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Liebig, Justus

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Summary Article: Liebig, Justus (1803–1873): Baron, von Liebig from The Hutchinson Dictionary of Scientific Biography
Justus von Liebig
Image from: Justus von Liebig in The Cambridge Dictionary of Scientists
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Subject: biography, chemistry

Place: Germany

German organic chemist, one of the greatest influences on 19th-century chemistry. Through his researches and those of his ex-students, he had a profound influence on the science for nearly a hundred years. To the schoolchild of today he is best known for the piece of chemical apparatus that he made popular and which still bears his name (the Liebig condenser). A better measure of his status is the fact that his students, assistants and co-workers included such famous chemists as Edward Frankland, Joseph Gay-Lussac, August von Hofmann, Friedrich Kekulé von Stradonitz, Friedrich Wöhler, and Charles Wurtz.

Liebig was born in Darmstadt, Hesse, on 12 May 1803. His father sold drugs, dyes, pigments, and other chemicals and carried out his own chemical experiments, to which Liebig was introduced as a boy. When he was 15 years old he was apprenticed to an apothecary and first went to university to study under Karl Kastner at Bonn (where Liebig was arrested for his liberalist political activity) and then he accompanied Kastner to Erlangen University, where he gained his PhD in 1822 when he was still only 19 years old. Financed by the Grand Duke of Hesse, Liebig went to Paris for two years where Alexander von Humboldt obtained a position for him in Joseph Gay-Lussac's laboratory at the Arsenal. He also made the acquaintance of Louis Thénard, who with von Humboldt recommended the 21-year-old Liebig for the chair of chemistry at the small University of Giessen. He stayed there for 27 years, 1825–52, building up a prestigious teaching laboratory. In 1840 he founded the journal Annalen der Chemie and was made a baron in 1845. Then in 1852 he moved to the University of Munich but, because of failing health, did less active research himself and concentrated on lecturing and writing. He remained there for the rest of his life and died in Munich on 18 April 1873.

In the early 1820s Liebig investigated fulminates, at the same time that Wöhler was independently working with cyanates. In 1826 Liebig prepared silver fulminate (modern formula AgCNO) and Wöhler made silver cyanate (AgNCO). When they reported their results they assigned the same formula to the two different compounds, which stimulated Jöns Berzelius's work that led to the concept of isomers.

Liebig and Wöhler became friends and continued their researches together. In 1832, from a study of oil of bitter almonds (benzaldehyde; phenylmethanal), they discovered the benzoyl radical (C6H5CO–). They showed that benzaldehyde can be converted to benzoic acid and made a number of other related compounds, such as benzyl alcohol and benzoyl chloride. The benzene ring had, in fact, conferred unusual stability to the benzoyl grouping, allowing it to persist in the various reactions. Liebig and Wöhler introduced the idea of compound radicals in organic chemistry, although they found no other radicals that as convincingly supported their theory and found themselves in an acrimonius dispute over the matter with Berzelius and Jean Baptiste Dumas. They had, however, tried to introduce a degree of systematization into the confused field of organic chemistry. To facilitate this work, many new methods of organic analysis were introduced by Liebig, and he devised ways of determining hydrogen, carbon, and halogens in organic compounds.

From 1838 Liebig's work centred on what we would now call biochemistry. He studied fermentation (but would not acknowledge that yeast is a living substance, a view to which Berzelius also subscribed, and which brought them both into contention with Louis Pasteur) and analysed various body fluids and urine. He calculated the calorific values of foods, emphasizing the role of fats as a source of dietary energy, and even developed a beef extract – long marketed as Liebig extract. Liebig also applied his chemical knowledge to agriculture. He demonstrated that plants absorb minerals (and water) from the soil and postulated that the carbon used by plants comes from carbon dioxide in the air rather than from humus in the soil. He also thought, incorrectly, that ammonia in rainwater passed into the soil and provided plants with their sole source of nitrogen. He thus advocated the use of artificial fertilizers in agriculture instead of animal manure, although his original formulation omitted essential nitrogen compounds.

In later life his rather rigid views made Liebig even more dogmatic in his statements – often labelled as arrogance by both his friends and his antagonists – but by then he was an established authority and his opinions were seldom questioned.

It is said that he could be grossly unfair, stimulating controversy and admitting an error only when it no longer mattered; only his lifelong friend Wöhler seems to have continued to have survived Liebig's irascibility.

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