[15.2] organohalogen compounds in the environment Flashcards
what is the ozone layer?
a region of earth’s stratosphere that absorbs most of the sun’s UV radiation
how is O₃ formed?
- UV radiation splits an oxygen molecule into two oxygen atoms
- free oxygen atoms react with oxygen molecules to form ozone
- dynamic equilibrium is established
- O₂ + O ⇌ O₃
what are organohalogens?
compounds that contain at least one halogen atom bonded to a carbon chain
what are some uses of organohalogens?
- general solvents (CHCl₃)
- dry cleaning solvents (C₂H₂Cl₂ or C₂HCl₃)
- making polymers (C₂H₃Cl or C₂F₄)
- flame retardents (CF₃Br)
- refrigerants (F₂CCl₂₃ or HCClF₂ or HCCl₂F)
examples of chlorofluorocarbons (CFC’s) and their uses
contain Cl, F and C
- dichlorofluoromethane - CHFCl₂, refrigerant
- trichlorofluoromethane - CF₃Cl, aerosol propellant + blowing agent
- bromochlorodifluoromethane - CBrClF₂, fire extinguishers
- trichlorotrifluoroethane - CCl₂FCClF₂, dry cleaning solvent + degreasing agent
why were the CFC’s chosen?
because of their low reactivity, volatility and non-toxicity
draw the lewis structure of ozone, what angle does it have?
- google it but structure is basically ^ with one double bond and one single bond
- don’t forget + sign
- bond angle is 116.8°
why have organohalogen compounds become a focus of concern?
- because they are rarely found in nature and are not broken down naturally
- scientists found evidence that CFCs were harmful to the atmosphere and governments banned them
- CFCs were replaced with HCFCs, but they were still found to be damaging to the ozone layer
- CFCs can still be a problem because they last a long time in the atmosphere
- some old CFC devices are still being used despite being against the law
why is it important that the ozone later absorbs harmful UV radiation?
- ozone at group level is poisonous
- UV-B from the sun causes DNA damage and is therefore harmful to living organisms
- excessive exposure to UV-B results in sunburn and contributes to skin cancer
what is the mechanism by which CFCs deplete the ozone layer?
- initiation: UV causes homolytic fission in CFC to form radicals: CF₂Cl₂ —> CF₂Cl● + Cl●
- propagation: Cl radical reacts with ozone
- Cl● + O₃ —> ClO● + O₂
- ClO● + O —> Cl● + O₂
- overall equation: O₃ + O —> 2O₂
is CFC responsible for all ozone depleting reactions?
no. nitrogen oxide radicals, formed during thunderstorms or aircraft breakdown give NO (nitrogen monoxide) which also catalyses the breakdown of ozone by a mechanism similar to Cl● radicals
how is NO formed in an aeroplane engine?
- extreme conditions in engine are enough to break N≡N bond
- N₂ + O₂ —> 2NO
equations for the breakdown of ozone catalysed by nitrogen oxide radicals, NO●
- nitrogen monoxide reacts with ozone: O₃ + ●NO —> ●NO₂ + O₂
- nitrogen monoxide is regenerated: ●NO₂ + O —> O₂ + ●NO
