Part 20: The Chem in SpaceChem - Round 6, Intermezzo

The Chem in SpaceChem, Intermezzo: Resonance structures

There's actually a lot of molecules that have something like that: there are multiple structure formulas that make sense, and neither is fully correct. We can try and give each bond a 'whole number' bond order (single, double, triple and so on) but nature isn't that exact. A bond is formed by a pair of electrons, but in some cases electrons are free to move around among multiple bonds, contributing to each of them. The electrons are 'delocalized'.

In that case a normal structural formula doesn't work. When I first learned of this, I was quite baffled. It showed me that chemistry isn't tidy, not even at the molecular level. Anyway, there are two main ways to draw a structural formula for such a molecule.

The first is drawing all 'contributing' structures and put double-headed arrows between them. These structures are called 'resonance structures', which is a stupid name because there is no resonance going on. The real structure is a hybrid of all resonance forms. Resonance structures are not isomers, and you cannot put double arrows between the structures, because those denote equilibrium reactions. Use a single double-headed arrow.


The second way is to draw the hybrid structure directly. For instance, in benzene, the six-membered carbon ring with 3 'double bonds', It turns out that the lengths of all C-C bonds in benzene are equal, 140 pm. A normal single C-C bond is 147 pm, a normal C=C bond is 135pm. Each bond is actually of order 1.5. That's because the electrons that make up the double bonds are delocalized among all C's in the ring. The hybrid structure shows this as a circle within the hexagon, or as dashed second bonds on each C-C.


By the way, depending on the molecule, one resonance structure can be more contributing than the other(s), in which case the real bond order could be considered some other decimal, such as 1.61.

_{I found the latter picture online, I'm guessing those bond orders are correct but there's no way I can be certain.}