In the mid-19th century, with the rapid development of petroleum industry and coking industry, the research of organic chemistry also flourished. We know that benzene is an important organic chemical raw material. It is an aromatic liquid extracted from coal tar. At that time, chemists were faced with a difficult problem, that is, how to understand the structure of benzene.
Benzene molecule contains 6 carbon atoms and 6 hydrogen atoms. The valence of carbon is tetravalent and the valence of hydrogen is monovalent. Then, one carbon atom should be combined with 4 hydrogen atoms, and 6 carbon atoms should be combined with 12 hydrogen atoms (because there is also a combination between carbon atoms and carbon atoms). How could benzene be combined with six carbon atoms and six hydrogen atoms? Chemists are puzzled.
At this time, Kekule also began to explore this problem. His mind is always full of six carbon atoms and six hydrogen atoms of benzene. He often sleeps only three or four hours a day and never stops working. He drew all kinds of chemical structural formulas on the blackboard, on the floor, on his notebook and on the wall, and envisaged dozens of possible arrangements, but they could not stand scrutiny and were denied by himself.