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4.2.2 Haloalkanes

Definitions

Term Definition
Nucleophile An atom or group of atoms which is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond
Nucleophilic substitution A reaction in which a nucleophile is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond
Hydrolysis A reaction with water that breaks a chemical compound into two compounds, the H and OH in a water molecule becomes incorporated into the two compounds

Haloalkane reactions

Reactivity of haloalkanes

  • Reactivity: alkenes > haloalkanes > alkanes
  • Halogen atoms are more electronegative than carbon atoms so the carbon-halogen bond is polar
  • \(\delta+\) on carbon can attract nucleophiles (contain a lone pair of electrons)
  • The nucleophile replaces the halogen atom
  • A new compound with a different functional group is formed

Hydrolysis mechanism

  • Nucleophilic substitution
  • \(OH^-\) normally from \(NaOH\)
  • \(OH^-\) (nucleophile) approaches the carbon atom attached to the halogen from the opposite side of halogen to minimise repulsion
  • A lone pair on OH attracted & donated to \(\delta+\) carbon atom
  • New bond formed between oxygen atom of OH and the carbon atom
  • Carbon-halogen bond breaks by heterolytic fission
  • Alcohol + halide ion formed
  • Exported image

Trend in reaction rates of hydrolysis of primary haloalkanes

  • \(C-F\) has the greatest bond enthalpy (strongest), \(C-I\) has the lowest bond enthalpy (weakest)
    • Going down the table = larger halogen atom = longer bond = bond becomes weaker
  • Rate: iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes
    • Rate increases as the bond enthalpy of carbon-halogen bond decreases
    • \(C-I\) bond is the weakest (lowest bond enthalpy), \(C-F\) bond is the strongest (highest bond enthalpy)
    • Less energy is needed to break the carbon-halogen bond to start the reaction so the activation energy is lower

Measuring rate of hydrolysis

  • Set up 3 test tubes of 1 \(cm^3\) ethanol and couple drops of 1-chlorobutane / bromobutane / iodobutane
    • Slow down the reaction so we can measure the reaction time easier
  • Put the test tubes + a test tube with silver nitrate in water bath at 60°C
  • Allow them to reach constant temperature (60°C)
  • Add 1 \(cm^3\) of silver nitrate to each test tube quickly + start stop watch
  • Observe time taken for precipitate to form
    • Chlorine = white, bromine = cream, iodine = yellow
  • Speed: iodobutane > bromobutane > chlorobutane

Organohalogen compounds and the ozone layer

CFCs

  • Shorthand for chlorofluorocarbons
    • Compounds containing carbon with chlorine and fluorine atoms attached

Uses of CFCs

  • CFCs are non-flammable and not very toxic so they have a lot of uses
    • Refrigerants
    • Propellants for aerosols
    • Generating foamed plastics
    • Solvents for dry cleaning and for general degreasing purposes

Problems associated with CFCs

  • Global warming
  • Breakdown of ozone layers in the atmosphere

Ozone layer

  • Ozone continually formed and broken down by the action of UV radiation
  • Initially very high energy UV breaks oxygen molecules into oxygen radicals: \(O_2 \rightarrow 2O\)
  • A steady state then set up where rate of ozone formation is the same as the rate of ozone being broken down: \(O_2 + O \rightleftharpoons O_3\)
  • Equilibrium disturbed by human activities e.g. production and use of CFCs

How CFCs break down ozone

  • CFCs remain stable until they reach the stratosphere
  • In the stratosphere UV breaks carbon-halogen bond by homolytic fission to form radicals (initiates the breakdown of ozone)
    • Photodissociation (Initiation): e.g. \(CF_2Cl_2 \rightarrow CF_2Cl\bullet + Cl\bullet\)
  • Chlorine radical formed is a very reactive intermediate and can react with an ozone molecule
    • Propagation step 1: \(Cl\bullet + O_3 \rightarrow ClO\bullet + O_2\)
    • Propagation step 2: \(ClO\bullet + O \rightarrow Cl\bullet + O_2\)
    • (Overall: \(O_3 + O \rightarrow 2O_2\))
    • There is a significant amount of \(O_3\) and free oxygen atoms in the upper atmosphere for reaction
  • Chlorine radical can go on in chain reaction to break down other ozone molecules

How nitrogen oxide break down ozone

  • Reaction with NO
    • Initiation: \(NO \rightarrow N\bullet + O\bullet\)
    • Propagation step 1: \(N\bullet + O_3 \rightarrow \bullet NO + O_2\)
    • Propagation step 2: \(\bullet NO + O \rightarrow N\bullet + O_2\)
    • Overall: \(O_3 + O \rightarrow 2O_2\)
  • Reaction with \(NO_2\)
    • Initiation: \(NO_2 \rightarrow NO\bullet + O\bullet\)
    • Propagation step 1: \(NO\bullet + O_3 \rightarrow NO_2\bullet + O_2\)
    • Propagation step 2: \(NO_2\bullet + O \rightarrow NO\bullet + O_2\)
    • Overall: \(O_3 + O \rightarrow 2O_2\)

Alternatives for CFCs

  • Replace the C-Cl bond with stronger C-F bond
    • Hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) can be used
    • Still volatile, non-toxic and non-flammable
    • Still damage the ozone layer
  • Replace the C-Cl bond with a C-H bond
    • Use hydrocarbons
    • The C-H bond is much weaker and the molecules don't persist until they reach the upper atmosphere
    • They are very flammable