A cheap way to increase the Octane rating of Petrol.
In short, the Octane rating is the measure of how hard it is for the petrol to ignite, and therefore a higher rated fuel can be used at higher compression ratios, to give more power. (More information is on its own node).
TetraEthyl Lead, when added to petrol, was a quick and easy way to raise the Octane rating. Unfortunately, of course, it also ended up belched out into the atmosphere. Lead is a cumulative neurotoxin.
Over the past 20-30 years, better (safer) ways of increasing the Octane rating have been developed, and these are used in unleaded petrol. These are actually more expensive, and less effective, than tetraethyl lead, but in many countries where both leaded and unleaded petrol are available, government taxes are set in such a way that the cost to the consumer is cheaper for unleaded.
For comparison, 4* petrol in the UK is (was) guaranteed to have an Octane of at least 97, and at one point (perhaps until the 1960's), you could get 5* petrol which was about 99 or 100. Premium Unleaded (which is the normal - "normal unleaded" isn't sold) is only about 95, although most cars that were made to run on 4* can be adjusted to run on Premium Unleaded by changing the compression ratio. There is also Super Plus unleaded which has an Octane of 98 - higher than traditional 4* petrol. Most cars made for leaded petrol can run on Super Plus without adjustment. However, it's more expensive than premium unleaded (but my Dad claims the fuel economy improvement more than makes up for this).
Tetraethyl lead has some other properties as well though, to do with engine lubrication. Therefore, even though most cars can be adjusted to run on unleaded, always check with your dealer first. Your mileage may vary.
Physically, tetraethyl lead is 4 (tetra) ethyl (c2h5) groups, centred around a lead atom.
This is an ethyl group.
H H
| |
C--C--H
| |
H H
Notice the left hand Carbon atom is missing a bond. In the case of ethane, another hydrogen atom is here. For an ethyl group, this is where it attaches to other things. So the entire structure is
H
|
H--C--H
|
H--C--H
|
H H | H H
| | | | |
H--C--C-------Pb-------C--C--H
| | | | |
H H | H H
|
H--C--H
|
H--C--H
|
H
In actual fact, the molecule is roughly tetrahedral, with the lead atom in the middle and the four ethyl groups coming off towards the four corners.