Chemistry · Haloalkanes And Haloarenes · NEET
A polyhalogen compound is a carbon compound that has more than one halogen atom (Cl, Br, I, F) joined to carbon. If it has one halogen it is a monohalogen compound; two makes it a dihalogen compound; three or more makes it polyhalogen. Examples you must know for NEET: chloroform CHCl3, iodoform CHI3, carbon tetrachloride CCl4, freon CCl2F2 and DDT. NCERT devotes a full section (6.8) to these, so 1 to 2 marks come from here almost every year.
Both are trihalomethanes. Chloroform is CHCl3 (trichloromethane), a sweet-smelling colourless liquid used as a solvent and once as a general anaesthetic. Iodoform is CHI3 (triiodomethane), a yellow solid with a strong smell used as an antiseptic. Easy trap: NEET may swap their formulas or their uses. Remember chloroform = liquid solvent/anaesthetic, iodoform = yellow solid antiseptic.
NCERT states the antiseptic action of iodoform is NOT due to iodoform itself. When iodoform touches skin and organic matter it slowly breaks down and releases free iodine, and this free iodine is the real germ-killer. Because of its bad smell, iodoform has been replaced by other iodine formulations. So the mark-scoring line is: antiseptic property is due to liberation of free iodine, not the molecule.
Freons are the chlorofluorocarbon (CFC) compounds of methane and ethane. Freon-12 is CCl2F2, the most common industrial freon. NCERT says it is manufactured from carbon tetrachloride (CCl4) by the Swarts reaction, where fluorine replaces chlorine using metal fluorides like SbF3. Freons are stable, unreactive, non-toxic, non-corrosive and easily liquefied, so they were used as refrigerants and aerosol propellants.
Freons (CFCs like CCl2F2). They are so stable that they float up unchanged to the stratosphere. There, UV light breaks the C-Cl bond and releases chlorine free radicals, which destroy ozone (O3). A thinner ozone layer lets more UV reach Earth, raising skin cancer and eye disease. NEET links freon directly to ozone depletion, so if a question asks which compound depletes ozone, the answer is CFC/freon.
DDT (p,p'-dichlorodiphenyltrichloroethane) was the first chlorinated insecticide and controlled mosquitoes (malaria) and lice (typhus) after World War II. But it is not biodegradable — it stays in soil, water and animal fat for a long time (bioaccumulation), and many insects became resistant to it. Because it builds up in the food chain and harms fish, birds and humans, its use was banned in many countries. Muller won the 1948 Nobel Prize for finding its insecticidal action.
In CCl4 chlorine is more electronegative than carbon, so each Cl is taken as -1 and there are four of them, total -4. The molecule is neutral, so carbon must be +4. This is a favourite NEET numerical: in CH4 carbon is -4 (H is less electronegative, +1 each), and in CCl4 it is +4, so making CCl4 from CH4 changes carbon from -4 to +4.
What is the change in oxidation number of carbon in the following reaction? CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)
The number of chlorine atoms present in the organic products X and Y of the following reactions, respectively, are: C6H6 + 6Cl2 --(anhyd. AlCl3, dark, cold)--> X ; C6H6 + 3Cl2 --(UV, 500 K)--> Y
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Chloroform (CHCl3), iodoform (CHI3), carbon tetrachloride (CCl4), freons such as Freon-12 (CCl2F2), and DDT. Learn each one's formula plus one key use and one hazard.
In the presence of light and air, chloroform is slowly oxidised to phosgene (COCl2), a highly poisonous gas. To stop this, chloroform is kept in dark, closed bottles filled to the top, often with a little ethanol added to destroy any phosgene formed.
No. Carbon tetrachloride was once used in fire extinguishers, but at high temperature it can form poisonous phosgene, and it also depletes ozone and may cause liver cancer. So it is no longer used for this purpose.
The Swarts reaction replaces chlorine in an alkyl chloride with fluorine using metal fluorides like SbF3 or AgF. Freon-12 (CCl2F2) is made from carbon tetrachloride (CCl4) by this reaction, which is why it links polyhalogen compounds to halogen exchange.
Freons are very stable and reach the stratosphere unchanged. UV light breaks their C-Cl bond, releasing chlorine radicals that convert ozone (O3) into oxygen (O2). One chlorine radical can destroy many ozone molecules, thinning the ozone layer.
