Freons, DDT and Carbon Tetrachloride

Polyhalogen Compounds

Carbon is a very important element of the periodic table because of its valency and its ability to form long compounds. The entire branch of organic chemistry is the study of carbon-hydrogen compounds. Polyhalogen compounds are carbon compounds which have more than one halogen atom attached to it. Halogens are the group 17 elements and are the most electronegative elements of the periodic table.  Some of the more important poly-halogen compounds are methyl chloride, chloroform, carbon tetrachloride, iodoform, DDT, benzene hexachloride, CFCs a.k.a Freon’s, etc. They have wide-spread applications in the fields ranging from agriculture to medicine. In this article, we are primarily focusing on poly-halogens Freons, DDT, and carbon tetrachloride.

Freons

Chlorofluorocarbons also known as Freon’s are considered to be important for poly-halogens. It is obtained when we replace the hydrogen atoms of methane or CH4 with fluorine and chlorine atoms. The properties of CFCs can vary with the presence of different numbers of chlorine and fluorine atoms. The naming of chlorofluorocarbons follows a unique system called the rule of 90. The concerned CFC is named CFC-n, where n is obtained as follows. The number of carbon atoms, hydrogen atoms, and fluorine atoms is written in that order and 90 is subtracted from it to obtain n.

So for example, if the formula of the concerned CFC is CCl3F, then it is named as CFC-11.

Here the number 11 gives information about the constituents of the compound as we can find the number of chlorine, fluorine, and hydrogen atoms from this number. The chlorofluorocarbons were initially manufactured as a refrigerant as the refrigerants used till then were toxic to humans and accidents were common. As these are non-toxic they are now used in sprays, perfumes, aerosols, etc. Their production and use as refrigerants have reduced considerably these days due to the fact that these are not very much environmentally friendly.

DDT 

DDT is the short form of dichloro-diphenyl-trichloroethane, an organochlorine. It is a colorless crystalline solid and has no taste or smell. It is primarily used as an insecticide and a pesticide. The manufacture of DDT uses a Friedel Crafts reaction using chloral and chlorobenzene in the presence of an acidic catalyst.   

It comprises a benzene ring with a chlorine atom in its para position and diphenyl trichloroethane in its ortho position. Because of its structure, this component exhibits isomerism ortho para and para-para isomerism. These isomers are however considered as impurities and do not exhibit any kind of insecticidal properties. 

DDT is not soluble in water due to the presence of the rings but is soluble in organic solvents, fats, and oils. Though it is not used in the agricultural sector very prevalently still it is extensively used in the manufacture of various aerosols and vaporizers, lotions, emulsifiable concentrates, etc. However, for this purpose, a mixture of different isomers of DDT is used.

Carbon Tetrachloride

The chemical formula of carbon tetrachloride is CCl4. We obtain carbon tetrachloride when we replace all the hydrogen atoms of CH4 with chlorine atoms. This is a clear, volatile liquid with a sweet smell. This is denser than water with density = 1.581 g/cubic centimeter and is not soluble in water.

Earlier this was used as refrigerants, coolants, in cleaning products, in fire extinguishers, etc. However, it is toxic upon inhalation and can cause irritation to the skin. Hence, these uses were banned. This is now used primarily as a fumigant and for the manufacture of other chemicals. It is also used in various reactions as a chlorine source like the ‘Appel Reaction’.

It is also used as a source of proton in NMR spectroscopy. Historically this was manufactured by the reaction of chlorine and chloroform, however now the chlorination of various carbon compounds primarily hydrocarbons are used.

This has a tetrahedral structure. Since this is the same structure of methane, carbon tetrachloride is also called halo methane. Hence this is a non-polar, covalent compound. Upon crystallization, this exhibits monoclinic crystal lattice.

Other than being toxic, it is also bad for the environment as it acts as a greenhouse gas and leads to the depleting of the ozone layer.


FAQ (Frequently Asked Questions)

1. Explain the Disadvantages of Freon?

Freon’s or chlorofluorocarbons are mainly used in refrigerators and air conditioners as a refrigerant. However, the world is striving to produce and use less Freon because of its adverse effect on the ozone layer. As we know ozone is an O3 molecule. The chemical formula of CFC is CFCl3 and is stable until normal circumstances. However, at the ozone layer, the U.V radiation from the sun breaks the C-Cl bond to form a chlorine-free radical. This reacts with the ozone layer to form an oxygen molecule and an oxygen free-radical and this process continues as a chain reaction.

2. What are the Uses of DDT? Also, Discuss the Disadvantages.

DDT also known as dichloro-diphenyl-trichloroethane can be used as an insecticide in the agricultural field and for pest control. It is easily manufactured, chemically stable, and cheap.  The use of this is limited nowadays because of the disadvantages and the threat this compound poses to the environment. This persists in the environment for up to 15 years. If reached animal or human tissues through food or water it will deposit in fatty tissues. DDT can reach the top layer of the atmosphere and pollute the air.