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Polyhalogen Compounds: Methyl Chloride, Chloroform, Iodoform, Carbon Tetrachloride, Freons, DDT, Practice problems and Frequently Asked Questions

Polyhalogen Compounds: Methyl Chloride, Chloroform, Iodoform, Carbon Tetrachloride, Freons, DDT, Practice problems and Frequently Asked Questions

Everyone is familiar with DDT (p,p’-Dichlorodiphenyltrichloroethane), a widely used pesticide.

Did you know that DDT was sold as a miracle chemical after World War II?

This is because DDT was a very simple solution to all kinds of pest problems, small or large. It was widely used as a pesticide. DDT is one of the only polyhalogenated compounds.

Polyhalogenated compounds have a variety of uses.

Table of content:

  • Polyhalogen compounds
  • Methyl chloride
  • Chloroform
  • Iodoform
  • Carbon tetrachloride
  • Freon’s
  • DDT
  • Practice problems
  • Frequently asked questions

Polyhalogen compounds:

Polyhalogenated compounds are carbon compounds with multiple halogen atoms (group 17 elements of the latest periodic table). Common uses for polyhalogenated compounds are in agriculture and the medical industry. They are commonly used for many purposes such as solvents, anesthetics, pesticides, etc.

Methylene chloride(Dichloromethane):

Dichloromethane, also known as methylene chloride, is one of the polyhalogenated compounds that exists in the form of a solvent. It is widely known for its use for many purposes, As a paint remover, as a process solvent for pharmaceutical manufacturing processes, as an aerosol propellant, as a cleaning solvent, and as a final solvent.

Low levels of methylene chloride in the air can cause slight hearing and visual impairment. High levels of methylene chloride in the air can cause nausea, dizziness, tingling and numbness on a person's fingers and toes. Direct contact of this solvent with human skin is extremely harmful to health. It can cause harmful effects such as slight redness of the skin and a severe burning sensation. Corneal burns can result from direct eye contact.

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Trichloromethane, also called chloroform, is a colorless polyhalogenated compound that exists in the form of a liquid. It has a unique sweet scent. Chloroform boiling point is 334 K. However, chloroform can slowly oxidize in the air and form the toxic gas "phosgene" (COCl2) in the presence of light.

CHCl3+12O2  COCl2+HCl

Therefore, chloroform bottles should be stored away from sunlight. The brim must be completely filled to keep out the air in the bottle. The chemical carbonyl chloride common name is known as phosgene.(COCl2). However, adding a small amount of ethanol to chloroform can convert toxic phosgene into a non-toxic compound called ethyl carbonate.

Chloroform acts as a solvent for many substances such as fats, iodine and alkaloids. One of the main uses of chloroform today is the production of Freon refrigerant R-22. Chloroform suppresses the central nervous system. Originally, chloroform was used as a general anesthetic for surgical purposes. However, much more secure options like ether are now available.

Breathing 900 ppm chloroform can cause fatigue, dizziness and headaches. Chronic exposure to compounds can cause liver damage (where chloroform is metabolized to phosgene) and kidney damage. The development of scars is one of the effects of immersing the skin in chloroform.

Preparation of chloroform:

Chloroform is made in the lab by distilling a combination of ethanol and bleaching powder solution.

CaOCl2+H2O  CaOH2+Cl2

CH3CH2OH+Cl2 → CH3CHO + 2 HCl

CH3CHO + 3Cl2 → CCl3CHO+3HCl

2CCl3CHO+Ca(OH)2 → 2CHCl3+(HCOO)2Ca

Chloroform laboratory uses:

Chloroform is used as a primary amine detector for the isocyanide test (also known as the carbylamine test). In the test, chloroform is heated with a mixture of amine and alcoholic NaOH. This reaction produces the foul-smelling isocyanide gas. Helps in the detection of aliphatic and aromatic primary amines.


The IUPAC name for iodoform is triiodomethane. It is a pale yellow solid with a unique odor. Initially, polyhalogenated compounds were used as preservatives. However, the antiseptic properties of this compound are due to the release of free iodine rather than iodoform, resulting in the release of an unpleasant odor. Therefore, today, other iodine-containing preparations are used in place of iodoform.

Preparation of Iodoform:

In the lab, iodoform is made by heating ethanol or acetone with iodine and alkali solution.

CH3CH2OH+ 4I2+6 NaOH   CHI3+5 NaI+HCOONa+5H2O

Carbon tetrachloride(Tetrachloromethane):

It is one of the most common polyhalogenated compounds. The most common use of carbon tetrachloride (CCl4) is in the production of refrigerants. It also functions as a propellant for aerosol cans. An additional use for this polyhalogenated compound is the synthesis of chemicals such as chlorofluorocarbons.

It is an important solvent for pharmaceutical manufacturing processes and other general solvent purposes. Initially, carbon tetrachloride was widely used in the industry as a cleaning solution and degreasing agent in the mid-1960s. It also functions as a cleaning agent, stain remover, and household fire extinguisher.

There is evidence that carbon tetrachloride can cause liver cancer in humans. The most common symptoms of carbon tetrachloride exposure are drowsiness, nausea, dizziness, and vomiting, which can cause permanent damage to nerve cells. It can also quickly lead to coma, unconsciousness, and, in severe cases, death. Carbon tetrachloride overexposure can cause irregular heartbeats or a sudden stop. On contact, it might irritate the eyes..

The release of CCl4 into the air can lead to ozone depletion. Ozone depletion is one of today's biggest environmental problems. Depletion of the ozone layer can lead to exposure to UV light, which can cause additional problems such as eye diseases, various illnesses, skin cancers, and immune system disorders.


Freon is a general term for chlorofluorocarbon derivatives of methane and ethane. These compounds are inherently stable, non-corrosive and non-toxic. These derivatives are gases that can be easily liquefied. The most common Freon is Freon 12 (CCl2F2). Freon can be formed from carbon tetrachloride through a reaction process known as the "Swartz reaction".

Freon acts as an aerosol propellant. It is also useful for air conditioning and cooling. Almost all types of Freon gradually settle in the atmosphere without changing their shape. This causes radical chain reactions that cause problems with the natural ozone balance of the atmosphere.


DDT is available in a variety of forms, including granules, powders and aerosols. In 1873, DDT was first manufactured as a chlorine-based organic pesticide. Paul Miller of Geigy Pharmaceuticals, Switzerland, first discovered the effectiveness of DDT as an insecticide in 1939. In 1948 he was awarded the Nobel Prize in Physiology or Medicine for the discovery of DDT.

After World War II, the use of DDT has increased significantly. This is due to the ability of DDT to act on mosquitoes that cause malaria and lice that spread typhoid fever. But by the late 1940s, everyone began to notice the shortcomings of DDT abuse.

Gradually, insects began to develop resistance to compounds. It was also very toxic to fish. In addition, the chemical stability of DDT and its ability to be fat-soluble exacerbated the problem. Rapid metabolism of DDT becomes more difficult in animals, leading to the accumulation and storage of DDT in adipose tissue. Therefore, continuous intake of DDT can lead to steady accumulation of compounds over time. The US government banned DDT in 1973.

According to the Stockholm Convention on Persistent Organic Pollutants, DDT was banned from agricultural use until 2001. However, it is still used in developed and developing countries.

Practice problems:

Q.1. Among the following, which chloro compound can be used as fire extinguisher

(A) CH3Cl                                                       (B) COCl2

(C) CCl4                                                           (D) CHCl3

Answer: (C)

Solution: Because its vapors (incombustible vapors) are denser than air which leads to a smoother flame.

Q.2. The iodoform antiseptic property is due to which of the following

(A) Liberation of iodine                                          (B) Smell of iodine

(C) Color of iodine                                                  (D) None of the above

Answer: (A)

Solution: Iodoform liberates iodine, when it comes in contact with skin antiseptic property of iodine is due to the liberation of I2 not because of iodoform itself.

Q.3. Depletion of ozone is believed to increase

(A) Population                                             (B) Ultraviolet rays

(C) Pollution                                                (D) Greenery

Answer: (B)

Solution: Ozone layer depletion increases the amount of ultraviolet rays that reaches the surface of earth. Because all sunlight contains some ultraviolet rays even with normal stratospheric ozone levels, it is always important to protect your skin and eyes from the sun.

Q.4. Which of the following is true?

(A) Freon does not initiate radical chain reactions in the stratosphere
(B) Iodoform initiates radical chain reactions in the stratosphere
(C) Freon initiates radical chain reactions in the stratosphere
(D) Chloroform initiates radical chain reactions in the stratosphere

Answer: (C)

Solution: Freon (CFCl3-Freon 11) initiates radical chain reactions in the stratosphere, which can upset the ozone layer.

Frequently asked questions:

Q1. Why is chloroform kept in dark bottles?
Chloroform is oxidized to phosgene in the presence of air. Phosgene is carbonyl chloride, and represented as COCl2. To prevent the production of phosgene, chloroform is stored in dark bottles.

Q2. At room temperature, ethyl chloride is a gas, whereas ethyl iodide is a liquid. Explain?
The molar mass of C2H5I is 155.97 g mol-1

the molar mass of C2H5Cl is 64.57 g mol-1.

Ethyl iodide has a higher molecular weight than ethyl chloride, so it has a higher van der Waals force. Therefore, ethyl iodide is a liquid at room temperature.

Q3. Name the groups that will show positive iodoform test?
The iodoform test is used to determine if a carbonyl compound of structure RCOCH3 or an alcohol of structure RCH(OH)CH3 is present in a particular unknown substance.

The reaction of iodine, base, and methyl ketone produces a yellow precipitate with a "Sweetish" odor. It is also positive for certain secondary alcohols that contain at least one methyl group at the alpha position.

Q4. Why are freons used as coolants in air conditioners?
Freon is the refrigerant used in most AC systems. It allows it to generate cold air and is sometimes referred to as a "refrigerant" because of its unique properties. It is cold and gaseous at room temperature, but becomes very hot when compressed.


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