august kekulé

Friedrich August Kekule von Stradonitz (also August Kekulé) (1829 – 1896) was a German organic chemist. One of the most prominent chemists in Europe from the 1850s until his death, especially in the theoretical realm, he was the principal founder of the theory of chemical structure.

The theory of chemical structure (1857-1858) was a descripton of the ability of carbon atoms to link to each other tetravalently. The idea of the self-linking of carbon atoms provided the first formulae where lines symbolise bonds connecting the atoms. For organic chemists the use of structural formulae to explain the formation of molecules provided dramatic new clarity of understanding, and as a consequence the field of organic chemistry developed explosively from this point.

However, Kekulé's most famous work was based on the structure of benzene. Early suggestions at the time had been based on a linear chain of 6 carbon atom with 4 double bonds. The empirical formula for benzene had been long known, but its highly unsaturated structure was challenging to determine. The study of aromatic compounds was in its earliest years, and too little evidence was then available to help chemists decide on any particular structure. In 1865 Kekulé published a paper in French suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German (his native language) on the same subject.

When Kekulé spoke of the creation of the theory, he said that he had discovered the ring shape of the benzene molecule after having a day-dream of a snake (dirty boy! Freud would have a field day) seizing its own tail. This vision, he said, came to him after years of studying the nature of carbon-carbon bonds.

Although it was an exciting and well thought out idea at the time, further evidence soon came to light that demonstrated that Kekulé was perhaps mistaken in his proposed structure of benzene. You will need to be able to describe and explain this evidence for your module 4 exam.

The evidence includes:

a) problem with bond lengths
b) lack of reaction with bromine (benzene will not undergo electophilic addition)
c) problem with enthalpy of hydrogenation data

Check your notes to make sure you can explain each piece of evidence.