In Chapter 2 we suggested you draw saturated carbon chains as zig-zags and not in straight lines with 90° or 180° bond angles. This is because we know they are zig-zags. The X-ray crystal structure of the ‘straight’ chain diacid, hexanedioic acid, is shown below. You can clearly see the zig-zag chain, the planar carboxylic acid groups, and even the hydrogen atoms coming towards you and going away from you. It obviously makes sense to draw this molecule realisti cally, as in the second drawing.

X-ray crystal structures are determined by allowing a sample of a crystalline compound to diffract X-rays. From the resulting diffraction pattern, it is possible to deduce the precise spatial arrangement of the atoms in the molecule—except, usually, the hydrogen atoms, which are too light to diffract the X-rays and whose position must be inferred from the rest of the structure. This is one question that X-ray answers better than any other method: what shape does a molecule have? Another important problem it can solve is the structure of an important new unknown compound. There are subterranean bacteria, for example, that use meth ane as an energy source. It is amazing that bacteria manage to convert methane into anything useful, and, of course, chemists really wanted to know how they did it. In 1979 it was found that the bacteria use a coenzyme, given the trivial name ‘methoxatin’, to oxidize methane to methanol. Methoxatin was a new compound with an unknown structure and could be obtained in only very small amounts. It proved exceptionally difficult to solve the structure by NMR but eventually methoxatin was found by X-ray crystallography to be a polycyclic tricarboxylic acid.