Bicyclic structures like norbornane don't undergo E2 eliminations, but why is this the case? I understand that it can't do E1 as it can't form a stable carbocation (the bridgehead carbons that is) but I don't see the issue for E2. Can you explain?

  • 1
    $\begingroup$ Elimination reactions (including E2) are not forbidden on bicyclic molecules, they are just forbidden at the bridgehead (due to geometry issues that prevent effective orbital interactions). $\endgroup$
    – Ben Norris
    Jan 19 '15 at 18:36
  • $\begingroup$ I am aware of this - apologies that I didn't make it clear. I just don't understand why these carbons can't undergo E2 eliminations. $\endgroup$
    – RobChem
    Jan 19 '15 at 18:38
  • 4
    $\begingroup$ for E2 mechanism to work, the pair of leaving groups (commonly hydrogen and halogen) must be in antiperiplanar conformation (i.e. when loonking along axis of the carbon-carbon bond that would be double after elimination, they must be at 12 and 6 o'clock). In small bicyclic systems assuming such conformation is problematic. Also, formation of 2-nornbornil cation is well documented and E1 elimination for 2-nornbornyl species is known. Also, it is prohibited to form double forms at bridgehead, as the resulting system has too much angular strain. $\endgroup$
    – permeakra
    Jan 19 '15 at 18:40
  • $\begingroup$ Firstly, you have to specific the position of elimination. Like Ben Norris said, double bond can not be formed on bridge head in strained bicyclic structure simply because the geometry does not allow a planer double bond. For E1, cation can be generated but double bond can not be formed, it end up a SN1 reaction. For the concerted E2, it simply can never happen because the product can not be formed. $\endgroup$
    – Coconut
    Jan 19 '15 at 22:14

Elimination via a E2 mechanism cannot take place at the bridgehead carbon of a bicyclic compound due to the lack of the required trans geometry between the leaving group at the bridgehead carbon and any of the six β-hydrogens.

On the other hand, as you have correctly observed, a E1 mechanism has to be ruled out as well due to the strain at the bridgehead carbocation. In fact bicyclic structures prevent the tertiary bridgehead carbon becoming planar, which implies that is very high in energy because the nonplanar structure forces the cation to be an empty "sp3 orbital" instead of an empty "p orbital". This observation was made for the first time by Bredt, therefore is generally known as "Bredt's rule".


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