Hexamethylbenzene
Hexamethylbenzene, also known as mellitene, is a hydrocarbon with the molecular formula C12H18 and the condensed structural formula C6(CH3)6. It is an aromatic compound and a derivative of benzene, where benzene's six hydrogen atoms have each been replaced by a methyl group. In 1929, Kathleen Lonsdale reported the crystal structure of hexamethylbenzene, demonstrating that the central ring is hexagonal and flat and thereby ending an ongoing debate about the physical parameters of the benzene system. This was a historically significant result, both for the field of X-ray crystallography and for understanding aromaticity. Hexamethylbenzene can be oxidised to mellitic acid, which is found in nature as its aluminium salt in the rare mineral mellite. Hexamethylbenzene can be used as a ligand in organometallic compounds. An example from organoruthenium chemistry shows structural change in the ligand associated with changes in the oxidation state of the metal centre, though the same change is not observed in the analogous organoiron system. In 2016 the crystal structure of the hexamethylbenzene dication C6(CH3)62+ was reported in Angewandte Chemie International Edition, showing a pyramidal structure in which a single carbon atom has a bonding interaction with six other carbon atoms. This structure was "unprecedented", as the usual maximum valence of carbon is four, and it attracted attention from New Scientist, Chemical & Engineering News, and Science News. The structure does not violate the octet rule since the carbon–carbon bonds formed are not two-electron bonds, and is pedagogically valuable for illustrating that a carbon atom "can [directly bond] with more than four atoms". Steven Bachrach has demonstrated that the compound is hypercoordinated but not hypervalent, and also explained its aromaticity. The idea of describing the chemical bonding in compounds and chemical species in this way through the lens of organometallic chemistry was proposed in 1975, soon after the dication C6(CH3)62+ was first observed.
