In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the homologous series of haloalkanes and CFCs.
Haloalkanes
Haloalkanes can be primary, secondary or tertiary.
Haloalkanes can undergo nucleophillic substitution reactions in which the halogen is swapped for another atom.
A nucleophile is an electron pair donor. They always have a lone pair of electrons.
Nucleophillic substitution
The nucleophiles attack the carbon atom.The carbon has a partial positive charge due to the difference of electronegativity between the carbon and the halogen.
The rate of substitution depends on the strength of the bond between the carbon and the halogen. C-F has such a high bond enthalpy it is almost unreactive. Iodoalkanes have the quickest reactions.
Haloalkane → alcohol
Conditions and reagents: in aqueous solution, heat under reflux; potassium hydroxide
Chlorofluorocarbons
Chlorofluorocarbons were developed for use as aerosols as they have low reactivity, volatility and are non-toxic. However, many of these have been stopped due to their detrimental impact on the ozone layer. Now there have been chlorine-free compounds that have replaced them.
HFCs are now used in fridges and air conditioners as they are safer as they do not contain the C-Cl bond.
Global warming
The ozone layer is a naturally occurring layer of oxygen in the upper atmosphere which filters out much of the sun's harmful UV radiation – can mutate DNA and cause cancerous cells.
The ozone is continually being broken down and reformed by the action of UV light. The UV splits O2 into radicals which react and form O3. There is a constant amount of ozone in the atmosphere.
Due to CFCs' low reactivity they reach high levels of the stratosphere and they manage to be broken down by UV light
The free radical chorines catalyse the decomposition of the ozone. They are regenerated in the reaction meaning they can go on to break down much more oxygen.
CFCs are still a concern because CFCs are still entering the atmosphere from disused items an are still used by some countries. CFCs have a long lifetime in the atmosphere and it takes a long time for them to decompose.
Drafted by Eunice (Chemistry)
References
https://docbrown.info/page06/halogenoalkanes4.htm
https://www.masterorganicchemistry.com/2011/09/26/introduction-to-substitution-reactions/