Halogenoalkanes

• A safer alternative to chlorofluorohydrocarbons are hydrofluorohydrocarbons
• Free radicals are formed by homolytic fission (homolysis) of a covalent bond
• Initiation step: Cl2 = Cl. + Cl.
• Propagation step:

  Cl. + CH4 = CH3. + HCl

  Cl2 + CH3. = CH3Cl + Cl.
• Termination steps:

  Cl. + Cl. = Cl2

  Cl. + CH3 = CH3Cl

  CH3. + CH3. = C2H6
• If an excess of a halogen is present, further substitution takes place.
• O3 is ozone
• Chlorine peroxide (Cl2O2) catalyses reactions in the ozone layer
• Nucleophile - has a lone pair of electrons, which can be used to form a covalent bond with an electron deficient carbon atom. Negatively or partially negatively charged.
• Hydroxide (:OH-), ammonia (:NH3-) and cyanide (:CN-) are nucleophiles.
• Warm, aqueous conditions are needed for nucleophilic substitution.
• Heterolytic fission happens when the pair of electrons from the covalent bond move onto the halogen atom
• In nucleophilic substitution, ethanol solubilises the haloalkane so it can react with the aqueous hydroxide ions.
• A base is a proton (H+) acceptor.
• :OH- can act as a base, as the lone pair of electrons on the oxygen atom forms a co-ordinate bond with the hydrogen ion.
• :OH- is a base in the elimination of halogenoalkanes
• Conditions of the elimination of halogenoalkanes is reflux and dissolved potassium hydroxide in ethanol.
• The hydroxide ion in the elimination of halogenoalkanes is attracted to the hydrogen on the carbon next to the carbon bonded to the halogen.
• Primary halogenoalkanes undergo mainly substitution reactions.
• Secondary halogenoalkanes undergo both substitution and elimination reactions.
• Tertiary halogenoalkanes undergo mainly elimination reactions.
• Water as a solvent encourages substitution reactions.
• Ethanol as a solvent encourages elimination reactions.
• Higher temperatures encourage elimination reactions.
• Higher concentrations of sodium or potassium hydroxide solution favours elimination.
• Potassium is better than sodium in hydroxide solutions, because potassium has a larger radius and gives up an electron more easily, splitting from :OH- ions
• The elimination of halogenoalkanes can lead to isomers.