CHEMISTRY HALOALKANES AND HALOARENES

Polyhalogen Compounds

Polyhalogen compounds: Carbon compounds containing more than one halogen atom per
molecule.

Polyhalogen compounds are useful in various industries and in agriculture.

Some important polyhalogen compounds:

Dichloromethane
(Methylene chloride)

p,p'-Dichlorodiphenyl- Trichloromethane
trichloroethane (DDT) (Chloroform)

Polyhalogen
Compounds

Triiodomethane
Freons
(Iodoform)

Tetrachloromethane
(Carbon tetrachloride)

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Dichloromethane (Methylene chloride)

Uses:
Dichloromethane (methylene chloride) is used as a

1. Solvent for paint removers

2. Propellant in aerosols
3. Process solvent in the manufacture of drugs
4. Metal cleaning and finishing solvent

Harmful effects:

1. It endangers the human central nervous system.

2. Exposure to lower levels of methylene chloride in air can lead to slightly impaired hearing and
vision.
3. High levels of methylene chloride in air cause dizziness, nausea, tingling and numbness in the
fingers and toes.
4. In humans, direct skin contact with methylene chloride causes intense burning and mild redness
of the skin.
5. Direct contact with the eyes can burn the cornea.

Trichloromethane (Chloroform)

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Uses:

1. Chemically, chloroform is used as a solvent for fats, alkaloids, iodine and other substances.
2. The major use of chloroform today is in the production of the freon refrigerant R-22.
3. It was once used as a general anaesthetic in surgery but has been replaced by less toxic, safer
anaesthetics such as ether.

Harmful effects:

1. As might be expected from its use as an anaesthetic, inhaling chloroform vapour depresses the
central nervous system.
2. Breathing about 900 parts of chloroform per million parts of air (900 ppm) for a short time can
cause dizziness, fatigue and headache.
3. Chronic chloroform exposure may cause damage to the liver (where chloroform is metabolised to
phosgene) and to the kidneys. Some people develop sores when the skin is immersed in
chloroform.
4. Chloroform is slowly oxidised by air (oxygen) in the presence of light to an extremely poisonous
gas, carbonyl chloride, also known as phosgene.

2CHCl3  O2 
light
 2COCl2  2HCl
Phosgene

It is therefore stored in closed dark-coloured bottles which are completely filled so that air is kept
out.

Triiodomethane (Iodoform)

Uses:

 It was used earlier as an antiseptic, but the antiseptic properties are due to the liberation of free
iodine and not due to iodoform itself.

Drawback:

 Because of its objectionable smell, it has been replaced by other formulations containing iodine.

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Tetrachloromethane (Carbon tetrachloride)

Uses:

1. It is produced in large quantities for use in the manufacture of refrigerants and propellants for
aerosol cans.
2. It is also used as feedstock in the synthesis of chlorofluorocarbons and other chemicals, in
pharmaceutical manufacturing and general solvent use.
3. Until the mid-1960s, it was also widely used as a cleaning fluid, both in industry, as a degreasing
agent, and in the home, as a spot remover and fire extinguisher.

Harmful effects:

1. There is evidence that exposure to carbon tetrachloride causes liver cancer in humans.
2. The most common effects are dizziness, light headedness, nausea and vomiting, which can
cause permanent damage to nerve cells.
3. In severe cases, these effects can lead rapidly to stupor, coma, unconsciousness or death.
Exposure to CCl4 can make the heart beat irregularly or stop.
4. The chemical may irritate the eyes on contact. When carbon tetrachloride is released into the air,
it rises to the atmosphere and depletes the ozone layer.
5. Depletion of the ozone layer is believed to increase human exposure to ultraviolet rays, leading to
increased skin cancer, eye diseases and disorders, and possible disruption of the immune
system.

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Freons

 The chlorofluorocarbon compounds of methane and ethane are collectively known as freons.
 They are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases.
 They are manufactured from tetrachloromethane by Swarts reaction.
 By 1974, the total freon production in the world was about 2 billion pounds annually.

Uses:

1. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes.
2. Freon 12 (CCl2F2) is one of the most common freons in industrial use.
3. Most freons, even those used in refrigeration, eventually make their way into the atmosphere
where it diffuses unchanged into the stratosphere.

Harmful Effect:

 In stratosphere, freons can initiate radical chain reactions which can upset the natural ozone
balance.

p,p′-Dichlorodiphenyltrichloroethane (DDT)

DDT, the first chlorinated organic insecticide, was originally prepared in 1873.
However, it was not until 1939 that Paul Muller of Geigy Pharmaceuticals in Switzerland discovered the
effectiveness of DDT as an insecticide.
Paul Muller was awarded the Nobel Prize in Medicine and Physiology in 1948 for this discovery.

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Paul Muller

Uses:

 The use of DDT increased enormously worldwide after World War II, primarily because of its
effectiveness against the mosquito which spreads malaria and lice which carry typhus.

Harmful Effects:

Problems related to extensive use of DDT began to appear in the late 1940s.
1. Many species of insects developed resistance to DDT.
2. It has a high toxicity towards fish.
3. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not
metabolised very rapidly by animals. Instead, it is deposited and stored in the fatty tissues. If
ingestion continues at a steady rate, DDT builds up within the animal over time.

The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of
the world.

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