2012: Advent Chemistry: Cyclohexane

by on December 19, 2012

Today’s molecule, cyclohexane, was responsible for the second largest peacetime explosion in British history.

Cyclohexane, as the name suggests, consists of six carbons in a ring, with their associated hydrogens, and nothing else. It isn’t interesting primarily for its reactions, but for its shape.

Cyclohexane, unlike its aromatic equivalent benzene, isn’t planar. It doesn’t sit flat, because being flat produces too much ring strain. Carbon has a preferred angle for its bonds to sit at, around 109.5 degrees. That number isn’t just pulled out of the air – 109.5 degrees is the angles between the points of a triangular pyramid, a tetrahedron, and a tetrahedron is the lowest energy arrangement for four bonds to assume. And as I’m sure you remember, carbon usually has four bonds. It has four bonds in cyclohexane, but a tetrahedral arrangement of the bonds on each carbon and a hexagon of carbons aren’t compatible.

So it doesn’t sit flat as a hexagon. It bends. You end up with an arrangement in which the carbons are, in fact, tetrahedral, and it’s a horror to explain. Luckily, we have a picture.

The chair form of cyclohexane

That structure is called the chair conformation, which apparently looks like a chair, though I’ve never been able to see how. It’s the slightly lower-energy conformation. The other, with both of the pointy ends sticking up, is called the boat conformation, and does look a bit like a boat.

This comes back to what we were talking about yesterday, the problems of stability and energy levels. Chair conformation is slightly lower energy, because the hydrogen atoms have less tendency to bump into each other, so more of the molecules assume that form. The conformations of cyclohexane affect the conformations of its derivatives, and not all of those can interconvert – change from one form to the other – so it’s important to know which form of cyclohexane will react faster in your particular circumstances, and whether you can control those rates of reaction to get the product you want.

Cyclohexane, beside being the precursor for a whole lot of other chemicals, is explosive. The Flixborough disaster occurred when a cloud of cyclohexane vapour mixed with air formed at a factory and met some kind of spark. It was a big boom.

Don’t mix oil and air, kids.

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