Halogenoalkanes

• Halogenoalkanes contain polar bonds due to the halogens being more electronegative than carbon atoms, forming ∂+ and ∂- regions.
• Nucleophiles are species that are 'positive liking', containing a lone electron pair that is attracted to ∂+ regions of molecules.
• Some of the most common nucleophiles are CN-, NH3, and OH-.
• Nucleophilic Substitution is the reaction mechanism that shows how nucleophiles attack halogenoalkanes, producing alcohols or amines.
• The nucleophile attacks the ∂+ carbon and the electrons are transferred to the chlorine in the mechanism for alcohols.
• The intermediate formed in the mechanism for amines has an N+ atom, so electrons are transferred to it causing a hydrogen to be lost too.
• The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy, meaning it can be broken more easily.
• Nucleophilic substitution reactions can only occur for 1o (primary) and 2o (secondary) halogenoalkanes.
• When a halogenoalkane is heated to high temperatures under alcoholic conditions, elimination occurs, resulting in the formation of a carbon-carbon double bond, an alkene.
• Elimination reactions can only occur from 2o and 3o (tertiary) halogenoalkanes.
• Ozone in the atmosphere absorbs UV radiation, and CFCs (chloro-fluoro carbons) also absorb UV radiation, breaking down the carbon-halogen bonds to form free radicals that can catalyse ozone depletion.
• CFC-free solvents are now being produced to prevent them entering the atmosphere, helping to minimise ozone depletion and global warming.