What is aromaticity?

If you’re inclined to resist the popular assertion that chemistry is just applied quantum physics, you need do no more than invoke the notion of aromaticity. Of all chemistry’s messy, ill-defined concepts, none is more so than this. It shows that chemistry is in some ways closer to sociology or zoology: populated by individuals – molecules – that convenience impels us to classify and group without being quite sure if our categories are sound.

The very word betrays its shaky status, being one of the more obvious misnomers of the field. It stems, of course, from the central role of the benzene ring, the hexagonal core of a slew of organic compounds notable for their pungency by the time August Kekulé proposed the cyclic structure in 1865. Michael Faraday immediately noted the almond scent of benzene when he first isolated it in 1825. (He called it bicarburet of hydrogen, believing the ratio of carbon to hydrogen to be 2:1 because of the erroneous atomic weight then assigned to hydrogen.) Kekulé presented the structure in an odd ‘sausage’ format in his 1865 paper; only in 1872 did he show the familiar rings, noting that there were two equivalent configurations of the alternating single and double carbon–carbon bonds.