13.3 & 13.4 Reactions of Alkenes and Electrophillic Addition

13.3 & 13.4 Reactions of Alkenes and Electrophillic Addition

1. Addition reactions of the alkenes

   • Hydrogen Halides
     • Alkenes react with gaseous hydrogen halides at room temp to form haloalkanes
       • If alkene is gaseous, reaction take place when two gases are mixed
       • If alkene is liquid, the the hydrogen halide is bubbled through it
       • Alkenes can also react with concentrated HCl or HBr (which are solution of the hydrogen halides in water)
   • Each of these reactions involves the ADDITION of a small molecule across the double bond, causing the pi bond to break, and new bonds to form
   • Hydrogenation of Alkenes
     • Nickel Catalyst
     • 423K
     • Alkene + Hydrogen = Alkane
     • Hydrogenation of Propene >>
   • Hydration reactions of Alkenes
     • Alkene + Water (steam) --> Alcohol
       • Requires the presence of a phosphoric acid catalyst (H3PO4)
       • Can form a Primary or secondary alcohol
   • Halogenation
     • Rapid addition reaction with the halogens: Cl or Br at room temperature
       • Haloalkane

2. Electrophilic addition in Alkenes

   • Alkenes usually take part in addition reactions to form saturated compounds = ELECTROPHILLIC ADDITION
     • Double bond in an alkene represents region of high electron density due ton presence of pi-electrons
       • High electron density attracts electrophiles
         • Electrophile = An atom that is attracted to an electron-rich centre and accepts an electron pair. Usually a positive ion or molecule containing an atom with a partial positive charge.
     • ^^ The mechanism

3. Markownikoff's Rule

   • You can have a primary or secondary carbocation. This is similar to a primary or secondary alcohol.
     • Carbocation Stability
       • The stability increases as you go from primary carbocation, through to tertiary carbocation.

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