Polyhalogen Compounds

CHEHaloarenes & Polyhalogen Compounds

Compounds carrying more than one halogen — chloroform, iodoform, carbon tetrachloride, freons, DDT — are industrially and environmentally important, and carry frequently-asked questions on their uses, storage, and hazards.

Chloroform oxidation to phosgene

2\,CHCl_3 + O_2 \xrightarrow{\text{light}} 2\,COCl_2 + 2\,HCl

phosgene

COCl_2

is toxic; ethanol is added to convert any phosgene to harmless diethyl carbonate

Common polyhalogen formulae

CHCl_3 \;(\text{chloroform}) \quad CHI_3 \;(\text{iodoform}) \quad CCl_4 \quad CCl_2F_2 \;(\text{Freon-12})

IUPAC: trichloromethane, triiodomethane, tetrachloromethane, dichlorodifluoromethane

Chloroform ( $CHCl_3$ ): kept in dark, well-filled bottles because air and light slowly oxidise it to poisonous phosgene ( $COCl_2$ ); ~1% ethanol is added as a stabiliser.
Iodoform ( $CHI_3$ ): a yellow solid, antiseptic because of the iodine it liberates; the iodoform test detects $CH_3CO\text{-}$ and $CH_3CH(OH)\text{-}$ groups.
Carbon tetrachloride ( $CCl_4$ ): a solvent and fire extinguisher (Pyrene), but it damages the ozone layer.
Freons (CFCs such as $CF_2Cl_2$ ): refrigerants and aerosol propellants that destroy stratospheric ozone.
DDT: the first chlorinated insecticide — non-biodegradable, it accumulates in food chains and is now banned in many countries.
p-Dichlorobenzene is used as a moth repellent and deodorant.
The bottle is filled to the brim to exclude air, and dark glass blocks light, because both air and light are needed to oxidise chloroform to phosgene; ethanol works by reacting with any phosgene formed.
Iodoform was once a popular antiseptic; its action is due to the iodine liberated on contact with skin, not the compound itself, and it has a characteristic smell.
$CCl_4$ vapour is dense and non-flammable, smothering flames, but at high temperature it can form phosgene, and it is now restricted as an ozone-depleting substance.
In the stratosphere, UV light splits CFCs to release chlorine radicals, each of which destroys many ozone molecules in a chain reaction, thinning the ozone layer.
IUPAC names: $CHCl_3$ trichloromethane, $CHI_3$ triiodomethane, $CCl_4$ tetrachloromethane, $CCl_2F_2$ dichlorodifluoromethane (Freon-12), DDT p,p'-dichlorodiphenyltrichloroethane.
Many polyhalogen compounds are persistent (non-biodegradable) and bioaccumulate up the food chain, which is the central environmental concern with DDT and CFCs.

2\,\mathrm{CHCl_3} + \mathrm{O_2} \xrightarrow{\text{sunlight}} 2\,\mathrm{COCl_2} + 2\,\mathrm{HCl}

ConditionsSlow oxidation in air and sunlight

\mathrm{COCl_2}

is phosgene, a poisonous gas. Chloroform is therefore stored in dark brown bottles filled to the brim (excluding air) with about

1\%

ethanol added, which converts any phosgene formed into harmless ethyl carbonate.

Compound	Formula	The examinable point
Chloroform	$\mathrm{CHCl_3}$	Oxidises in sunlight to phosgene; stored with ethanol in dark bottles
Iodoform	$\mathrm{CHI_3}$	Antiseptic, from its liberated iodine — the smell is the giveaway
Freon-12	$\mathrm{CCl_2F_2}$	Refrigerant; releases $\mathrm{Cl}$ radicals that destroy ozone catalytically
DDT	$\mathrm{C_{14}H_9Cl_5}$	Non-biodegradable, so it bioaccumulates up the food chain

The four polyhalogen compounds the syllabus names, and the one fact asked about each.

Where the marks go

Saying chloroform is stored in dark bottles to prevent evaporation — the real reason is to prevent light-induced oxidation to phosgene.
Crediting iodoform's antiseptic action to the whole molecule rather than to the iodine it liberates.
Calling $CCl_4$ a greenhouse gas when the syllabus point is that it (and CFCs) deplete the ozone layer.
Writing the wrong IUPAC names — chloroform is trichloromethane, not trichloroethane; iodoform is triiodomethane.
Forgetting the role of the added ethanol (it converts phosgene to harmless diethyl carbonate) and only mentioning the dark, full bottle.

How the board asks it

Give reasonschloroform oxidation to phosgene; storage precautions1 mkAsked 2024
Give reasons: Chloroform is stored in dark-coloured bottles filled completely up to the brim, with about $1\%$ ethanol added.
Define / stateuses of polyhalogen compounds1 mk
State one important use each of $CCl_4$ and $p$ -dichlorobenzene.
Give reasonsozone depletion by CFCs via chlorine radicals
Give reasons: Freons such as $CF_2Cl_2$ are responsible for the depletion of the stratospheric ozone layer.
Structure / namingIUPAC names of polyhalogen compounds
Give the IUPAC names of $CHCl_3$ and $CHI_3$ .
Assertion–ReasonDDT non-biodegradability and bioaccumulation
Assertion: DDT has been banned in many countries. Reason: DDT is non-biodegradable and accumulates up the food chain. Select the correct option.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.