Polyhalogen Compounds

What Polyhalogen Compounds Are

In the classification of organohalogen compounds, those containing a single halogen atom are monohalogen compounds, while carbon compounds containing more than one halogen atom — di-, tri-, tetra- and so on — are grouped together as polyhalogen compounds. Many of these are useful in industry and agriculture, but the very properties that make them useful — chemical stability, low reactivity, fat solubility — also make several of them persistent pollutants. NCERT specifically notes that halogenated compounds persist in the environment because they resist breakdown by soil bacteria.

The six compounds in §6.8 split neatly into two themes. Dichloromethane, chloroform, iodoform and carbon tetrachloride are simple halomethanes studied mainly for their uses and toxicity. Freons and DDT are studied for their environmental impact — ozone depletion and bioaccumulation respectively. The table below maps each compound to the single fact NEET most often tests.

Dichloromethane (Methylene Chloride)

Dichloromethane, $\ce{CH2Cl2}$, common name methylene chloride, is the simplest of the group with two halogens. It is widely used as a solvent — as a paint remover, as a propellant in aerosols, and as a process solvent in the manufacture of drugs. It also serves as a metal cleaning and finishing solvent.

Its toxicology is what NEET tends to probe. Methylene chloride harms the human central nervous system. Exposure to lower levels in air can lead to slightly impaired hearing and vision; higher levels cause dizziness, nausea, and tingling and numbness in the fingers and toes. In humans, direct skin contact causes intense burning and mild redness of the skin, and direct contact with the eyes can burn the cornea. The pattern to remember is that the harm scales with the level of exposure — mild sensory effects at low concentration, systemic and corrosive effects at high concentration — and that the chief target organ is the nervous system rather than the liver, which distinguishes it from the chloroform that follows.

Chloroform — Phosgene and Dark Bottles

Trichloromethane, $\ce{CHCl3}$, is universally known as chloroform. Chemically it is employed as a solvent for fats, alkaloids, iodine and other substances, and its major modern use is in the production of the freon refrigerant R-22. It was once used as a general anaesthetic in surgery but has been replaced by safer, less toxic anaesthetics such as ether. As expected of an anaesthetic, inhaling chloroform vapours depresses the central nervous system; chronic exposure can damage the liver — where chloroform is metabolised to phosgene — and the kidneys.

The single most examined fact in this entire subtopic is the oxidation of chloroform. Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as phosgene ($\ce{COCl2}$).

$\ce{2CHCl3 + O2 ->[light] 2COCl2 + 2HCl}$

For this reason chloroform is stored in closed, dark coloured bottles filled completely so that air is kept out. The NIOS supplement adds the laboratory refinement that a small amount of ethanol is added to the chloroform: any phosgene that does form is converted into harmless ethyl carbonate.

$\ce{COCl2 + 2C2H5OH -> CO(OC2H5)2 + 2HCl}$

Iodoform (Triiodomethane)

Triiodomethane, $\ce{CHI3}$, common name iodoform, is a pale yellow solid with a distinct, objectionable smell. It was used earlier as an antiseptic, but the crucial NEET point is that the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself. Because of its smell it has since been replaced by other formulations containing iodine.

Although the detailed reaction belongs to carbonyl chemistry, the chapter notes the diagnostic iodoform test: yellow iodoform is produced when ethanol or a methyl ketone is warmed with iodine and alkali, so the test detects the $\ce{CH3-CO-}$ group or the $\ce{CH3-CH(OH)-}$ group. Two structural details are worth fixing in memory. First, iodoform is the only common solid among the four halomethanes discussed here — dichloromethane, chloroform and carbon tetrachloride are liquids — and its pale yellow colour with the characteristic smell is itself a recognition cue. Second, the same compound that acts as a mild antiseptic is also the visible end-product of an analytical test, so the formula $\ce{CHI3}$ links the polyhalogen syllabus to the carbonyl syllabus.

$\ce{CH3COCH3 + 3I2 + 4NaOH -> CHI3 (v) + CH3COONa + 3NaI + 3H2O}$

Carbon Tetrachloride

Tetrachloromethane, $\ce{CCl4}$, common name carbon tetrachloride, is produced in large quantities for the manufacture of refrigerants and aerosol propellants, and as a feedstock in the synthesis of chlorofluorocarbons. Until the mid-1960s it was also widely used as a cleaning fluid, as a degreasing agent in industry, and at home as a spot remover and fire extinguisher.

Its hazards are twofold. There is evidence that exposure causes liver cancer in humans; the common effects are dizziness, light-headedness, nausea and vomiting, which can cause permanent nerve-cell damage, and severe exposure can lead to stupor, coma or death and can make the heart beat irregularly or stop. Equally important for NEET, when carbon tetrachloride is released into the air it rises to the atmosphere and depletes the ozone layer. Depletion of ozone increases human exposure to ultraviolet rays, leading to increased skin cancer, eye diseases and possible disruption of the immune system.

Freons and Ozone Depletion

The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases — an almost ideal profile for a refrigerant, which is exactly why they were used for refrigeration, air conditioning and as aerosol propellants. Freon-12, $\ce{CCl2F2}$, is the most common industrial freon and is manufactured from tetrachloromethane by the Swarts reaction.

The same stability is their undoing. Most freon eventually makes its way into the atmosphere, where it diffuses unchanged into the stratosphere. There it absorbs ultraviolet light, a C–Cl bond breaks, and a chlorine radical is released that can initiate radical chain reactions upsetting the natural ozone balance. The defining feature of the mechanism is that the chlorine radical is regenerated, so one chlorine atom destroys many ozone molecules.

$\ce{CF2Cl2 ->[UV] CF2Cl^. + Cl^.}$

$\ce{Cl^. + O3 -> ClO^. + O2} \qquad \ce{ClO^. + O -> Cl^. + O2}$

DDT — The First Chlorinated Insecticide

DDT — p,p'-dichlorodiphenyltrichloroethane — was the first chlorinated organic insecticide. It was originally prepared in 1873, but it was not until 1939 that Paul Müller of Geigy Pharmaceuticals in Switzerland discovered its effectiveness as an insecticide, work for which he received the Nobel Prize in Medicine and Physiology in 1948. Use of DDT increased enormously worldwide after World War II, chiefly because of its effectiveness against the mosquito that spreads malaria and the lice that carry typhus.

Problems appeared by the late 1940s. Many insect species developed resistance, and DDT was found to have high toxicity towards fish. Its chemical stability and fat solubility compounded the problem: DDT is not metabolised rapidly by animals but is deposited and stored in fatty tissues, so with continued ingestion it builds up within the animal over time. The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world.

Dipole Moment and Environmental Summary

One favourite NEET angle on this group is molecular geometry. Among $\ce{CH2Cl2}$, $\ce{CHCl3}$ and $\ce{CCl4}$, carbon tetrachloride is perfectly tetrahedral and symmetric, so its four C–Cl bond dipoles cancel exactly and its net dipole moment is zero. Dichloromethane has the highest dipole moment of the three because its lower symmetry leaves the bond dipoles uncancelled, with chloroform intermediate.

Pulling the environmental thread together: a number of polyhalogen compounds — dichloromethane, chloroform, iodoform, carbon tetrachloride, freon and DDT — have valuable industrial applications, yet several of them cannot be easily decomposed and even cause depletion of the ozone layer, proving to be environmental hazards. That dual character — usefulness shadowed by toxicity or persistence — is the unifying idea NEET wants you to carry out of §6.8.