Electrophillic addition

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Electrophillic addition

The reaction and mechanism for but-2-ene and hydrogen bromide (polar):

  • Bromine is more electronegavtive than hydrogen (hydrogen bromide is polar) so it contains a dipole... H+, Br-
  • The electron pair in the pi bond is attracted to the partially positive hydrogen. 
  • This causes the double bond to break.
  • A bond forms between the hydrogen and a carbon atom (his was part of the double bond)
  • H-Br bond breaks by heterolytic fission, the electron pair goes to the Br.
  • A bromide ion forms (Br-)
  • A carbocation forms (positively charged carbon atom.)
  • Finally the Br- ion reacts with the carbocation

Image result for addition reaction of but-2-ene and hydrogen bromide (http://alevelchem.com/img/electrophilic_addition_hydrogen_bromide.gif)

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Electrophillic addition

The reaction and mechanism for but-2-ene and bromide (non-polar):

  • the pi electrons interact with the electrons in the Br-Br bond
  • this causes polarisation of the bond (induced dipole) Br+-Br-
  • the electron pair in the pi bond are attracted to the Br+ 
  • the doubke bond is broken
  • the Br-Br bond breaks by heterolytic fission and the electron pair goes to Br- end
  • a bromide ion and a carbocation are formed
  • the Br- ion reacts with the carbocation

Image result for addition reaction of but-2-ene and bromide

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Markownikoff's rule

  • hydrogen attaches itself to the carbon atom with the greatest/smallest number of hydrogen atoms attached 
  • a primary carbocation is when the positive charge is on a cabron at the end of the chain
  • a secondary carbocation is when the positive charge is on a carbon with two carbon chains attached.
  • the major product is the one where the carbocation has the most carbon atoms on it, this yield is higher
  • the minor product is the one where the carbocation has  the least carbon atoms on it, this yield is lower
  • inductive effect= is the ability of neighbouring atom to donate electorn density to stabilise the + charge 
  • the more alkyl groups attached (removal of H e.g. a C is there instead) to the positively-charged carbon atom, the more the charge is spread out
  • this makes the ion more stable 
  • stability increases: primary, secondary and tertiary ( as there are less hydrogens and more alkyls) 
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Polymerisation in alkenes

Polymers are extremely large molecules fomed from many repeat units of smaller molecules  (monomers). unsaturated alkene molecules undergo addition polymerisation= produces long saturated chains (no double bonds)

    • a repeat unit is the specific arrangement of atoms that repeats over and over
    • the repeat unit is always written is square brackets
    • addition polymers have large molecular masses
    • synthetic polymers are named after the monomer that reacts alongside the prefix 'poly'
  • polyethene is the most common e.g. supermarket bags, shampoo bottles
  • polychlorothene/poly vinyl chloride (PVC) can be prepared to make ampolymer that is flexible or rigid.
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