X-ray crystallography proved benzene ring is flat and symmetrical
According to chemistry lore, the idea for the ring-based structure of benzene came to German chemist Friedrich August Kekulé in a daydream, in which he envisioned a snake eating its own tail. Although Michael Faraday first isolated benzene in 1825, chemists had not nailed down its structure.
Kekulé’s dream led him to propose that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds—a theory that he published in 1865.
But questions still remained. In what configuration was this ring? Was it puckered or bowed or flat? Did the molecule have three distinct double bonds?
Most chemists subscribed to the theory that benzene was flat, but it wasn’t until crystallographer Kathleen Lonsdale entered the picture in 1929 that the mystery was finally solved.
Lonsdale obtained her bachelor’s degree in physics at age 19, in 1922. She was the first woman tenured professor at
University College London and the first woman president of the British Association for the Advancement of Science. In 1956 she was named Dame of the British Empire.
In an essay about her life, Lonsdale wrote, “It had been my intention to give up scientific research work and settle down to become a good wife and mother, but my husband would have none of it.”
In the 1920s, she set up a crystallography lab in the physics department at
Leeds University, where her husband was also a scientist.
Lonsdale’s historic relationship with hexamethylbenzene began when she was given some crystals of the compound. Unlike benzene itself, hexamethylbenzene has just one molecule per unit cell, making it easier to distinguish the orientation of the molecule’s central benzene ring. Lonsdale’s X-ray crystallography experiments showed without doubt that the benzene ring was not only flat, but also had an evenly distributed cloud of electrons, sharing the three double bonds (Nature 1928, DOI: 10.1038/122810c0).
Because the benzene ring is the foundation of aromatic compounds, the discovery made possible innumerable inroads into their chemistry.
Lonsdale’s discovery was also notable in that chemists were just starting to make progress in using X-ray crystallography to determine the structures of organic molecules. Organic molecules, with their relative lack of symmetry, were much harder to study than symmetric inorganic lattices. The first X-ray structure of an organic molecule, hexamethylenetetramine, had been reported a mere five years earlier, in 1923.— Elizabeth Wilson