6.1.4: HALOGENATION & FRIEDEL-CRAFTS

6.1.4: HALOGENATION & FRIEDEL-CRAFTS

1. Halogenation

   • alkenes have pi bond and so this region of high e- density means they will undergo addition reactions to become saturated (double bond breaks)
     • alkenes shaken with Br water causes coloured Br to become colourless
       • no reaction when benzene shaken with Br water
         • benzene must have lower e- density between C atoms than alkenes
   • when non polar molecules (eg. Br) approach benzene ring, dipole cannot be included to start reaction due to there not being enough e- density
     • using halogen carrier means a stronger electrophile can be generated

2. Friedel- Crafts

   • developed technique for aromatic electrophilic substitutions
     • alkylation
       • H replaced for alkyl chain by breaking C-H bond and forming C-C bond
       • strong acid used as catalyst (accepts e- pair)

3. Acyl Chlorides

   • R-COCl functional group
     • very reactive
   • halogen carrier substitutes one hydrogen
     • carbonyl group withdraws e- from ring
       • less reactive ketone made

4. Haloalkanes

   • mixed with halogen carrier (eg. iron (III) chloride)
     • acts as catalyst; so regenerated
     • carbocation formed which undergoes electrophilic substitution with the benzene ring
   • mixture of products likely to be made
     • separated by fractional distillation/ chromatography
     • caused by each substitution making the delocalised pi e- more nucleophilic and so more susceptible to electrophilic attack
       • increase in reactivity due to alkyl chain donating e- to ring