Orientation of Double Bond
- No matter what the mechanism, a double bond does not go to a bridgehead carbon, unless the ring sizes are large enough.
- No matter what the mechanism, if there is a double bond (C=C or C=O) or an aromatic ring already in the molecule, which can be in conjugation with the new double bond, the conjugated product usually predominates, sometimes even when the stereochemistry is unfavourable.
- In the E1 mechanism, the leaving group is gone before the choice is made as to which direction the new double bond takes: Zaitsev’s rule operates.
- For the anti-E2 mechanism a trans-β proton is necessary. Where trans-β hydrogens are available on two or three carbons, two types of behaviour are found, depending on the substrate structure and the nature of the leaving group.
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