Lead Bromide (Volatile lead?)

Gasoline and diesel engines operate using very different philosophies. In a gas engine, a spark ignites a compressed fuel-air mixture; in a diesel, air is compressed and gets very hot, and fuel is injected, resulting in ignition.

In the case of gasoline, the activation energy to start the fire is supplied by a high-energy spark. With diesel, the heat of compression must be sufficient to start the fire. For this reason, gas has an "octane number" (a measure of how difficult it is to ignite) and diesel has a "cetane number" (a measure of how easy it is to ignite). Low-octane gas can burn unevenly and explode under compression, resulting in "pinging" and engine damage. Low-cetane diesel might result in trouble starting your truck.

To raise cetane, tossing in some oxidizer is a common strategy. The downside here is that you have oxidizer in your fuel, which can result in gum/polymer formation. To raise octane, you need an ingredient that slows the process of combustion (that is, raises the activation energy).

For a long time, this meant tetraethyl lead:


The radicals produced in the combustion of tetraethyl lead slow combustion. To avoid lead deposits in engines, dibromoethane and dichloroethane were used:


This results in gaseous lead compounds, PbCl2 and PbBr2. If this is all starting to sound perverse, you will be heartened to know that Thomas Midgley, who pioneered TEL use, also pioneered CFCs.

It wasn't too long until a young chemist named Clair Patterson noticed we were pumping out an awful lot of lead into the air. Patterson noticed this when he was trying to monitor lead isotopes in ancient rocks to determine the age of the earth. He later worked around the problem and determined the age of the earth to three significant figures, a number that has stood for over five decades.

The lead thing troubled him, though, and he spent the next couple decades fighting to get TEL removed from gasoline. The additive lobby passed the time trying to discredit Patterson, and Midgley did yeoman's work putting on shows trying to reassure reporters TEL was safe. From Bill Bryson's A Short History of Nearly Everything:

As rumors circulated about the dangers of the new product, ethyl's inventor, Thomas Midgley, decided to hold a demonstration for reporters to allay their concerns. As he chatted away about the company's commitment to safety, he poured tetraethyl lead over his hands, then held a beaker of it to his nose for sixty seconds, claiming all the while that he could repeat the procedure daily without harm. In fact, Midgley knew only too well the perils of lead poisoning: he had himself been made seriously ill from overexposure a few months earlier and now, except when reassuring journalists, never went near the stuff if he could help it.

Dedication that'd make a tobacco company proud.

Patterson would have been 86 Monday.


More like this

Dedication that'd make a tobacco company proud.

Nicely put.

The history of TEL is covered in this Damn Interesting article.

'mazing. Throughout history there's been some cognitive dissonance between how lead is used and it's known toxicity. Much of the lead is still around in the soil and everything, too, this is a study on lead in dust done in my own city in (dunno when it was done, though, I seem to remember 1999):

Here's an older study from the UK in which the authors find that there's enough plenty enough lead in street dust to cause full blown lead poisoning on a regular basis, and the definition of lead poisoning was much more lax then :

The gaseous lead compounds produced in combustion are actually PbCl2 and PbBr2. Most lead(IV) compounds are terribly unstable at high temperatures.

I think we can all agree that Thomas Midgley would have been better off not playing with chemicals... CFCs and tetraethyl Lead considered...

I've heard that the levels of lead in water and soil are vastly higher now than before humans started digging it up and using it for all sorts of stuff --- I mean, many orders of magnitude higher. I think people have done studies of ice cores in Greenland or Antarctica, or something like that. But, I can't find this information now. Does anyone know the details?

Here's one fact I just read: between the late 1800s and 1989, about 10 million metric tons of lead were dug up and dispersed into the biosphere worldwide - about equal amounts in the form of lead paint and leaded gasoline. Lead-based paint peaked in the 1920s, and leaded gasoline peaked in the 1970s.

TEL is a powerful radical scavanger so one can use very little of it with a great effect, it was a cheap and profitable solution - the whole idea of using TEL and branched hydrocarbons, alkyl benzenes and MTBE and alcohol (that have high octane number) is that the radicals formed in the process of their burning are a lot less reactive (than you would get with low-octane unbranched stuff) so the radical chan process propagates slower hence you get even burn.

It also depends how the engine is designed, the race cars and propeller planes need high octane fuel but with a modern family car you wont notice much decrease in lifetime or performance with a low-octane grade gas.

Removing lead helped greatly to improve engine lifetime and led-free gas made the catalytic converters feasible so it helped to reducing the smog in cities.

John Baez,

You meant Clair patterson's work on lead in the atmosphere (presumably partly from TEL)? "In 1965, when the tests were conducted, lead levels were roughly 1,000 times higher than they had been in the pre-Ethyl era. He also compared modern bone samples to that of older human remains, and found that modern humans' lead levels were hundreds of times higher."

More about Clair patterson and what he did,

Again amazing how it was only one person that did so much of the work to show how much damage was being done. Has there EVER been a mistake of such huge magnitude and over such a long time? I mean the medical community KNEW it was poisonous from day 1, not quite how much so, but more than close enough.

Deren - thanks a lot for those links! Yes, that's the sort of thing I was thinking about, though I think there's some newer data too.

A toast to Clair Patterson!

Throw some more lead-contaminated dirt on the grave of Thomas Midgely.