Why is chloroform kept in dark brown bottles filled to the brim, and what is phosgene doing in NCERT?

It's 2 am and I can't sleep so I'm revising polyhalogen compounds under a blanket with a torch (my mum thinks I'm sleeping).

NCERT says chloroform is stored in closed dark brown bottles, completely filled so that air is kept out. And then there is a line about it being slowly oxidised by air in the presence of light to carbonyl chloride, also called phosgene, which is a poisonous gas. Why is that so bad, and how does filling the bottle completely help?

I read that a little ethanol is added too. That confused me even more. How does ethanol help if the problem is oxygen?

Also the other compounds in this section: carbon tetrachloride, iodoform, freons and DDT. I know each has some use, but my mind blurs them all into one big "harmful stuff" blob. Can somebody help me separate them with one line each, including what the main problem with each was?

I'd also like to remember the reaction of chloroform with oxygen. Is it a balanced equation I need to write or just the product?

7 answers & replies

[+]Lakshmi TripathiDropperSample· 9d ago

Reaction: $2CHCl_3 + O_2 \rightarrow 2COCl_2 + 2HCl$ . Phosgene is highly poisonous. Light speeds up the oxidation, so the bottle is dark brown, and the bottle is filled to the top so that there is no air space to supply oxygen.

About ethanol: about 1% is added as a stabiliser. If any phosgene does form, it reacts with the alcohol to give ethyl carbonate, which is not poisonous: $COCl_2 + 2C_2H_5OH \rightarrow (C_2H_5O)_2CO + 2HCl$ . So there are two lines of defence, keep oxygen out and neutralise any phosgene that forms. And yes, the balanced equation is worth writing.

[+][–]Mehak AroraDropperSample· 9d agoAuthor
Thank you so much, the two lines of defence idea is easy to remember. Now I can finally try to sleep 😴

[+]Farhan ShaikhDropperSample· 9d ago

CCl4 and freons:

Carbon tetrachloride: used as a solvent, in making refrigerants and as a fire extinguisher (the vapours are heavy and cover the flames). It damages the liver on exposure and also contributes to ozone depletion.
Freons: chlorofluorocarbons like $CCl_2F_2$ (Freon-12), used as refrigerants and aerosol propellants. In the stratosphere sunlight breaks the C–Cl bonds, the chlorine radicals destroy ozone. That's why they are being phased out.

[+]Kabir Joshi11thSample· 9d ago

I thought dark bottle is because chloroform is light sensitive and the brim part is so it doesn't evaporate out. Isn't that the reason?

[+]Aman YadavDropperSample· 9d ago

DDT. First chlorinated organic insecticide, very effective against mosquitoes and crop pests. Problem: it is not metabolised by the animal body, gets stored in fatty tissues and moves up the food chain. Banned in many countries for agricultural use. That is what NCERT wants.

[+]Siddharth RaoDropperSample· 9d ago

Iodoform: yellow solid used as an antiseptic. The antiseptic action isn't because of iodoform itself but because it liberates free iodine. It's also the product of the iodoform test.

Kabir, close but the brim is not about evaporation. The danger is air: oxygen + light turn chloroform into phosgene. If the bottle is full there's no oxygen to react with. That's the point the question is checking.

[+][–]Kabir Joshi11thSample· 8d ago
oh right, oxygen not evaporation. got it, thanks

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