1861

Structural Formula

Josef Loschmidt (1821–1895)

Chemists write down the structures of molecules in ways that many people find strange or even intimidating. For advertising directors and the like, nothing (well, except possibly an Erlenmeyer flask) says chemistry like a complicated structural formula.

But the rules for drawing molecular structures aren’t hard. Each line represents a chemical bond, and if an atom isn’t labeled, it’s a carbon atom. Also, organic chemists tend to leave hydrogen atoms off their structural formulas, since they are everywhere. There can be single, double, and triple bonds to certain elements, and chemists often draw aromatic rings with a circle in them to show they’re different (see Benzene and Aromaticity). This notation, though, assumes that everyone knows what the underlying structures really are, which makes Josef Loschmidt’s work remarkable. An Austrian chemist whose work ranged over several fields, Loschmidt made his real mark on science with his 1861 book Chemische Studien (Chemical Studies), which presented a variety of his new structural notations for molecules. He drew the various atoms as different-sized circles, using shading to indicate different elements, and while many of them look strange to a modern chemist at first, they make reasonable sense after just a short inspection. Although he got several structures wrong, his notation for benzene and other aromatic rings is a strikingly modern circle with atoms or groups bonded to its edges, which is especially impressive given that this was four years before these cyclic structures were accepted as real.

Drawing molecular structures out this way is tremendously helpful to chemists; a structural formula conveys a lot of information in a small space and can tell an experienced chemist a great deal about how the molecule it represents will react, what its physical properties are probably like (even down, in some cases, to its smell), and how one might go about making or using it. Chemists take this sort of notation for granted, although we really shouldn’t, as the rest of the nineteenth century was occupied with questions about how molecules were really put together.

These modern-day structural formulas for various common compounds bear only a passing resemblance to Loschmidt’s versions.