September’s Molecule of the Month

Tetraethyl lead (TEL)

If you have ever bought fuel from a petrol station, or even been sat in the car whilst the driver is filling it up, you will likely have noticed two different types of fuel: diesel and unleaded petrol. Have you ever wondered why it is called unleaded? After all, you never see leaded petrol on sale.

A petrol pump with unleaded fuel just like every petrol station in the U.K.

The truth is, however, that petrol used to have lead in it. In fact for many, many years the only petrol available contained lead. The source of this lead came from the molecule tetraethyl lead (TEL for short) added to the petrol to improve its quality. The story and science behind this most infamous and controversial of molecules is one worth TEL-ling.

Leaded petrol pumps were bright red!

Tetraethyl lead is a small molecule with a very simple structure. It consists of a single lead atom attached to four short chains made of carbon and hydrogen called ethyl groups (C2H5)[1] . As for its appearance, it is a colourless liquid, described as having a sweet odour.

It was during the Victorian age that two scientists claimed to have made TEL. The first discovery, made in 1853/4, is usually given to the German chemist Karl Jacob Löwig. Despite the discovery, little attention was given to TEL and it found no use throughout the rest of the 19th and early 20th Century. This is how TEL could and perhaps should have remained, nothing more than a boring molecule used occasionally in research labs.

Tetraethyl lead (TEL)

This all changed in the 1920s when Thomas Midgeley Jr, a chemist working for General Motors, realised that adding TEL into petrol improved its octane rating and reduced the amount of knocking. Why was this good?

Normally in an engine the fuel / air mixture is ignited by a device called a spark plug. In addition, a pushable cylinder called a piston presses against the fuel / air mixture. This increases the pressure, causing the temperature to rise, and can result in the fuel igniting when it shouldn’t – a problem known as pre-ignition. Furthermore, the early ignition of fuel can also lead to a series of noises coming from the engine – known as knocking.

The octane rating of a fuel describes its performance and how good it is at preventing knocking. The higher the octane number, the more pressing from the piston the fuel can take without pre-igniting.[2] This was the appeal of TEL, a cheap, efficient molecule that improved fuel’s octane rating. But before you rush to drop lumps of lead into your petrol, be warned, lead is toxic and can severely damage both the environment and your health!

A toxic green compound that has made a company an awful lot of money with their oil based products.
So even though we might pollute the Earth, affect people’s health and lower the IQ of children, the money’s pretty good…

TEL itself is an unpleasant molecule, but things only get worse in a car engine. The attachment (bond) between the lead and carbon atoms is weak and so when exposed to the hot temperatures inside an engine begins to fall apart – we call this decomposition. The decomposition of TEL, with some help from oxygen, produces carbon dioxide, water and lead, which can react further to produce lead oxide (PbO). From a car makers point of view, lead and lead oxide are not good and can ruin the engine. To deal with this, another molecule specifically designed to soak up lead and its oxide is added to the fuel. This produces the molecules lead chloride (PbCl2) and lead bromide (PbBr2), which are then finally released from the car in exhaust fumes.

So what’s the problem with this chemical?

It makes people angry, violent and it hinders the development of intelligence in children! The real problem is that they found the perfect distribution tool for a chemical weapon – millions of hot exhaust pipes belching out gases on every road up, down and around the world. Before long, traces of lead were being found in food, drink, soil and air. It was clear that lead, mainly from car exhausts, was damaging the environment and affecting human health. Something needed to be done!

The greatest tragedy is perhaps that the hazards of TEL and other lead compounds were known about long before its inclusion in fuel. Early in the 1920s, before TEL was first put into petrol, Alice Hamilton had studied the effects of lead in the body and confirmed its poisonous nature. Alice Hamilton (1869 – 1970) was an American researcher, medical investigator and Public Health official. She was shocked at the idea of putting lead in petrol and warned the public against the health risks, stating the risks were much greater than the benefits. Even with her vocal opposition, officials were convinced by the motor industry that the amount of lead being released from cars posed no harm.

It wasn’t until the late 1960s that Derek Bryce-Smith (1926 – 2011), an academic at the University of Reading, lead a renewed campaign against the use of TEL and highlighted its dangers. His research showed that lead pollution was reducing the IQ (intelligence) of children. To begin with, these concerns were ignored by the motor industry. However, upon going public with his research-based concerns, the intense public pressure eventually resulted in the creation of unleaded petrol and the slow decline of TEL.

A portrait of Derek Bryce-Smith - the organic chemist from University of Reading who opposed lead in petrol for years
Stylised image of Derek Bryce-Smith, whose campaign against leaded petrol resulted in the introduction of unleaded petrol and the need to phase out TEL.

The overwhelming evidence for the damage TEL was causing finally lead to action. In the US in the 1970s, the newly formed Environment Protection Agency (EPA) began taking action to reduce and phase out lead in petrol. The Clean Air Act of 1996, formally and finally banned completely the use of leaded fuel in any on-road vehicle. In the UK, leaded petrol was totally banned in 2000, as well as throughout the European Union. In fact, by 2011 the majority of countries across the world had banned and removed leaded petrol. Recent studies from different parts of the world have shown the amount of lead in people’s (especially children’s) blood to have dropped significantly since such bans were put into force.

The campaign for cleaner emissions from cars meant that the newly invented catalytic converters were in demand. The evidence that they were unable to work with leaded petrol only contributed more to TEL’s decline.

The Campaign for lead-free air (CLEAR) won a battle against the government to get lead removed from petrol.

Despite efforts around the world to completely remove leaded petrol, there are still countries that have still been using it within the last five years. These countries are however in the process of phasing leaded petrol out. So does this mean a total worldwide ban is nearly complete and the use of TEL is finally over? Unfortunately not!

Whilst the main focus on TEL has been on leaded petrol for road vehicles, it is still widely used in aircraft fuel. More controversially, was the revelation that a British company still making and exporting TEL to several countries around the world.

Tetraethyl lead demonstrates the delicate balance between the benefits and risks that molecules can present. Even today, new products are constantly being assessed, with critics pointing to potential harmful impacts whilst manufacturers strive to assure the public all is safe. The problems of molecules such as TEL also result in a negative public view regarding chemistry and a view that we are constantly and carelessly damaging the world. The research and public campaigning by scientists such as Alice Hamilton and Derek Bryce-White show there are those determined to ensure science helps us, but not at the risk of our health or environment. Luckily, the ban and removal of TEL from cars has helped reduce levels and improve health, although higher concentrations do still remain in places.

Some molecules will never change their behaviour and lead compounds will always be toxic and harmful. Since the demise of TEL, a variety of new chemicals have been created and tested to improve the octane rating of petrol and make it a better anti-knocking agent. Even without lead, the role of petrol and the emissions from car exhausts is still under much debate. Moving forward, the ultimate aim is the invention of fuel or vehicles that no longer requires petrol as we know it, and instead of emitting harmful carbon and nitrogen oxides produces only water. Will we get there before the damage becomes too great? Only time will TEL.

[1] An ethyl group (C2H5) is made up of two carbons atoms attached to each other. One of the carbon atoms is also attached to three hydrogens, whilst the other carbon only has two attached to it. The carbon atom attached to only two hydrogens has a gap (since carbon can have four attachments or bonds) that allows it to attach to another atom. In the case of TEL this atom is lead. It could, however, be another carbon atom, a hydrogen (to make ethane) or many other different atoms. BACK TO POST

[2] The octane rating for a fuel is determined by comparing it to a mixture of two molecules. The first is iso-octane, which has an octane rating of 100, and the second is n-heptane with an octane rating of 0. Most vehicle petrol in the UK has an octane rating of 95. BACK TO POST

This article was written by Marcus Taylor.

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