Moritz Malischewski & Konrad Seppelt

TEAR up that old textbook. A pyramid-shaped carbon molecule that contradicts one of the most basic chemistry lessons we learn at school has been studied for the first time. It contains a carbon atom that bonds to six other atoms instead of the four we have been told carbon is limited to.

Atoms form molecules by sharing electrons. Carbon has four electrons that it can share with other atoms. But in certain conditions, carbon can be stretched beyond this limit, says Moritz Malischewski, a chemist at the Free University of Berlin who synthesised and studied the molecule, called hexamethylbenzene.

Typically, this compound resembles a ship’s wheel, consisting of six carbon atoms arrayed in a hexagonal ring, with extra carbon-atom arms protruding from the ring’s outer edge. In an experiment in 1973, German chemists took away two of the compound’s electrons, and evidence suggested that the positively charged version then collapsed in on itself and formed a pyramid. In this arrangement, there are six electrons available to connect the top of the pyramid to the five carbons in the rest of the ring and the extra arm, Malischewski says. But no one double-checked the molecule’s shape, until now.


It is an unusual, unstable arrangement that exists only at low temperatures inside extremely acidic liquids. So Malischewski spent six months tinkering with a potent acid to produce the compound and derive a few milligrams of crystals that could then be viewed using X-rays.

The X-ray diffraction pattern showed the unmistakeable five-sided pyramidal shape (Angewandte Chemie, dx.doi.org/f3s9kw).

Quantum calculations and other experiments suggested a six-bond carbon atom was possible, but the crystal structure serves as photographic proof, says Dean Tantillo at the University of California, Davis. “It sheds light on the nature of bonding and the limits of our understanding of organic chemical structures,” he says.

“It is all about the challenge and the possibility to astonish chemists about what can be possible”

In normal temperature and humidity, the molecule would break down immediately, so it is unlikely to have any practical applications, such as producing new types of carbon nanotubes.

But Malischewski says he was just intrigued by the question of whether the molecule could even exist. “It is all about the challenge and the possibility to astonish chemists about what can be possible,” he says.

This article appeared in print under the headline “Carbon rips up chemistry textbooks”