds have been examined, and the chance that every one contains two or more atoms becomes negligible.

Hydrogen and oxygen

Hydrogen gas has \(M=2\), hydrogen chloride \(M=36.5\) with 1 part hydrogen, water \(M=18\) with 2, ammonia \(M=17\) with 3. The smallest amount is 1, so hydrogen gas contains two atoms. Oxygen gas has \(M=32\); water contains 16 of oxygen, carbon dioxide 32, carbon monoxide 16. So O = 16 and free oxygen is O\(_2\). The atom is the quantity that enters compounds whole; the molecule of an element can contain several.

Specific heats and the metals

For the general reader. Most metals form no gases, so their molecules cannot be weighed. Cannizzaro used a second rule, found by Dulong and Petit in 1819: one atom of any solid element holds about the same amount of heat. He showed that this rule and the gas rule agree wherever both can be applied, and then used the heat rule alone for the metals. The result doubled the atomic weights of most metals in Gerhardt’s system and made their formulas consistent.

Combining weights give \(A\) up to a small integer factor; the specific heat gives \(A\) roughly, to perhaps 10%. Together they fix \(A\) exactly. Cannizzaro’s crucial step was the cross-check. Where an element has both volatile compounds and a measurable solid specific heat, as for iodine, mercury and arsenic, the two methods agree. That entitled him to trust specific heats where no vapour exists.

Mercury

Mercury was the hard case. Mercuric chloride vapour has \(M\approx271\) and contains 200 of mercury; mercury vapour itself has \(M\approx200\); the specific heat of mercury, 0.0333 cal g\(^{-1}\) K\(^{-1}\), gives \(A\approx190\). So Hg = 200 and its vapour is monatomic: the molecule of an element can contain one atom. Cannizzaro then compared the two chlorides. Calomel he wrote HgCl, with mercury taking the place of one hydrogen; corrosive sublimate HgCl\(_2\), with mercury taking the place of two. He set the “mercurous” radical beside hydrogen and the “mercuric” beside the organic radicals ethylene and propylene, which also replace two hydrogens, an analogy he said other chemists had missed. In the same way copper forms cuprous and cupric series. This was a clear statement of variable combining capacity, soon to be called valence. Later work showed that calomel is Hg\(_2\)Cl\(_2\), with a mercury–mercury bond, but the atomic weight 200 stands.

Anomalous densities and dissociation

For the general reader. Some vapours were too light. Ammonium chloride, phosphorus pentachloride and sulphuric acid behaved as if their molecules weighed half what the formulas said. Opponents of Avogadro took this as a refutation. Cannizzaro answered that these substances fall apart on heating into two smaller molecules. Count the pieces, and the hypothesis is saved. Henri Sainte-Claire Deville had just shown that hot molecules can dissociate, and later experiments proved the answer correct.

For ammonium chloride, \(M_0=53.5\) and \(M_{\text{obs}}\approx26.8\), so \(\alpha\approx1\): the vapour is \(\mathrm{NH_3}+\mathrm{HCl}\). Leopold Pebal confirmed this in 1862 by letting the hot vapour diffuse through an asbestos plug: the lighter ammonia passed faster and the gas on either side was no longer neutral. For phosphorus pentachloride the dissociation \(\mathrm{PCl_5}\rightleftharpoons\mathrm{PCl_3}+\mathrm{Cl_2}\) is partial and depends on temperature and pressure, exactly as an equilibrium should.

Equilibrium constant

At total pressure \(P\), the mole fractions are \(x_{\mathrm{AB}}=(1-\alpha)/(1+\alpha)\) and \(x_{\mathrm A}=x_{\mathrm B}=\alpha/(1+\alpha)\), so

The Cannizzaro reaction

For the general reader. In 1853 Cannizzaro treated benzaldehyde, the oil of bitter almonds, with potash in alcohol. Half of it became benzyl alcohol and half became benzoic acid. One molecule had given a hydrogen to another. Reactions in which one substance is both oxidized and reduced are called disproportionations, and this one became a standard tool and a teaching example of hydride transfer.

Mechanism

1. Hydroxide adds to the carbonyl carbon: \(\mathrm{RCHO}+\mathrm{OH^-}\rightleftharpoons\mathrm{RCH(OH)O^-}\).
2. The tetrahedral anion transfers a hydride ion directly to the carbonyl carbon of a second aldehyde molecule, giving \(\mathrm{RCOOH}\) and the alkoxide \(\mathrm{RCH_2O^-}\).
3. A fast proton transfer gives the carboxylate and the alcohol.

The rate law is usually \(v=k[\mathrm{RCHO}]^2[\mathrm{OH^-}]\), with an additional term in \([\mathrm{OH^-}]^2\) at high base, where the doubly deprotonated adduct is the donor. Run in D\(_2\)O, the alcohol carries no deuterium on carbon: the new C–H hydrogen came from the other aldehyde, not from the solvent. This rules out mechanisms through free hydrogen or solvent protons and supports direct hydride transfer.

The crossed reaction

Formaldehyde is the most easily oxidized aldehyde. Mixed with another non-enolizable aldehyde and excess base, it is sacrificed to formate and reduces the partner to the alcohol: \(\mathrm{ArCHO}+\mathrm{HCHO}+\mathrm{OH^-}\to\mathrm{ArCH_2OH}+\mathrm{HCOO^-}\). The industrial synthesis of pentaerythritol combines aldol additions with a final crossed Cannizzaro step.

Karlsruhe and the road to the periodic table

For the general reader. In September 1860 more than a hundred chemists from across Europe met at Karlsruhe, in the first international chemistry congress, to agree on atoms, molecules and formulas. They did not agree. But as the meeting closed, copies of Cannizzaro’s pamphlet were handed out. Several readers, among them Lothar Meyer and Dmitri Mendeleev, adopted its atomic weights. Within ten years those weights produced the periodic table.

The congress

August Kekulé, Karl Weltzien and Adolphe Wurtz organized the congress, held on 3–5 September 1860. Cannizzaro argued forcefully that there was one kind of molecule, the one fixed by Avogadro’s hypothesis, with no separate “physical” and “chemical” molecules, and that the anomalous densities were dissociations. The meeting voted on nothing of substance. Cannizzaro left at the end; his friend Angelo Pavesi, professor at Pavia, stayed and distributed copies of the Sunto. Meyer read it on the way home to Breslau and later wrote that the scales fell from his eyes. His 1864 book Die modernen Theorien der Chemie was built on Cannizzaro’s system. Mendeleev, a young Russian chemist then working in Heidelberg, attended the congress and later recalled Cannizzaro’s defence of Avogadro as decisive.

The Palermo school and the benzene ring

For the general reader. In Palermo Cannizzaro built a laboratory that drew researchers from across Europe. One of them, Wilhelm Körner, who came from Kekulé’s laboratory in 1867, found a purely chemical way to tell which positions on the benzene ring a substituent occupies, by counting how many new products each compound gives.

Proof. Number the ring 1 to 6 with X at the given positions and count the inequivalent free positions under the symmetries of the ring that fix the set of X positions. Para (1,4): positions 2, 3, 5, 6 are all equivalent, one product (1,2,4). Ortho (1,2): positions 3 ≡ 6 and 4 ≡ 5, two products (1,2,3 and 1,2,4). Meta (1,3): positions 2, 5 and 4 ≡ 6, three products (1,2,3; 1,3,5; 1,2,4). The counts 1, 2, 3 identify each isomer without any assumption about structure beyond the ring.

Cannizzaro’s own Palermo research dealt with aromatic alcohols and amines derived from benzyl alcohol. His later work in Rome determined the constitution of santonin, a drug from wormseed, and showed it to be one of the few natural products related to naphthalene. He also introduced the name hydroxyl for the OH group.

Palermo and Rome, 1860–1910

In 1860 Giuseppe Garibaldi’s volunteers took Palermo, and Cannizzaro, no longer under sentence of death, returned home to serve the new government. In October 1860 he was named to the Sicilian council for public education. In October 1861 he took the chair of inorganic and organic chemistry at Palermo and built a modern teaching and research laboratory. Palermo became, for a decade, the leading centre of chemistry in Italy. Adolf Lieben and Wilhelm Körner worked there, and Emanuele Paternò, who graduated in 1871 under Cannizzaro and succeeded him in the chair in 1872, came out of the school. Cannizzaro was rector of the university from 1866 to 1868, and during the cholera epidemic of 1866–67 he directed the city’s health office. In 1870 he helped found the Gazzetta Chimica Italiana, the national chemistry journal.

In 1871, after Rome became the capital of Italy, he moved to the chair of chemistry at the University of Rome and founded a chemical institute in the former convent of San Lorenzo. In the same year he was appointed a senator of the kingdom for his scientific merit. He was vice-president of the Senate from 1886 to 1897 and from 1898 to 1904, served on its finance commission and others, and worked on the reform of education and public health. He never gave up the laboratory and lectured until 1909.

Recognition abroad followed the triumph of his atomic weights. He gave the Faraday Lecture to the Chemical Society in London in 1872 and received the Copley Medal of the Royal Society in 1891. He became a member of the Accademia nazionale delle scienze in 1865 and of the Lincei in 1873, and was president of the Accademia nazionale delle scienze from 1903 until his death.

Death and legacy

Cannizzaro died in Rome on 10 May 1910, aged 83. In 1926, at the centenary of his birth, during the second national Italian congress of pure and applied chemistry, his remains were moved to the church of San Domenico in Palermo, the burial place of many famous Sicilians.

Legacy

• Atomic weights: the Sunto gave chemistry a single system of atomic weights and formulas, based on Avogadro’s hypothesis and checked by specific heats. It is essentially the system used today.
• Atom and molecule: he made the distinction clear, showing that elementary molecules may contain one (mercury), two (hydrogen) or more (sulphur) atoms.
• Dissociation: he explained the anomalous densities that had discredited Avogadro, preparing the ground for the chemical equilibrium studies of the following decades.
• The periodic table: the weights he championed at Karlsruhe were the data from which Meyer and Mendeleev built the periodic system.
• Organic chemistry: the Cannizzaro reaction remains in every organic textbook; the crossed variant is used in industry.
• Italian science: he founded the Palermo and Rome schools and the national chemistry journal, and shaped science policy for forty years.
• Honours: Faraday Lecture (1872), Copley Medal (1891), the lunar crater Cannizzaro, and the chemistry museum of the University of Palermo named after him.

Sources and further reading

Biography

1. Stanislao Cannizzaro, Wikipedia; Italian Wikipedia, for family, revolution, Palermo and the Senate.
2. Cannizzaro, Stanislao, Complete Dictionary of Scientific Biography, via Encyclopedia.com.
3. Stanislao Cannizzaro, Linda Hall Library, Scientist of the Day.
4. Chemistry collection, Sistema Museale, University of Palermo.
5. W. A. Tilden, Cannizzaro Memorial Lecture, Journal of the Chemical Society, Transactions 101 (1912).

Chemistry

1. S. Cannizzaro, Sunto di un corso di filosofia chimica, Il Nuovo Cimento 7 (1858) 321–366. English translation: Sketch of a Course of Chemical Philosophy, Alembic Club Reprint 18, Edinburgh, 1910.
2. S. Cannizzaro, Ueber den der Benzoësäure entsprechenden Alkohol, Liebigs Annalen 88 (1853) 129–130.
3. Review of the Alembic Club reprint, Nature 85 (1910).
4. A. J. Rocke, Chemical Atomism in the Nineteenth Century: From Dalton to Cannizzaro, Ohio State University Press, 1984.
5. M. J. Nye (ed.), The Question of the Atom: From the Karlsruhe Congress to the First Solvay Conference, Tomash, 1984.
6. Wilhelm Körner, Wikipedia, on the Palermo work on benzene isomers.
7. J. Clayden, N. Greeves and S. Warren, Organic Chemistry, Oxford University Press: the Cannizzaro reaction.