August Wilhelm von Hofmann (1818 - 1892) Content from the guide to life, the universe and everything

August Wilhelm von Hofmann (1818 - 1892)

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.
Some Prominent 19th Century German Chemists
Friedrich Wohler | Baron Justus von Liebig | Leopold Gmelin | Friedrich August Kekulé
Johann von Baeyer | Robert Bunsen | August Wilhelm von Hofmann

August Wilhelm von Hofmann was born at Gießen on 8 April, 1818, and in 1836 he entered the University of Gießen intending to study Law. However, at Gießen, he attended lectures by Liebig, which caused him to change his course to Chemistry. Hofmann's work led to the establishment of the coal tar and dye industries, the latter being exploited by one of his students, William Perkin, who developed the first synthetic purple dye based on anilines (aminobenzenes).

The Royal College of Chemistry, UK

In 1845, Hofmann accepted the invitation of Prince Albert, the Prince Consort, to fill the first Chair of Chemistry at the newly-founded Royal College of Chemistry in London1 where he taught for some 20 years, training chemists who were destined to become the leading English chemists of the Victorian era.

Whilst still a student of Liebig's at Gießen, Hofmann had been involved in extracting substances from coal tar and had isolated from it a substance which he recognised as being identical with a compound first obtained by Otto Unverdorben, for which he retained the name, aniline2.

On coming to London in 1845, Hofmann continued to investigate coal-tar, and one of his earliest results was to discover that it contained benzene, although nobody at that time knew its structure. The discovery that benzene is a fairly abundant constituent of coal tar was of huge importance because, in 1834, Eilhard Mitscherlich had shown that when benzene is treated with concentrated nitric acid, nitrobenzene is formed. This was introduced in 1847 as a cheap scenting material, particularly of soap under the name of essence of mirbane. Furthermore, since it had been shown that nitrobenzene could be reduced to aniline, benzene from coal tar became the raw material for the production of aniline.

Prominent among Hofmann's students was William Henry Perkin (1838 - 1907) who studied the relationship between quinine - long used as a medicine in its natural form as the bark from the cinchona tree - and a substance derived from aniline. This research led him in 1856 to discover, quite by chance, the mauve dye mauveine, thus leading to the foundation of the aniline dye industry.

In 1850, the year after the German chemist Charles Wurtz had discovered the 'compound ammonias', or amines, Hofmann not only prepared the amines by direct action of ammonia on the halogen compounds of the hydrocarbon3 (alkyl) radicals, methyl, ethyl, etc, but also obtained the alkyl derivatives of aniline. It thus became clear that these compounds were derived from ammonia or aniline by substitution of one or more hydrogen atoms by alkyl radicals.

At the time Hofmann came to London, organic chemistry was still in its infancy, being mainly concerned with classification. The attention of chemists was taken up with the theory of compound radicals. Virtually nothing was known of the internal constitution (atomic arrangements) of compounds. Hence, although the preparation of the new dyes, opened up by Perkin, was vigorously pursued, chemists realised that further development could only be achieved through a knowledge of their chemical composition, and Hofmann played a leading part in this. Thus, in 1862 he showed that the dye, magenta, is the salt of a base which he called rosaniline.

In 1864, Hofmann confirmed an observation of Nicholson that magenta cannot be made by oxidation of pure aniline, but only of commercial aniline in which the isomeric ortho- and para toluidines4 are present as impurities.

Hofmann's Violets

In 1863, Hofmann showed that the aniline blue of Girard and de Laire is a triphenyl derivative of rosaniline and, as a result of this was led to try the effect of alkyl iodides on rosaniline. He thus discovered that different alkyl groups could be introduced into the rosaniline molecule to produce dyes of various purple or violet colours - 'Hofmann's violets'.

Although Hofmann was able to throw some light on the relationship between magenta and a number of other dyes, which were shown to be derivatives of rosaniline, the constitution of rosaniline itself was unknown. This was going to require the principles of chemical structure, to be laid down later by Couper and Kekulé. It was two other German chemists, Emil and Otto Fischer who, in 1878, showed that the parent of rosaniline, magenta etc, was a hydrocarbon, triphenylmethane.

Hofmann later went on to help elucidate the constitution of another group of dyes, the azo-dyes, one example of which is aniline yellow.

In 1865, Hofmann became 'seized with a profound homesickness for the spiritual heights of a German university', and returned to fill the Chair of Chemistry at the University of Berlin. Here he continued to exert a great influence on the development of organic chemistry, particularly in the field of synthetic dyes. In a tribute to his memory the Hofmann-Haus was opened in Berlin in 1900, to become the home and centre of German chemical science.

In 1902, the German Chemical Society had a medal made in his honour, and it was given as a prize for outstanding achievements in chemistry. In 1952 it was taken over by the Society of German Chemists (and is now given to people internationally) who aren't chemists but have earned special merits for the field of chemistry. Hofmann was raised to the rank of a nobleman of Prussia (von Hofmann) on his 70th birthday and he died on 5 May, 1892.

1This later became absorbed into what is now Imperial College of Science Technology and Medicine.2From the Portuguese, anil, applied to indigo, itself derived from the Arabic an-nil, 'the blue substance'. It was first obtained by Unverdorben by heating indigo. It's modern systematic name is aminobenzene.3Now known, appropriately enough, as 'Hofmann's Method'. This is an ammonolysis. A halogenoalkane and an ethanolic solution of ammonia are heated in a sealed tube at 100°C. A mixture of all three classes of amines is obtained, primary, secondary and tertiary, together with some 'quaternary ammonium' compound.4The toluidines, also called tolylamines, are the mono-substituted amino derivatives of toluene (methylbenzene).

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