Reaction mechanisms

  1. Using diagrams show the two steps in the electrophilic addition of bromine to ethene using the cyclic ion intermediate mechanism
    • Image Upload 2As the bromine molecule approaches double bond in ethene it becomes polarised.  The δ+ Br atom of the bromine molecule attacks the ethene and a cyclic ion intermediate is formed, along with a bromide (Br-) ion.  The bond in the bromine molecule breaks  - it has undergone heterolytic fission.
    • Image Upload 4  The Br- ion attacks the cyclic intermediate ion from the die opposite to where the Br atom is attached.  This is because the bromine atom in the intermediate is large and so prevents access to that side of the cyclic ion intermediate.
  2. In step one of the reaction between ethene and bromine, does the bromine molecule act as a nucleophile or an electrophile?
    An electrophile (as the positive Br atom is seeking out the negatively charged double bond in ethene)
  3. In step two of the reaction between ethene and bromine, does the bromine ion act as a nucleophile or an electrophile?
    A nucleophile (as the Br- ion seeks out the positive centre of the cyclic ion intermediate)
  4. Using diagrams show two steps in the reaction between hydrogen bromide and propene using the carbocation mechanism
    • Image Upload 6The Hδ+ atom attacks the double bond in propene and bonds to one of the carbon atoms of the double bond forming an intermediate known as a carbocation.  At the same time the bond in the H-Br molecule breaks heterolytically forming a Br- ion as well.
    • Image Upload 8  The Br- ion attacks the carbocation intermediate (it can attack from either side of the carbocation meaning there is two possible products).
  5. Define an SN1 mechanism
    Substitution nucleophilic 1st order
  6. Define an SN2 reaction
    Substitution nucleophilic 2nd order
  7. Which reaction mechanism involves two steps?  SN1 or SN2?
  8. Which reaction mechanism includes a transition state? SN1 or SN2?
  9. Which step in an SN1 mechanism is the rate determining step?
    The first step (the step involving the halogenoalkane as the reaction is first order with respect to the halogenoalkane)
Card Set
Reaction mechanisms
Advanced Higher Chemistry - Unit Three - Reaction Mechanisms