Polyhalogen compounds: Everyone knows that D.D.T is a very common pesticide. Did you know that D.D.T was marketed as an amazing chemical after World War II? This is because D.D.T is a very simple solution for all types of pests, small and large. D.D.T is just one of the polyhalogen compounds. Polyhalogen compounds have many uses. To learn more about polyhalogen compounds and their uses, read this article.

Define Polyhalogen Compounds

Carbon compounds containing more than one halogen atom are called polyhalogen compounds. These compounds are useful in industry and agriculture fields. The uses and environmental effects of some polyhalogens are discussed below:

Dichloromethane (Methylene Chloride) CH₂Cl₂

Methylene chloride is a colourless, sweet-smelling liquid. It is a volatile liquid having a low boiling point of \(312.75\,{\rm{K,}}\) and its specific gravity is \(1.37.\)

Harmful Effect of Dichloromethane

Methylene chloride is known to have harmful effects on the human central nervous system. Exposure to low concentrations of methylene chloride can result in mild hearing and vision loss. However, exposure to high concentrations of methylene chloride may cause dizziness, nausea, tingling, and numbness in the fingers and toes. Additionally, in humans, direct contact of methylene chloride with the skin causes severe stinging and slight reddening. Direct contact with the eyes can even burn the cornea. It has been observed that the corneas of animals were damaged when exposed to methylene chloride fumes.

Uses of Dichloromethane

1. It is used as a solvent in paint remover and also as a metal cleaner and finish solvent.
2. Due to its low boiling point and low flammability, it is an effective solvent for extraction in pharmaceutical products in the manufacture of medicines and in the food industry.
3. It is also used as a propellant in aerosols.
4. lt is used as a refrigerant and dewaxing agent.

Chloroform (Trichloromethane) CHCl₃

The molecular formula of chloroform is \({\rm{CHC}}{{\rm{l}}_3}.\) The IUPAC name for \({\rm{CHC}}{{\rm{l}}_3}\) is trichloromethane. A \({\rm{CHC}}{{\rm{l}}_3}\) molecule is formed by overlapping a hybrid \({\rm{s}}{{\rm{p}}^3}\) orbital of the \({\rm{C}}\) atom with the \(1{\rm{s}}\) orbital of the \({\rm{H}}\) atom, and further, three hybrid \({\rm{s}}{{\rm{p}}^3}\) orbitals of the same \({\rm{C}}\) atom overlap with the partially-filled \({\rm{p}}\)-orbital of three \({\rm{Cl}}\) atoms. \({\rm{CHC}}{{\rm{l}}_3}\) has a tetrahedral structure.

Preparation of Chloroform

Chloroform is prepared in the laboratory by distilling a mixture of ethanol with a suspension of bleaching powder.

\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {\rm{C}}{{\rm{l}}_2} \to {\rm{C}}{{\rm{H}}_3}{\rm{CHO}} + 2{\rm{HCl}}\)

\({\rm{C}}{{\rm{H}}_3}{\rm{CHO}} + 3{\rm{C}}{{\rm{l}}_2} \to \mathop {{\rm{C}}{{\rm{l}}_3}{\rm{CCHO}}}\limits_{{\rm{Trichloroacetaldehyde}}} + 3{\rm{HCl}}\)

\({\text{2C}}{{\text{l}}_{\text{3}}}{\text{CHO + Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}\xrightarrow{{{\text{Distil}}}}\mathop {{\text{2CHC}}{{\text{l}}_{\text{3}}}}\limits_{{\text{Chloroform}}} {\text{ + }}\mathop {{{\left( {{\text{HCOO}}} \right)}_{\text{2}}}{\text{Ca}}}\limits_{{\text{Calcium}}\,{\text{formate}}} \)

Properties of Chloroform

1. It is a colourless oily liquid with a peculiar smell and a burning taste.
2. It is heavier than water.
3. It is sparingly soluble in water but easily soluble in organic solvents such as ethanol and ether.
4. It is a poisonous compound. Inhalation of vapours causes unconsciousness. For this reason, chloroform is used as an anaesthetic.
5. Chloroform is slowly oxidised into poisonous compound phosgene in the presence of air or light.
\({\text{CHC}}{{\text{l}}_{\text{3}}}{\text{ + }}\frac{{\text{1}}}{{\text{2}}}{{\text{O}}_{\text{2}}}\xrightarrow[{{\text{Light}}}]{{{\text{Air}}\,{\text{or}}}}\mathop {{\text{COC}}{{\text{l}}_{\text{2}}}}\limits_{{\text{Carbonyl}}\,{\text{chloride}}\,{\text{(Phosgene)}}} {\text{ + HCl}}\)

Uses of Chloroform

• 1. In industry, chloroform is used as an important solvent, especially for fats, alkaloids, iodine, waxes, rubber, etc.
• 2. It is used as an anaesthetic. But nowadays, it has been replaced by other anaesthetics like ether; it is naturally very toxic.
• 3. It is used as a laboratory reagent.
• 4. It is used in the manufacture of chloropicrin, chloretone, etc.
• 5. It is used in medicines.
• 6. Used in Freon Refrigerant Manufacture, \({\rm{R}} – 22.\)

Environmental Effect of Chloroform

• 1. Inhaling chloroform vapours stresses the central nervous system.
• 2. Inhaling approximately \(900{\mkern 1mu} \,{\rm{ppm}}\) chloroform can cause dizziness, tiredness, and headaches even for a short time.
• 3. Prolonged exposure to chloroform can damage the liver (where chloroform is metabolized to phosgene) and kidneys.
• 4. Direct and prolonged contact with chloroform leads to wounds on the skin.
• 5. Chloroform slowly oxidizes in the air in the presence of light to form an extremely toxic gas called phosgene, also called carbonyl chloride.

Iodoform (Tri-iodomethane) CHI₃

The molecular formula of iodoform is \({\rm{CH}}{{\rm{I}}_3},\) and its IUPAC name is triiodomethane. During the formation of iodoform \(\left( {{\rm{CH}}{{\rm{I}}_3}} \right),\) one \({\rm{s}}{{\rm{p}}^3}\) hybrid orbital of the \({\rm{C}}\) atom overlaps with the \({\rm{1s}}\) orbital of the \({\rm{H}}\) atom, and the other three \({\rm{s}}{{\rm{p}}^3}\) hybrid orbitals of the same \({\rm{C}}\) atom overlap with partially filled \({\rm{p}}\)-orbitals of iodine.

Preparation of Iodoform

Iodoform is prepared in the laboratory by heating ethanol or acetone with iodine and \({\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}\) or alkali solution.

\(\mathop {{\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH}}}\limits_{{\text{Ethanol}}} {\text{ + 4}}{{\text{I}}_{\text{2}}}{\text{ + 6NaOH}}\xrightarrow{{{\text{Heat}}}}\mathop {{\text{CH}}{{\text{I}}_{\text{3}}}}\limits_{{\text{Iodoform}}} {\text{ + 5NaI + HCOONa + 5}}{{\text{H}}_{\text{2}}}{\text{O}}\)

Properties of Iodoform

• 1. Iodoform is a pale yellow crystalline solid with a melting point of \(392\,{\rm{K}},\) which has a characteristic unpleasant odour.
• 2. It is insoluble in water but easily dissolves in ethyl alcohol and ether. It has an antiseptic effect due to the release of iodine.

Uses of Iodoform

• 1. It is used as an antiseptic because of the iodine released and not the iodoform itself. But because of its very unpleasant odour, it has now been replaced by other formulations containing iodine.
• 2. It is used in the manufacture of pharmaceutical products.

Carbon tetrachloride (Tetrachloromethane) CCl₄

Carbon tetrachloride is a colourless oily liquid with a sickly odour, insoluble in water but soluble in organic solvents such as ether, alcohol; its boiling point is \(350\,{\rm{K}}.\) It is flammable.

Uses of Carbon Tetrachloride

The main uses of \({\rm{CC}}{{\rm{l}}_4}\) are as follows:

• 1. It’s utilised extensively in the production of refrigerants and propellants for aerosol cans.
• 2. It is utilised as a starting material to produce chlorofluorocarbons (freons) and other chemical compounds.
• 3. It’s a solvent that’s employed in the production of pharmaceuticals.
• 4. It’s a solvent for oils, fats, and waxes, among other things.
• 5. Pyrene is a chemical that is used as a fire extinguisher.Carbon tetrachloride is used for dry cleaning.

The Hazardous Effect of Carbon Tetrachloride on Human Being

Exposure to carbon tetrachloride causes liver cancer in humans. Dizziness, lightheadedness, nausea, and vomiting are the most prevalent side effects, and they can damage nerve cells permanently.  These effects can quickly lead to stupor, coma, unconsciousness, or even death in severe cases. Exposure to carbon tetrachloride vapours can cause your heartbeat to become irregular or even stop. After eye contact, it may cause eye irritation.

Environmental Effect of Carbon Tetrachloride

It also has harmful effects on the environment. When carbon tetrachloride is released into the air, it rises into the atmosphere and depletes the ozone layer. The depletion of the ozone layer increases human exposure to ultraviolet radiation, leading to an increase in skin cancer, eye diseases and disorders and possibly weakened immune systems.

Freons and Ozone layer depletion

The chlorofluoro compounds of methane and ethane are collectively referred to as freons. These are extremely stable, low concentration, non-flammable, non-toxic, non-corrosive, easily liquefied, and strongly non-reactive compounds. They are stable even at high pressures and temperatures and are therefore used as refrigerants. Among these, dichlorodifluoromethane \(\left( {{\rm{CC}}{{\rm{l}}_2}{{\rm{F}}_2}} \right),\) known as Freon-\(12,\) is the most important compound in this class because of its industrial use. The action of antimony fluoride produces it on carbon tetrachloride in the presence of antimony pentachloride by means of the Swarts reaction.

Uses of Freons

1. Freons are used as refrigerants in refrigerators and air conditioners, hence the name Freons.
2. These have also been widely used as propellants for aerosols and foams for spraying deodorants, detergents, shaving creams, hairsprays, and insecticides.

Ozone Layer Depletion due to Freons

In the stratosphere, freons are subject to photochemical decomposition and moderate radical change reactions and deplete the protective ozone layer that surrounds our earth. Therefore, the use of freons as a propellant and refrigerant was drastically discouraged and prohibited in many countries.

Dichloro-Diphenyl Trichloroethane (D.D.T.)

D.D.T. is almost insoluble in the water but moderately soluble in polar solvents. It is a powerful insecticide, and it is widely used as an insecticide to kill mosquitoes and other insects. D.D.T. is synthesised by heating a mixture of chloral \(\left( {1\,{\rm{mol}}} \right)\) with chlorobenzene \(\left( {2\,{\rm{mol}}} \right)\) in the presence of concentrated \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}.\) D.D.T. was first produced in \(1874,\) but the effectiveness of D.D.T. as an insecticide was discovered in \(1939\) by Paul Muller at Geigy Pharmaceuticals in Switzerland. For this discovery, Paul Muller received the \(1948\) Nobel Prize in Medicine and Physiology.

Side Effects of D.D.T.

D.D.T is one of the most powerful and effective insecticides. The use of D.D.T. increased enormously after World War II due to its effectiveness against the malaria-transmitting mosquito and the typhus-transmitting lice. However, it has been cautioned against the excessive use of D.D.T.. leads to many problems.

Many insect species developed resistance to D.D.T, and also D D.T. was produced with high fish toxicity. Due to the chemical stability and fat solubility of D.D.T. It is not metabolized very quickly by animals, but it is deposited and stored in adipose tissue, creating alarming dangers, which is why its use has been banned in the United States. However, despite its dangerous side effects, D.D.T. is still widely used in India and other Asian countries because other cheaper insecticides are not available.

Summary

Polyhalogen compounds are carbon compounds that contain more than one halogen atom. Polyhalogen compounds have many applications, such as dichloromethane as a solvent in paint remover and as a metal cleaner and finish solvent. In industry, chloroform is used as an important solvent, especially for fats, alkaloids, iodine, waxes, and rubber. Etc. Iodoform is used as an antiseptic. Carbon tetrachloride is used in large quantities in the manufacture of refrigerants and propellants for aerosol cans, Etc.

FAQ’s on Polyhalogen Compounds and their uses

Q.1. Name the polyhalogen compound which is used as an antiseptic?
Ans: Iodoform is used as an antiseptic.

Q.2. Name the polyhalogen compound which is used as a solvent in industry and in dry cleaning?
Ans: Carbon tetrachloride is used as a solvent in industry and in dry cleaning.

Q.3. What are polyhalogen compounds?
Ans: Carbon compounds containing more than one halogen atom are called polyhalogen compounds.

Q.4. What are Polyhalogen compounds? Give two examples?
Ans: Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Two examples are dichloromethane and chloroform.

Q.5. What are Polyhalides? Explain with an example?
Ans: Metal halides containing more halogen atoms are known as polyhalides. For example, potassium triiodide \(\left( {{\rm{K}}{{\rm{l}}_3}} \right).\)

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