Wednesday, 14 February 2007


Thanks to modern imaging techniques you just have to look at this guy's face to know that tetraethyl-lead is bad to the bone. It's not because it's inorganic. Inorganic chemistry is nice once in a while. Actually I don't even know the difference. It think it's that inorganic chemists grease their joints?

Well, actually I wasn't going to make jokes here. What I wanted to do, is talk about anti-knocking agents, something that caught my interest studying for organic technology.

"Knocking" means that the gasoline air mixture inside the cylinder explodes before the ignition when the piston is at the wrong position. This wastes energy and damages the engine. As I understand it, knocking is caused by the formation of instable radicals when n-alkanes break apart. The octane rating is a measure for the autoignition resistance of gasoline.

Tetraethyl lead produces ethyl radicals that recombine with other radicals. Therefore it inhibits knocking and raises the octance number. By modern standards you can't imagine having things drive around that spread lead all over the country-side. Anyway, that's what it used to be like until about 20-30 years ago. The soil next to roads is still recovering. A problem with lead in modern cars is that it would poison the catalytic converter.

The modern solution is methyl-t-butyl-ether. I guess it has to do with the fact that a tertiary radical is formed when a radical attacks the oxygen. The tertiary radical may be stable until the ignition. A reason for taking MTBE is that it is cheap. It is formed after adding methanol and acid to the C4 fraction of steamcracking. Only isobutene will react and form MTBE. It is a step for separating the C4 fraction which contains the 7 possible alkanes and alka(di)enes. This is difficult to do through destillation.

I thought that MTBE was an interesting substance. But if I wouldn't have posted this I'd probably forget and then I would wonder what it was, next time I think about it. I will post more theoretical things soon. But probably not before the 25th.

No comments: