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Summary Article: Cannizzaro, Stanislao (1826-1910)
From The Hutchinson Dictionary of Scientific Biography

Place: Italy

Subject: biography, chemistry

Italian chemist who, through his revival of Avogadro's hypothesis, laid the foundations of modern atomic theory. He is also remembered for an organic reaction named after him, the decomposition of aromatic aldehydes into a mixture of the corresponding acid and alcohol.

Cannizzaro was born on 13 July 1826 in Palermo. He studied chemistry at the universities of Palermo, Naples, and Pisa, where in 1845 he became assistant to Raffaele Piria (1815-1865), who worked on salicin (preparing salicylic acid) and glucosides. In 1848 Cannizzaro joined the artillery to fight in the Sicilian Revolution, was condemned to death, but in 1849 escaped to Marseille and went on to Paris. There he worked with Michel Chevreul and F Cloëz (1817-1883). In 1851 he synthesized cyanamide by treating an ether solution of cyanogen chloride with ammonia, and in the same year became professor of physics and chemistry at the Technical Institute of Alessandria, Piedmont. It was there that he discovered the Cannizzaro reaction. He was appointed professor of chemistry at Genoa University in 1855, followed by professorships at Palermo 1861-71 and Rome. He became a senator in 1871 and eventually vice-president, pursuing his interest in scientific education. He died in Rome on 10 May 1910.

Cannizzaro's reaction involves the treatment of an aromatic aldehyde with an alcoholic solution of potassium hydroxide. The aldehyde undergoes simultaneous oxidation and reduction to form an alcohol and a carboxylic acid. It is an example of a dismutation or disproportionation reaction, and finds many uses in synthetic organic chemistry. Cannizzaro also investigated the natural plant product santonin, used as a vermifuge, which he showed was related to naphthalene.

His greatest contribution to chemistry was made in 1858 when he revived Avogadro's hypothesis and insisted on a proper distinction between atomic and molecular masses. The pamphlet he published was distributed at the Chemical Congress at Karlsrühe in 1860. Cannizzaro pointed out that once the molecular mass of a (volatile) compound had been determined from a measurement of its vapour density, it was necessary only to estimate, within limits, the atomic mass of one of its elemental components. Then by investigating a sufficient number of compounds of that element, the chances were that at least one of them would contain only one atom of the element concerned, so that its equivalent mass (atomic mass divided by valency) would correlate with its atomic mass. Despite objections by a group of French chemists led by Henri Saint-Claire Deville (who studied abnormal vapour densities of substances such as ammonium chloride and phosphorus pentachloride, and were reluctant to account for these in terms of thermal dissociation), Cannizzaro's proposal was soon widely accepted.

Cannizzaro's contribution to atomic theory paved the way for later work on the periodic law and on an understanding of valency. The Royal Society recognized its significance with the award in 1891 of its Copley Medal.

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