Halogenations are the swapping of one atom or molecule with halogen atoms. Fluorine, bromine, chlorine, and iodine are halogen atoms. Halogenations help to create useful compounds in practical life. In polyhalogenated compounds, there will be halogen substitution in multiple sites. The main uses of polyhalogenated compounds are in agricultural and industrial uses.

What is a polyhalogenated compound?

A polyhalogenated compound is a compound having substituted halogen in multiple sites. They form dangerous compounds like pesticides and DDT, which might be harmful to human health. For example, consider the chlorination of methane to study the formation of polyhalogenated compounds. After the propagation step in the chain reaction mechanism, chlorine may be again bonded to the chloromethane and form dichloromethane. It may be converted again to trichloromethane or even tetrachloromethane.

Types of polyhalogenated compounds

• Poly chlorinated compounds: halogen substitution with chlorine. Examples include hexachlorophene and dioxins
• Poly brominated: Halogen substitution with bromine. Examples include PBDEs, polybrominated biphenyls
• Perfluorinated compounds: halogen substitution with fluorine. Examples include perfluoro octane sulfonyl fluoride and perfluoro octane sulfonic acid 
• Poly Iodinated compounds: halogen substitution with iodine. Examples include organic pollutants like Iopromide

Uses of polyhalogenated compounds

Polyhalogenated compounds have a wide variety of uses in human life

• Used as solvents- dichloromethane used for paint removal as the solvent and also a propellant in aerosols.
• Anaesthetic – polyhalogenated compounds like chloroform are used as an anaesthetic.
• Chemicals – Polyhalogenated compounds like DDT are used as pesticides and are helpful in pest control.
• Antiseptics- polyhalogenated compounds like iodoform have an antibacterial effect.
• Aerosol propellants – freons are used as aerosol propellants.

Disadvantages of polyhalogenated compounds

Even though they are used in various fields in practical life, generally, they are dangerous. This causes various problems associated with it. Some of them can be explained as,

• DDT – a pesticide used widely in the 1940s, later banned on the finding of bioaccumulation in fatty tissues and toxicity to fishes.
• The pungent odour of iodoform made people use such a milder form of iodine for antiseptic purposes.
• Iopromide is a polyhalogenated compound causing environmental pollution.
• Tetrachloromethane causes ozone depletion, which eventually leads to skin cancer.
• A low amount of methylene chloride in the environment causes slight hearing and impairment in vision. Higher concentration causes dizziness and numbness of extremities of hands and legs. Direct contact with the skin causes intense burning and burning of the cornea.

In general, most of the polyhalogenated compounds are pollutants to the environment like Iopromide, perfluorooctanoate, Dichloro diphenyl trichloroethane and hexabromocyclododecane.

Important polyhalogenated compounds

Let’s take a look at some important polyhalogenated compounds.

• PFOA and PFOS (perfluorooctanoate and perfluorooctane sulfonate )

These compounds are used in consumer applications like clothing fabrics, leather, aviation hydraulic fluid etc. Subchronic exposure to PFOS leads to significant hepatotoxicity and reduction of cholesterol and thyroid hormone.

• Dichloromethane (methylene chloride)

It is a flammable, colourless liquid. It is used as paint remover, aerosol propellant, refrigerant and dewaxing agent. Disadvantages include slight hearing, impairment in vision, dizziness and numbness of extremities of hands and legs. Direct contact with the skin causes intense burning and burning of the cornea.

• Chloroform

It is a colourless, high density liquid. It is poisonous and used as an anaesthetic in mild concentrations. It is used in pharmaceuticals and the manufacturing of freons. Disadvantages include dizziness and headache. When chloroform is exposed to light, it is oxidised to a poisonous gas called phosgene. 

• Iodoform

Iodoform is widely used for its antiseptic property. Later it identified that the antiseptic property is due to the release of iodine, not due to iodoform. It had a pungent odour too. Therefore later, it is substituted with other forms of iodine for antiseptic purposes.

• Poly chlorinated biphenyl (PBCs)

It is a heat exchange fluid. It is also used as an additive in paints and plastics. Negative side effects include acne-like skin in adults and neurobehavioural and immunological changes in children.

• Tetrabromo bisphenol A (TBBPA)

It is used as a reactive flame retardant in laminates for printed wiring boards. It is also used as an additive flame retardant in plastics. Adverse effects include problems in primary hepatocytes, immunotoxicity and neurotoxicity.

• Tetrachloromethane

It is a colourless oily liquid used to make chloroform carbons. It is a solvent for oil, fat and waxes. It is used as a fire extinguisher and also contributes to dry cleaning.

• Dichloro diphenyl trichloroethane (DDT)

It is a strong pesticide widelt\y used in the 1940s. Later due to the arising DDT resistant insects, the core problem became more adverse. Later it was banned due to its underlying problems. It is toxic to fishes and bioaccumulates in fatty tissues.

How to break down these compounds?

Sometimes these toxic compounds formed as side products in chain reactions. Mostly they kept forming more than the targeted products. To avoid this, there are certain ways.

• In the chlorination of methane, we can introduce more methane molecules than chlorine. It prevents forming dichloromethane, trichloromethane and tetrachloromethane to an extent.
• Removal of halogen by electrolysis
• Using anaerobic bacteria

Conclusion

 In polyhalogenated compounds, there will be halogen substitution in multiple sites. The main uses of polyhalogenated compounds are in agricultural and industrial uses. Halogenations are the swapping of one atom or molecule with halogen atoms. Fluorine, bromine, chlorine and iodine are the halogen atoms. There are four types of polyhalogenated compounds – polychlorinated, polybrominated, Perfluorinated and poly iodinated.