Halogenoalkanes

• general formula

  CnH2n+1X
• functional group

  C-X
• Why are halogenoalkanes more reactive than alkanes?

  due to the polar bond of C-X
• uses

  • refrigerants
  • solvents
  • pharmaceuticals
• two reactions halogenoalkanes take part in

  nucleophilic substitution

  elimination
• nucleophile

  electron pair donor
• why halogenoalkanes take part in nucleophilic substitution reactions?

  because of the polar carbon-halogen bond
• outline nucleophilic substitution 


  the nucleophile donates a pair of e- to the delta+ c atom to form a new bond

  the c-halogen bond breaks by heterolytic fission and a halide ion is formed
• common nucleophiles

  OH-, CN-, NH3
• Reaction of halogenoalkanes with aqueous hydroxides

  hydrolysis reaction

  to form alcohols

  reagent: NaOH

  conditions: warm, aqueous

  General equation: RX +NaOH -> ROH + NaX
• Mechanism of halogenoalkanes and OH- (nucleophilic substitution)

  the OH- acts as a nucleophile

  
• reaction with ethanolic potassium cyanide

  to form nitriles

  reagent: KCN

  conditions: warm, ethanolic

  general equation: R-X + KCN -> R-CN +KX

  reacting a halogenoalkane with cyanide increases the length of the carbon chain
• mechanism of chloroethane and KCN

  to name: ethane nitrile

  
• reaction with ammonia

  halogenoalkanes react with excess ammonia to form amines

  reagent: NH3

  conditions: excess NH3, warm, ethanol solvent, sealed tube

  general equation: R-X + 2NH3 -> R-NH2 + NH4+X-
• mechanism of halogenoalkane and excess ammonia

  product: amino methane

  
• which is the strongest carbon-halogen bond?

  C-F
• How can the relative rates of hydrolysis of chloro-, bromo- and iodoalkanes be determined experimentally?

  all are warm to 60 degrees celsius with aqueous silver nitrate using ethanol as a mutual solvent

  rate of reaction can be measured by timing how long it takes for a ppt of the silver halide ion to form
• to ensure a fair test, it's important to use?

  • equal amount , in mol, of each halogenoalkane
  • halogenoalkanes with same chain length
  • use a water bath to maintain a constant temperature
• colour of halogenoalkane and silver nitrate ppt

  1-chlorobutane: white ppt

  1-bromobutane: cream ppt

  1-iodobutane: yellow ppt
• which precipitate forms fastest and why?

  silver iodide

  relative rate: iodoalkanes>bromoalkanes>chloroalkanes

  the C-I bond has a lower bond enthalpy so takes less energy to break the C-Br or C-Cl.
• why can this reaction not be used to determine the rate of hydrolysis of hydrolysis of 1-fluorobutane?

  AgF is soluble, so only a colourless solution would be seen
• elimination


  a reaction in which an atom or group of atoms is removed from a reactant
• what do halogenoalkanes form in an elimination reaction?

  Alkenes
• elimination reaction

  reagent: NaOH/KOH

  conditions: ethanolic, hot/reflux

  • the halogen atom is always removed
  • a H+ ion from an adjacent C atom is also removed
  • a double bond forms between the two C atoms
• elimination mechanism

  role of OH-: base (accepts H+ from RX)

  
• What are chlorofluorocarbons?

  CF2Cl2 for example, had uses in solvents, refrigerants. they were suitable for this because they were non-toxic, had low reactivity. However, they caused the depletion of the ozone layer
• Ozone 


  O3, forms in upper atmosphere from the reaction of oxygen atoms with oxygen molecules

  O2 + O -> O3

  it's beneficial because it absorbs UV radiation that can cause skin cancer
• What do CFCs break down to form?

  chlorine radicals, the UV radiation provides energy to break the C-Cl bond

  CF3Cl2 -> CF3C*2 + Cl* ( F* don't form as C-F is very strong)
• 2 equations to show the breakdown of ozone

  O3 + Cl* -> CL*O + O2

  O3 + CL*O -> CL* + 2O2
• chlorine radicals act as catalysts because they are regenerated in 2nd step. one chlorine radical can break down many O3 molecules
• Overall:

  2O3 -> 3O2
• Why are HFCs alternatives to CFCs?

  will not form chlorine radicals because they don't contain chlorine