Haloalkanes and Haloarenes

Polyhalogen Compounds

Everyone is aware of DDT- a very common pesticide. Did you know DDT was sold as a wonder-chemical post World War II? It is because DDT was a very simple solution for all sorts of pest problems, be it small or large. It was in widespread use as a pesticide. DDT is nothing but one of the polyhalogen compounds. Polyhalogen compounds have a wide variety of usage. Let’s learn more about them.

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Polyhalogen Compounds

Polyhalogen compounds are Carbon compounds having more than one halogen (elements present in the group 17 of the modern periodic table) atom. Common uses of polyhalogen compounds are in agriculture and industrial sectors. They are popularly used for many purposes such as solvents, anaesthetics pesticides, etc.

Few very important polyhalogen compounds are methylene chloride, chloroform, carbon tetrachloride, iodoform, DDT, benzene hexachloride among many others. In this article, we will discuss some of the most prominent polyhalogen compounds.

1) Methylene chloride (Dichloromethane)

Dichloromethane, also known as methylene chloride, is one of the polyhalogen compounds present in form of solvent. It is popularly known for its uses in many purposes such as a paint remover, as a process solvent during drug manufacturing processes, as a propellant in aerosols, as a cleaning solvent, and as a finishing solvent.

Low levels of methylene chloride exposure in the air can cause slight hearing and vision impairment. High levels of methylene chloride exposure in the air can lead to nausea, dizziness, tingling, and numbness in an individual’s fingers and toes. Direct skin contact of humans with this solvent is highly injurious. It can cause harmful effects such as mild redness of the skin, intense burning, etc. Direct eye contact can lead to burning of the cornea.

Polyhalogen compounds

2) Chloroform (Trichloromethane)

Trichloromethane, also known as Chloroform, is a colourless polyhalogen compound present in liquid form. It has a characteristic sweet smell. The boiling point of chloroform is 334 K. However, chloroform can slowly oxidize by air and form a poisonous gas “phosgene” in the presence of light.

Therefore, it is necessary to store chloroform bottles away from sunlight. It should be completely filled to the brim in order to keep the air out of the bottle. Phosgene is the common name for the chemical carbonyl chloride, (COCl2). However, the addition of little ethanol to chloroform can convert toxic phosgene to a nontoxic compound known as ethyl carbonate.

Chloroform act as a solvent for many substances such as fats, iodine, alkaloids, etc. One of the major use of chloroform in the present day for the manufacturing of Freon refrigerant R-22. Chloroform depresses the Central Nervous System. Initially, chloroform was used as the general anaesthetic for surgical purposes. However,  now much safer options are available like ether.

Breathing 900 parts per million concentration of chloroform can lead to fatigue, dizziness, and headache. Chronic exposure of the compound can cause liver damage (due to the metabolism of chloroform and formation of poisonous gas phosgene) and kidney damage. Development of sores is one of the effects on immersion of skin in the chloroform.

Laboratory Uses of Chloroform

Chloroform act as a detection agent of primary amines during isocyanide test, also known as Carbylamine test. In the test, chloroform is heated with the mixture of amines with alcoholic NaOH. The reaction results in the formation of a foul-smelling isocyanide gas. It helps in the detection of aliphatic and aromatic primary amines.

Polyhalogen compounds

3) Iodoform (Triiodomethane)

The IUPAC name of Iodoform is triiodomethane. It is a pale yellow solid with the presence of a distinct smell. Initially, the polyhalogen compound was used as an antiseptic. However, the antiseptic property of the compound was due to the release of free iodine rather than iodoform which led to the release of objectionable smell. Therefore, now a day other iodine-containing formulations are in use rather than iodoform.

Polyhalogen compounds

4) Carbon tetrachloride (Tetrachloromethane)

It is one of the most common polyhalogen compounds. The most common use of carbon tetrachloride (CCl4) is in the manufacture of refrigerants. It also acts as propellants for aerosol cans. Additional use of this polyhalogen compound is for the synthesis of chemicals such as chlorofluorocarbons.

It is an important solvent in the pharmaceutical manufacturing process and for other general solvent purposes. Initially, in the mid of 1960’s, carbon tetrachloride was widely available as cleaning fluid and degreasing agent in industries. It also acts as a cleaning agent, spot remover, and as a fire extinguisher in the home.

Evidence proves that carbon tetrachloride can cause liver cancer in humans. The common symptoms of carbon tetrachloride exposure are lightheadedness, nausea, dizziness, and vomiting that can lead to permanent damage to the nerve cells.  It can also lead rapidly to coma, unconsciousness and sometimes death in severe cases. Overexposure to carbon tetrachloride can lead to irregular heart beating or sudden stop. It can irritate the eyes on contact.

The release of CCl4 into the air can result in ozone depletion. Ozone depletion is one of the major environmental concerns today. Depletion of the ozone layer can lead to exposure of the UV rays which may further cause many problems such as eye diseases, different disorders, skin cancer, and disturbance of the immune system.

Polyhalogen compounds

5) Freon (CFC’s)

Freon is the collective term for chlorofluorocarbon derivatives of methane and ethane. These compounds are stable, non-corrosive, and non-toxic in nature. These derivatives are easily liquefiable gases. The most common Freon is Freon 12 (CCl2F2). Freon form from tetrachloromethane by a reaction process known as “Swarts reaction.”

Freons can act as aerosol propellants. It also helps in air conditioning and refrigeration purposes. Total Freon production was approximately 2 billion pounds per year by the year 1974. Almost all types of freon gradually settle into the atmosphere without changing its form. This initiates radical chain reactions that cause the problem in the natural ozone balance of the atmosphere.

Polyhalogen compounds

6) DDT (p,p´-Dichlorodiphenyltrichloroethane)

DDT is available in many forms including granules, powder, aerosols, etc. In 1873, DDT was prepared as the first chlorine-based organic insecticides. Paul Miller of Geigy Pharmaceuticals in Switzerland first found the efficacy of DDT as an insecticide in 1939. He received Nobel Prize in Medicine and Physiology in 1948 for the discovery of DDT.

Post World War II there was an enormous increase in the use of DDT. It is because of the capacity of DDT to act against mosquitoes that cause malaria and lice that spreads typhus. However, everyone started observing the disadvantages of the overuse of DDT by late 1940s.

Gradually, insect started developing resistance against the compound. Moreover, it was highly toxic to fishes.  Additionally, the chemical stability of DDT and its capacity of fat solubility increased the problem. Rapid metabolism of DDT is difficult by animals as a result of which accumulation and storage of DDT in the fatty tissues occur. Therefore constant ingestion of DDT can lead to a steady buildup of the compound over time. US government put a ban on DDT in the year 1973.

According to Stockholm Convention on Persistent Organic Pollutants, DDT was banned from using for agricultural purposes by the year 2001. However, it is still in use in developing and underdeveloped countries.

Polyhalogen compounds

Here’s a Solved Question for You

Q: Name the groups that will give a positive iodoform test?

Solution: Iodoform test is positive for the CH3 – C = O (acetyl) or CH3–CH-OH (alcohol) group.

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