Polyhalogen compounds and the environment

Several halogens on one carbon

A polyhalogen compound has two or more halogen atoms on the same carbon or on a few neighbouring carbons. They are useful solvents, refrigerants and pesticides. The same stability that makes them useful lets them stay in the environment.

Dichloromethane

Dichloromethane, $\text{CH}_{2}\text{Cl}_{2}$, is also called methylene chloride. It boils at $313\ \text{K}$. It is a solvent for paint removal and for metal cleaning, a propellant in aerosols and a solvent in making drugs.

Harm. It affects the central nervous system. Low levels in air can slightly impair hearing and vision. Higher levels cause dizziness, nausea and tingling in fingers and toes. On the skin it burns and reddens, and in the eye it can burn the cornea. The International Agency for Research on Cancer lists it as probably carcinogenic to humans.

Trichloromethane (chloroform)

Chloroform, $\text{CHCl}_{3}$, boils at $334\ \text{K}$ and has a density of $1.49\ \text{g/mL}$.

Preparation. It is made from ethanol or from propanone (acetone) with bleaching powder. Bleaching powder supplies chlorine, which oxidises and chlorinates. Ethanol first gives trichloroethanal (chloral), $\text{CCl}_{3}\text{CHO}$. Calcium hydroxide from the bleaching powder then splits it into chloroform and calcium formate. It can also be made by the chlorination of methane.

Uses. It is a solvent for fats, alkaloids and iodine. Its main use today is making the refrigerant R-22, chlorodifluoromethane. It was once a general anaesthetic, and it was replaced by safer ones such as ether.

Harm. It affects the central nervous system, the liver and the kidneys. In air and light it is slowly oxidised to phosgene, the poisonous gas $\text{COCl}_{2}$.

$2\text{CHCl}_{3}\ +\ \text{O}_{2}\ \xrightarrow{\text{light}}\ 2\text{COCl}_{2}\ +\ 2\text{HCl}$

So chloroform is stored in dark brown bottles that are filled to the top, to keep out air. A little ethanol, about 1 percent, is often added. It turns any phosgene into harmless diethyl carbonate.

Triiodomethane (iodoform)

Iodoform, $\text{CHI}_{3}$, is a yellow crystalline solid with a sharp smell. It melts at $392\ \text{K}$. It is prepared by warming ethanol or propanone with iodine and sodium hydroxide, or sodium carbonate. A yellow precipitate shows that a compound contains the $\text{CH}_{3}\text{CO}-$ group or the $\text{CH}_{3}\text{CH}(\text{OH})-$ group. This is the iodoform test.

Iodoform was used as an antiseptic. The action comes from the iodine that it slowly releases, and not from iodoform itself. Because of its smell, it has been replaced by other antiseptics.

Tetrachloromethane (carbon tetrachloride)

$\text{CCl}_{4}$ boils at $350\ \text{K}$ and has a density of $1.59\ \text{g/mL}$.

Preparation. (1) Carbon disulphide reacts with chlorine in the presence of a catalyst such as aluminium chloride or iodine: $\text{CS}_{2}\ +\ 3\text{Cl}_{2}\ \rightarrow \ \text{CCl}_{4}\ +\ \text{S}_{2}\text{Cl}_{2}$. (2) Methane is chlorinated with excess chlorine in light.

Uses. It was used to make refrigerants and propellants, as a solvent for fats, oils and resins, and as a cleaning fluid. It was also used in fire extinguishers, because its vapour is heavy and does not burn.

Harm. It damages the liver and kidneys, and in strong doses the nerves, with dizziness, nausea and coma. A very high dose can stop the heart. When released into the air it reaches the stratosphere and takes part in the destruction of the ozone layer. These dangers led to the end of most of its uses.

Freons

Freons are chlorofluorocarbons (CFCs), such as dichlorodifluoromethane, $\text{CCl}_{2}\text{F}_{2}$ (freon-12). They are made from tetrachloromethane by the Swarts reaction.

$\text{CCl}_{4}\ +\ 2\text{HF}\ \xrightarrow{\text{SbF}_{3}}\ \text{CCl}_{2}\text{F}_{2}\ +\ 2\text{HCl}$

They are unreactive, non-toxic, not flammable and easy to liquefy. They were the standard refrigerants and aerosol propellants for decades.

The ozone layer. A CFC is so stable that it survives until it rises to the stratosphere, about $15$ to $50\ \text{km}$ up. There, ultraviolet light breaks a C-Cl bond and frees a chlorine atom.

$\text{Cl}^{\bullet}\ +\ \text{O}_{3}\ \rightarrow \ \text{ClO}^{\bullet}\ +\ \text{O}_{2}$

$\text{ClO}^{\bullet}\ +\ \text{O}\ \rightarrow \ \text{Cl}^{\bullet}\ +\ \text{O}_{2}$

The chlorine atom comes out of the cycle unchanged, so it acts as a catalyst. It is removed only when it forms a stable compound, such as hydrogen chloride. One chlorine atom can destroy of the order of $10^5$ ozone molecules. The ozone layer absorbs much of the ultraviolet light from the sun. With less ozone, more ultraviolet light reaches the ground. It raises the risk of skin cancer and eye disease and can harm crops and sea plants. The ozone hole that opens over Antarctica each spring shows the damage.

The Montreal Protocol of 1987 phased out the CFCs. The replacements, hydrochlorofluorocarbons and then hydrofluorocarbons, break down more easily or contain no chlorine. The second group does not damage ozone, but it is a strong greenhouse gas, so it too is being reduced.

DDT

DDT is $p,p'$-dichlorodiphenyltrichloroethane, $\text{C}_{14}\text{H}_{9}\text{Cl}_{5}$, with a molar mass of $354.5\ \text{g/mol}$. It is made from two molecules of chlorobenzene and one of trichloroethanal, in the presence of concentrated sulphuric acid.

$2\text{C}_{6}\text{H}_{5}\text{Cl}\ +\ \text{CCl}_{3}\text{CHO}\ \xrightarrow{\text{conc}.\ \text{H}_{2}\text{SO}_{4}}\ (\text{ClC}_{6}\text{H}_{4})_{2}\text{CHCCl}_{3}\ +\ \text{H}_{2}\text{O}$

Uses. It was the first widely used synthetic insecticide. In the 1940s and 1950s it cut malaria and typhus by killing the insects that carry them, and it protected crops.

Harm. DDT is not easily broken down. It stays in soil for years. It dissolves in fat and is not excreted, so it builds up in the body of an animal, which is bioaccumulation. At each step of a food chain the concentration rises, which is biomagnification. Birds at the top of the chain, such as eagles and fish-eating birds, laid eggs with thin shells that broke. DDT is now banned for farming in most countries. It is still allowed in small amounts against disease-carrying mosquitoes under the Stockholm Convention.

Practise with new numbers

Try it in Python

The program gives the molar mass and the halogen content of six polyhalogen compounds. Then it estimates the ozone destroyed by the chlorine from $1.0\ \text{kg}$ of freon-12.

Common mistakes

• Thinking chloroform is safe to keep in a clear bottle. Light and air make phosgene.
• Saying the chlorine atom is used up. It is regenerated, so it is a catalyst.
• Mixing up the iodoform test with a test for any alcohol. It needs a CH3CO- or CH3CH(OH)- group.
• Forgetting that DDT builds up along the food chain.

Remember these

• $\text{CH}_{2}\text{Cl}_{2}$: solvent and paint remover. $\text{CHCl}_{3}$: solvent, R-22. $\text{CHI}_{3}$: antiseptic. $\text{CCl}_{4}$: solvent. $\text{CCl}_{2}\text{F}_{2}$: refrigerant.
• Chloroform makes phosgene in air and light, so it is kept in full dark bottles.
• Iodoform test: yellow precipitate from a CH3CO- or CH3CH(OH)- compound with $\text{I}_{2}$ and NaOH.
• A chlorine atom from a CFC destroys about $10^5$ ozone molecules by a chain reaction.
• DDT: made from chlorobenzene and chloral, persistent, bioaccumulates and biomagnifies.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is chloroform stored in dark bottles filled to the top?

2. Which reagents are used to make iodoform from ethanol?

3. How does a CFC damage the ozone layer?

4. What does biomagnification mean for DDT?

References

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes, on polyhalogen compounds.
• Morrison and Boyd, Organic Chemistry, the sections on polyhalogen compounds and chlorofluorocarbons.
• UNEP Ozone Secretariat, the Montreal Protocol on Substances that Deplete the Ozone Layer (1987).
• Stockholm Convention on Persistent Organic Pollutants (2001), for the status of DDT.