Halogenoalkanes are a homologous series of saturated carbon compounds that contain one or more halogen atoms. They are used as refrigerants, solvents, flame retardants, anaesthetics and pharmaceuticals but their use has been restricted in recent years due to their link to pollution and the destruction of the ozone layer.
They contain the functional group C-X where X represents a halogen atom, F,Cl, Br or I. The general formula of the series is CnH2n+1X.
The C-X bond is polar because the halogen atom is more electronegative than the C atom. The electronegativity decreases as you go down group 7 therefore the bond becomes less polar. Flourine has a 4.0 EN whereas iodine has a 2.5 EN meaning it is almost non-polar.
The two types of intermolecular forces between halogenoalkane molecules are Van Der Waals and permanent dipole-dipole interactions. As the carbon chain length increases, the intermolecular forces (due to VDWs) increase as the relative atomic mass increases due to more electrons creating induced dipoles. Therefore the boiling point of the halogenoalkanes increases since more forces must be broken.
Branched chains have lower boiling points than chains of the same length and halogen because the VDWs are working across a greater distance and are therefore weaker.
When the carbon chain length is kept the same, but the halogen atom is changed, despite the effect of the changing polar bond on the permanent dipole-dipole interactions, the changing VDWs have a greater effect on the boiling point. Therefore as RAM increases, the boiling point increases meaning an iodoalkane has a greater boiling point than a bromoalkane if they have the same carbon chain length.
Halogenoalkanes are insoluble or only slightly soluable in water despite their polar nature. They are soluble in organic solvents such as ethanol and can be used as dry cleaning agents because they can mix with other hydrocarbons.
Summary
Halogenoalkanes are saturated carbon compounds with one or more halogen atoms. Their general formula is CnH2n+1X, where X is a halogen. Their functional group is therefore C-X.
They are used as refrigerants, solvents, pharmaceuticals and anaesthetics but have been restricted due to their link to the depletion of the ozone layer.
C-X bonds are polar due to the halogen being more electronegative than the carbon. The polarity of the bond decreases down group 7.
Van der Waals and permanent dipole-dipole interactions are the intermolecular forces in halogenoalkanes.
When carbon chain length increases, boiling points increase due to RAM increasing and the number of Van Der Waals increasing too.
In branched halogenoalkanes, Van Der Waals are working across a greater distance therefore attraction is weaker and boiling points are lower than an identical unbranched chain.
When the halogen is changed, the boiling point increases down the group due to the effect of a greater RAM - more VDWs mean more intermolecular forces to break.
Halogenoalkanes are insoluble in water but soluble in organic solvents like ethanol.
Bonus: free radical substitution reactions in the ozone layer
Ozone, O3, is an allotrope of oxygen that is usually found in the stratosphere above the surface of the Earth. The ozone layer prevents harmful rays of ultraviolet light from reaching the Earth by enhancing the absorption of UV light by nitrogen and oxygen. UV light causes sunburn, cataracts and skin cancer but is also essential in vitamin D production. Scientists have observed a depletion in the ozone layer protecting us and have linked it to photochemical chain reactions by halogen free radicals, sourced from halogenoalkanes which were used a solvents, propellants and refrigerants at the time.
CFCs cause the greatest destruction due to their chlorine free radicals. CFCs – chloroflouroalkanes – were once valued for their lack of toxicity and their non-flammability. This stability means that they do not degrade and instead diffuse into the stratosphere where UV light breaks down the C-Cl bond and produces chlorine free radicals.
RCF2Cl UV light —> RCF2● + Cl●
Chlorine free radicals then react with ozone, decomposing it to form oxygen.
Cl● + O3 —> ClO● + O2
Chlorine radical is then reformed by reacting with more ozone molecules.
ClO● + O3 —-> 2O2 + Cl●
It is estimated that one chlorine free radical can decompose 100 000 molecules of ozone. The overall equation is:
2O3 —-> 3O2
200 countries pledged to phase of the production of ozone depleting agents in Montreal, leading to a search for alternatives. Chemists have developed and synthesised alternative chlorine-free compounds that do not deplete the ozone layer such as hydroflurocarbons (HFCs) like trifluromethane, CHF3.
SUMMARY
Ozone, found in the stratosphere, protects us from harmful UV light which can cause cataracts, skin cancer and sunburn.
Ozone depletion has been linked to the use of halogenoalkanes due to their halogen free radicals.
CFCs were good chemicals to use because they have low toxicity and were non-flammable. The fact they don’t degrade means they diffuse into the stratosphere.
Chlorine free radicals are made when CFCs are broken down by UV light.
These go on to react with ozone to produce oxygen.
Chlorine free radicals are then reformed by reacting with more ozone.
It is a chain reaction that can deplete over 100 000 molecules of ozone.
There is a 200 country ban on their use and scientists have developed alternatives like hydrofluorocarbons to replace them
Happy studying!