Haloalkanes

• Haloalkanes:

  • Haloalkanes are a homologous series of organic compounds that contain a halogen
  • Like alcohols they are classified by the number of alkyl groups bonded directly to the halogen bearing carbon.
  • Primary - no or 1 alkyl group
  • Secondary - 2 alkyl groups
  • Tertiary - 3 alkyl groups
• In the presence of sunlight, alkanes react with halogens. High energy UV radiation present in sunlight provides the initial energy for a reaction to take place.
• Reactivity of haloalkanes:

  • Halogen atoms are more electronegative than carbon atoms. The carbon- halogen bond is polar
  • In haloalkanes , the carbon atom has a slightly positive charge & can attack species containing a lone pair of electrons
  • Species that donate a lone pair of electrons are known as nucleophiles
• Nucleophile —> An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair to form a new covalent bond
• Nucleophiles include:

  • Hydroxide ions, :OH-
  • Water molecules, H2O:
  • Ammonia molecules, :NH3
• Hydrolysis:

  • Hydrolysis reactions involve the splitting of molecules with water to form new molecules
  • It is a substitution reaction (atom / group replaced by another atom/ group) where a halogen is replaced by a hydroxyl.
• Rate of hydrolysis:

  • The rate of hydrolysis depends on the enthalpies as lower enthalpy will give a higher rate of hydrolysis;
  • Faster down group as weaker attraction with carbon
  • More alkyl groups present weakens the C-X bond
• Hydrolysis:

  1. The nucleophile, OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
  2. This direction of attack by the OH- ion minimises repulsion between the nucleophile and the delta negative halogen atom
  3. A lone pair of electrons on the hydroxide ion attracted & donated to the delta positive carbon atom
  4. A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
  5. The carbon- halogen bond breaks by heterolytic fission
  6. The new organic product is an alcohol. A halide ion is also formed
• • Iodoalkanes react faster than bromoalkanes
  • Bromoalkanes react faster than chloroalkanes
  • Fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond
  • 1-chlorobutane reacts slowest & the C-Cl bond is the strongest
  • 1-iodobutane reacts fastest & the C-I bond I the weakest
  • Rate of hydrolysis increases as the strength of the carbon- halogen bond decreases
• • Nucleophile —> an electron donator
• Nucleophilic substitution:

  Due to the electronegativity of halogens the C-X bond is polar.As the electrons in the C-X bond are attracted to the halogen this laves the carbon atom with a positive dipole.The carbon is referred to as an electron deficient centre

  1. The nucleophile attacks the electron deficient carbon by donating a pair of electrons to form a covalent bond
  2. The formation of the bond with the electron-deficient carbon causes the electron pair in the C-X bond to transfer to the halogen, breaking the bond
  3. The heterolytic fission of the C-X bond generates a halide ion
• • Organohalogen compounds are also used in many pesticides. Rarely found in nature. Not broken down naturally in the environment
• Ozone formation:

  • Ozone is constantly being formed and destroyed in an equilibrium high in the stratosphere due to UV radiation
  • As UV radiation is absorbed to split O2, the ozone layer in the stratosphere lowers the amount of UV reaching the surface.
  • However tropospheric ozone is linked to respiratory problems
• Ozone depletion - chlorine:

  • Chlorine containing compounds undergo homolytic fission under UV light to form chlorine radicals which attack O3
  • Initiation —> Homolytic fission of the C-Cl bond
  • Propagation —> Chlorine radicals break down ozone
• Ozon depletion -chlorine:

  • Overall the reaction can be summarised as O3 + O. —>2O2.
  • The chlorine radicals are acting as a catalyst so can be omitted
  • The oxygen radical in the second step of the propagation reaction is from the splitting of the O2 by UV
  • The reaction terminates once the radicals react together
• Ozone depletion - nitrogen oxide:

  • Under high temperatures and pressures experienced in an engine, N2 and O2 will react to form nitrogen oxides
  • As nitrogen (II) oxide is a radical (NO.) so can take part in the propagation reactions to break down O3.
• CFCs are very stable due to the strength of the carbon-halogen bonds within their molecules
• CFCs remain stable until they reach the stratosphere, where they begin to break down, forming chlorine radicals that catalyse the breakdown of the ozone layer
• The stability of CFCs in the troposphere is due to the strength of their carbon-halogen bonds, leading to a long residence time
• In the stratosphere, UV radiation breaks a carbon-halogen bond in CFCs by homolytic fission to form radicals, with the C-Cl bond having the lowest bond enthalpy and being the bond that breaks
• The process of UV radiation breaking down CFCs into radicals is called photodissociation