3.3.3 Halogenoalkanes

• Halogenoalkanes contain polar bonds as the halogens are more electronegative than carbon atoms.
• In polar bonds, electron density is drawn towards the halogen forming delta positive and delta negative regions.

  
• Nucleophiles contain a lone pair of electrons that is attracted to delta positive regions of molecules.
• Give examples of nucleophiles: CN-, NH3, OH-
• Nucleophiles must always be shown with a lone pair of electrons and a negative sign.
• Reactions of haloalkanes with OH- produces an alcohol.
• Reactions of haloalkanes with NH3 produce amines.
• The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy meaning it can be broken more easily.
• When a halogenoalkane is heated to high temperatures under alcoholic conditions, elimination occurs.
• Substitution reactions involve breaking the carbon-halogen bond.
• Bond energies can be used to explain different reactivities. C-I bond has the lowest bond energy so it is the weakest C-X bond meaning it will break easily. On the other hand C-F bond is the strongest meaning it is less likely to undergo substitution reactions.
• Reacting halogenoalkanes with aqueous silver nitrate solution will result in the formation of a precipitate.
• Chlorides form a white precipitate.
• Bromides form a cream precipitate.
• Iodides form a pale yellow precipitate.
• The formation of the pale yellow silver iodide is the fastest nucleophilic substitution reaction whereas the formation of silver fluoride is the slowest.
• Fluoralkanes are the least reactive.
• Iodoalkanes are the most reactive.
• Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens.
• The Carbon-Halogen bond is polar causing the C to have a delta + charge and the halogen a delta - charge.
• Nucleophilic substitution reactions are when a halogen is substituted for another atom or group of atoms and the products formed are alcohols, amines and nitriles.
• Elimination reactions are when a hydrogen halide is eliminated during the reaction and the key product during this reaction is an alkene.
• The nucleophile is the formation of alcohols in OH- .
• In the formation of alcohols an aqueous solution of sodium hydroxide or potassium hydroxide is used with ethanol.
• The formation of alcohols is very slow at room temperature
• The formation of an alcohol is a hydrolysis reaction and the rate of the reaction depends on the type of halogen in the haloalkane- the stronger the bond, the slower the reaction rate.
• In the formation of an alcohol the halogen is replaced with the nucleophile OH- .
• The hydroxide ion is a better nucleophile than water because it carries a full negative charge and in water the oxygen atom only carries a partial charge.
• Hydrolysis reaction

  
• In the formation of nitriles, the nucleophile is CN-.
• Chlorofluorocarbons are haloalkanes with only carbon, chlorine and fluorine atoms.
• Alternatives to Chlorofluorocarbons are Hydrofluorocarbons and hydrocarbons.
• Halogenoalkanes are involved in the depletion of ozone. Chlorine free radicals are formed in the atmosphere and these chlorine atoms catalyse the decomposition of ozone which leads to holes in the ozone layer.
• Ozone, O3 , is formed from oxygen free radicals and prevents a lot of UV radiation from the sun from reaching the earth. UV radiation causes sunburn and skin cancer.
• The equations of the formation of ozone are:

  O2 ---> O- + O-

  O2 + O- ----> O3
• Elimination reactions result in the formation of an alkene from a haloalkane.
• For elimination reactions the hydroxide ions must be dissolved in ethanol.
• In elimination reactions, the hydroxide ions act as a base e.g. KOH or NaOH and they accept a proton from the carbon atom.
• Nucleophiles are electron pair donors.
• Nucleophilic substitution with cyanide (CN-) results in the formation of nitriles.