Those which contain more than one halogen atom (group 17 elements of the modern periodic table) are Polyhalogen hydrocarbons compounds.

Useful in industry and agriculture many of these compounds some notable polyhalogen compounds are as follows:-

1. Dichloromethane

It is a geminal organic compound called Methylene dichloride or Methylene chloride. From oceanic sources, wetlands, macroalgae, and volcanoes.

Uses:-

• CH2Cl2 is used as a degreasing agent.
• Ethane foam blowing agent.
• In food, technology is used as a solvent.
• Used in aerosol formulations.
• Used as a solvent in the manufacturing of pharmaceutical products.

2. Chloroform

Trichloromethane or chloroform is an organic compound. It is an organic chemical compound initially employed as an ideal anesthetic. In 1831 it was first prepared. The chemical formula is CHCl3.

Uses:-

• Used as the background or reference, milk products are dissolved in chloroform and cholesterol or pure cholesterol powder extract.
• As an extraction dissolvable for fats utilized in the past.
• As a component of food for adhesive components, indirect food additive in food packaging materials.
• Used as an anesthetic and used in dentistry during a root canal.

3. Carbon Tetrachloride or Tetrachloromethane

• With a fragrant smell that can be detected at low levels of colourless liquid.
• Used in manufacturing as a cleaning agent of refrigerants. 
• Most potent hepatotoxins widely used in scientific research (toxic to the liver). 
• It is found in the atmosphere, and it depletes the ozone layer. This increases human exposure depletion of the ozone layer is believed to have ultraviolet rays, leading to increased skin cancer.

4. Iodoform

Soluble in solvents like ethanol, chloroform, and ether but Insoluble in water. It is a yellow crystalline solid to give sodium methanoate. It is hydrolyzed (sodium formate)

also known as triiodomethane.

The antiseptic properties due to the liberation of free iodine are not due to iodoform.

It has been replaced by other formulations that contain iodine.

5. Chlorofluorocarbon

• These compounds are referred to the compounds having mainly carbon, fluorine, and chlorine.
• Extremely stable, unreactive, non-toxic, non-corrosive.
• feron one of the most typical representatives is R-12 (CCl2F2).
• produced for aerosol propellants, refrigeration.

6. p, p’-Dichlorodiphenyltrichloroethane

• In 1873, the first chlorinated organic insecticides were prepared.
• crystalline, tasteless, and almost odourless organochloride and colourless
• in 1948, dichlorodiphenyltrichloroethane was awarded the Nobel Prize in Medicine and Physiology.
• it is used against the mosquito that spreads diseases like malaria and causes lice because of its curing nature.
• DDT got banned due to its actions such as chemical instability and fat solubility.

They have classified are:-

gem –dihalides

VIC – dihalides

Preparation

gem- Dihalides

With PCl5 treating acetaldehyde:-

to acetylene, adding hydrogen chloride.

2. VIC- Dihalides

on ethylene glycol action of PCl5 (or HCl)

to ethylene addition of chlorine.

Physical properties

1. Having relatively high boiling points, sweet-smelling, colourless liquids.
2. The boiling point of ethylene chloride is high than ethylidene chloride.

Chemical properties

Passes through Hydrolysis with aqueous KOH or NaOH.

with aqueous KOH give an aldehyde, or a ketone aqueous KOH gives glycols, 

VIC-Dihalides on hydrolysis.

(Dehalogenation) Reaction with Zinc:- 

on treatment with zinc dust in methanol give alkenes VIC- Dihalides and gem- Dihalides.

(Dehydrohalogenation) Reaction with Alcoholic KOH:-

On treatment with alcoholic KOH, give alkynes 

VIC- Dihalides or gem- Dihalides.

(Dichloromethane) Methylene Chloride:-

Chloroform reduction:-

Gives methylene chloride in the presence of Zn + HCl when chloroform reacts.

1. b) using H2/Ni

Methane chlorination

When methane chlorination takes place, it gives methylene chloride.

Uses:-

It is a metal cleaning solvent.

It is used as an aerosol spray.

It is used as a solvent when the paint is to be unpolished.

In the manufacture of drugs.

Trihaloalkene:- 

When three hydrogen atoms are removed from the three halogen atoms.

Example

CHCl3 chloroform.

Chloroform

On hydrolysis, it gives formic acid its dumas name is chloroform it is an important trihaloalkane.

Preparation:-

By the reaction in the laboratory, chloroform is prepared with bleaching powder followed by the distillation of the products. Bleaching powder act as a source.

There are three steps for performing this reaction:-

Step – 1: Oxidation

CH3CH2OH + Cl2 → CH3CHO + 2HCl

Ethyl alcohol        Acetaldehyde

Step – 2: Chlorination

CH3CHO + 3Cl2 → CCl3CHO + 3HCl

Acetaldehyde Trichloro acetaldehyde

Step – 3: Hydrolysis

2CCl3CHO + Ca(OH)2 → 2CHCl3 + (HCOO)2 Ca

Chloral                  Chloroform

Physical properties:-

It is a colorless liquid with a peculiar smell and even a burning taste.

Its vapors cause drowsiness it depresses the central nervous system. 

It is used as an anesthetic.

Chemical properties:-

Reduction:- When chloroform undergoes reaction with HCL and zn to form methylene chloride in the presence of ethyl alcohol.

Oxidation:-In the presence of air and light. To form phosgene carbonyl chloride.

For use as anesthetic make chloroform It is poisonous it is unfit. 

3) Nitration:-

When nitric acid reacts with its chloroform.

to form chloropicrin (Trichloro nitromethane) 

It is a soil sterilizing agent used as an insecticide.

 4) Carbylamine reaction

To give foul-smelling alkyl isocyanide (carbylamines) alcoholic caustic potash with an aliphatic or aromatic primary amine.

Tetra haloalkane

It is an excellent example of Carbon tetrachloride. 

Preparation

1. Methane chlorination 

With excess chlorine in the presence of sunlight, carbon tetrachloride is the primary product of the reaction of methane

2. with chlorine gas action of carbon disulphide:- as catalyst giving carbon tetrachloride

in the presence of anhydrous AlCl3.

Physical properties

• With its specific smell, it is a colourless liquid.
• In organic solvents, insoluble in water.

Chemical properties:-

1. I) Reduction:-

 To form chloroform by iron powder in dilute HCl medium reduced to Carbon tetrachloride.

1. ii) Oxidation:-

With hot water vapour producing the poisonous gas react to Carbon tetrachloride.

Freons (CFC)- CCl2F2

Physical Properties

• Unreactive 
• Non-corrosive 
• Non-toxic 
• Highly stable

Uses:-

• In refrigerators and air conditioners a used as refrigerants ferons.
• for aerosols and foams as fuel.
• to spray out deodorants, shaving creams.

(p-p’- dichlorodiphenyl trichloroethane)

In 1939 Paul Muller discovered the effectiveness of DDT. In 1873, the first chlorinated organic pesticide. 

 In 1948, Noble prize in medicine and physiology.

By heating a mixture of chlorobenzene in the presence of con.H2SO4  with chloral (Trichloro acetaldehyde).

Uses:-

1. To control certain insects which carry diseases like yellow fever and malaria.

2. To control some agricultural pests in farms.

3. Used at construction as pest control.

Conclusion

Those which contain more than one halogen atom (group 17 elements of the modern periodic table) are Polyhalogen hydrocarbons compounds. Soluble in solvents like ethanol, chloroform, and ether but Insoluble in water. It is a yellow crystalline solid to give sodium methanoate. It is hydrolyzed (sodium formate) also known as triiodomethane.