Tetrachloroethylene

Liquid Tetrachloroethylene in a clear glass bottle.

Tetrachloroethylene is a pleasant-smelling colourless liquid that is very widely used for dry-cleaning of clothes. It is also known as perchloroethylene and perc for short. It has the chemical formula C₂Cl₄ and it is composed of carbon and twice the chlorine. Structurally, tetrachloroethylene is ethylene with 4 chlorine atoms instead of hydrogens.

It is a liquid at 25 degrees Celsius, at standard atmosphere (a pressure).

Tetrachloroethylene was first made in 1839 in France but it did not gain popularity until the 1920s when it was used as a worm medication for humans and animals.

Tetrachloroethylene is not flammable or explosive, and it is not poisonous for humans.

History

Tetrachloroethylene was discovered by the French chemist Henri Victor Regnault in 1839. Regnault made it from hexachloroethane (a chlorinated derivative of ethane):

C₂Cl₆ → C₂Cl₄ + Cl₂

At first Regnault tried to make carbon tetrachloride and discovered that he made something different after boiling the substance.

The name "tetrachloroethylene" was first used by the English chemist William Odling in 1857.

Production

Tetrachloroethylene is produced mostly by mixing and heating small hydrocarbons with chlorine. Tetrachloroethylene is not the only product and there can be other chemicals in the resulted mixture. Tetrachloroethylene is distilled and cleaned.

Oxidation of 1,1,2,2-Tetrachloroethane can give tetrachloroethylene:

C₂H₂Cl₄ + [O] → C₂Cl₄ + H₂O

Properties

Perchloroethylene is a colourless liquid at room temperature. It has a mild, sweet odour that most people can smell even in small amounts. It is much heavier than water and it does not mix with water, because Perc is nonpolar while water is polar. It can mix with many other solvents. Perc boils at 121 degrees Celsius and freezes at –22 degrees Celsius.

3D model of tetrachloroethylene's molecular structure. Green balls represent chlorine atoms and black balls represent carbon atoms.

Its molecule is very similar to ethylene but with chlorines instead of hydrogens in ethylene (C₂H₄). Perchloro- in "perchloroethylene" means that the ethylene molecule is full of chlorines. Tetrachloro- in its other name, "tetrachloroethylene", means that it is an ethylene with 4 chlorines without meaning that it's fully chlorinated.

Uses

Dry cleaning

A freight train car carrying Tetrachloroethylene

Tetrachloroethylene has been used in dry cleaning since the 1930s, where it is known as perc. This name is a shortening of a different chemical name, perchloroethylene. It was introduced as a safe alternative to carbon tetrachloride (extremely poisonous to the liver and kidneys) and flammable solvents like gasoline and kerosene. Perc is still widely used in the world because of its effectiveness in cleaning and safety.

Years ago, dry cleaners used to throw away dirty perc after a few uses. The modern dry cleaning machines have mechanism to distill (boiling and condensing again) the perc for re-use.

As medicine

Perchloroethylene was previously used as a parasite medication for animals and humans that are suffering from intestine worms. This was discovered by an American veterinary doctor named Maurice Hall in 1925. Perc was used as a medication until more effective medications were discovered.

In science

Plan of the Homestake Experiment, the tank on the left contained 100,000 gallons of tetrachloroethylene.

In the 1960s, a group of American astrophysicist decided to look into a nuclear mechanism of the Sun. They designed an experiment called the Homestake experiment that used a large tank of tetrachloroethylene to detect neutrinos. The chlorine atoms in the tetrachloroethylene react with electron neutrinos:

³⁷Cl + ν_e → ³⁷Ar + e⁻

This nuclear reaction is the opposite of beta decay. The argon-37 atom can be removed from the solution by bubbling helium through it. By measuring the amount of argon made, scientists were able to calculate the number of neutrinos that passed through the tank.

Dangers

People who inhale it for a long time can become unconscious, and inhalation of lower amounts may have dissociative and hallucinogenic effects. Drinking tetrachloroethylene can cause nausea and dizziness. Its ability to remove oils is very strong and can hurt the skin. Despite its widespread use, poisonings and deaths are very rare.

It is not flammable or explosive, but it can be converted into phosgene (a very deadly gas) if it is thrown into a fire.

Cancer

Tetrachloroethylene is considered probably carcinogenic to humans (Group 2A) by the International Association for Research on Cancer. It has been suspected to cause bladder cancer, non-Hodgkin lymphoma, and multiple myeloma.

Tetrachloroethylene is known not to cause tumour formation in humans. It also does not trigger uncontrolled DNA synthesis in animal cells, which plays a major role in cancer formation.

A study published in 1999, which examined cancer cases among aircraft workers in the United States over a 36-year period, found no difference in cancer rates among those who worked with tetrachloroethylene compared to those who had not used tetrachloroethylene. No significant increase was found in the incidence of suspected lymphoma, leukemia, or liver cancer.

A study in Sweden found no difference in cancer rates between laundry workers that did or did not use tetrachloroethylene, but did not have enough data for a true risk assessment.

Popular Culture

• Tetrachloroethylene pollution was used as a plot point in the TV series Ghostwriter: "Over A Barrel" (episodes 27-30, 1993).

• In the third season of Stargate SG-1, episode 14 (Foothold, 5 November 1999), an alien invasion is hidden by a tetrachloroethylene leak.

• In the episode "Wild Women Do" of the animated TV series American Dad!, a dry cleaner becomes paranoid from sniffing tetrachloroethylene, which causes him to fear washing machines.

Similar compounds

• trichloroethylene
• carbon tetrachloride