Unit 2.6

• The conditions for elimination reactions are: reacting halogenalkanes in hot, concentrated sodium hydroxide in ethanol.
• Nucleophilic substitution reactions involve reacting primary halogenalkanes with dilute aqueous sodium hydroxide at room temperature.
• The 2 reactions that can occur when reacting halogenalkanes with sodium hydroxide are elimination reactions and nucleophilic substitution reactions.
• Halogenalkanes are alkane with a halogen (chlorine, bromine or iodine). They can be primary (with 1 R group), secondary (woth 2 R groups) or tertiary (with 3 R groups).
• The more electronegative halogen withdraws electron density from the carbon, making the the C-Halogen bond polar.
• Electrophilic addition of alkenes will form halogenalkanes when HX or X2 are added to the double bond of the alkene.
• The nucleophilic substitution of halogenoalkanes is carried out under reflux. This is when a condenser is placed in the flask when it is heated so that gases cool and condense back into the reaction at a certain temperature. This allows a constant temperature.
• Hydrolysis of the halogenoalkane using aqueous NaOH produces an aqueous halide ion. Addition of acidifies silver nitrate produces a precipitate which have different solubilities in aqueous ammonia. Chlorine dissolves in dilute NH3, bromine dissolves in concentrated NH3 and iodine is insoluble in NH3.
• Simple chloroalkanes such as tetrachloromethane (CCl4) are good solvents a d will dissolve many non-polar solutes. They dissolve grease well and used in dry cleaning.
• Chlorofluorocarbons (CFCs) are non-toxic, volatile liquids that are widely used as aerosols and refrigerants. However, when CFCs are released into the stratosphere, the intense UV radiation will form radicals from the C-Cl bond. These radicals react with ozone and will produce a 'hole' in the ozone layer.
• Ozone shields the earth from harmful UV radiation so less ozone results in increased levels of skin cancer. HCFs (hydrofluorocarbons) are useful but more expensive alternatives to CFCs.
• Halogenoalkanes have been used as anaesthetics for many years. Modern halogenoalkane anaesthetics are isoflurane and sevoflurane.
• Halogenoalkanes are effective pesticides. DDT was used to kill mosquitoes resulting in a decrease of deaths from malaria. However, DDT is very unreactive and therefore concentrated in the food chains, causing toxic effects in mammals.
• Some plastics are made from halogenoalkane monomers, for example PVC (polyvinylchloride) is made from chloroethene monomers.
• The slightly negative carbon in the carbon halogen bond is more susceptible to nucleophilic attack which leads to substitution reactions.
• A nucleophile is a species with a lone pair of electrons which can be donated to electron deficient species.
• A reflux is the continuous evaporation and condensation of reactants
• The test for halogenoalkanes is sliver nitrate, however it first needs to be hydrolysed via NaOH to seperate the halogen ions, and then neutralised with dilute nitric acid. Silver nitrate is added, and the precipitate colour identifies the halide present.
• Halogenoalkanes are used as solvents. They are polar compounds but insoluble in water, so they can be mixed with both polar and non polar organic compounds which can be used as solvents.
• Reflux maximises the yield, as the reaction is heated for long periods of time without being lost through evaporation.
• A nucleophilic substitution reaction involves 'substituting' the halogen of the halogenoalkane to an OH group, so the haligenoalkane produces an alcohol.