Alkenes- electrophillic addition reactions

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Alkenes- electrophillic addition reactions

E.g the addition of bromine to ethene

  • There are four electrons in the double bond of ethene- these give the region between the two carbon atoms a high negative charge density.
  • Electrophiles are attracted to this negatively charged regionin an alkene and accept a pair of electrons from thedouble bond at the start of the reaction.
  • When a bromine molecule approaches an alkene it becomes polarised as an induced dipole forms.
  • The electrons in the bromine moleculeare repelled back along the molecule.
  • The electron density is unequally distributed.
  • The bromine atom nearest to the alkenebecomes slightly positively charged.
  • It now acts as an electrophile. A pair of electrons from the alkene moves towards the slightly charged bromine atom and a C-Br bond is formed.
  • The carbon species is now positively charged;it is a carbocation.
  • The other bromine, now negatively charged, moves in rapidly to make another bond.
Room temperature and pressure
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