Wize University Organic Chemistry Textbook > Nucleophilic Substitutions and Elimination Reactions
Selectivity in Elimination Reactions
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Selectivity in Elimination Reactions: The E2 Reaction
Elimination reactions can display both regioselectivity (preferential elimination at one site over another) and stereoselectivity (preferential formation of one stereoisomer over another).
The E2 Reaction
E2 reactions require an anti-periplanar geometry of the proton and leaving group. For example:
When a non-bulky base is used, the Zaitsev product is formed. The Zaitsev product has the most substituted alkene. The trans product is also favoured, due to steric hinderence.
Wize Concept
If a bulky base is used (the bulky base you need to know is KOtBu!), the Hofmann product will be formed. This is the least substituted alkene!
Watch Out!
Remember, the hydrogen atom that is being removed and the leaving group must be anti-periplanar! So, if there is only one hydrogen atom that meets that requirement - that's the one that will be removed!
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Selectivity in Elimination Reactions: E1 Reaction
Elimination reactions can display both regioselectivity (preferential elimination at one site over another) and stereoselectivity (preferential formation of one stereoisomer over another).
Wize Tip
Remember, this reaction proceeds through a carbocation intermediate so we don't need to worry about anti-periplanar geometry!
When a non-bulky base is used, the Zaitsev product is formed. The Zaitsev product has the most substituted alkene. The trans product is also favoured, due to steric hinderence.
Wize Concept
If a bulky base is used (the neutral bulky base you need to know is HOtBu!), the Hofmann product will be formed. This is the least substituted alkene!
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Example: Selectivity in Elimination Reactions
Both alkyl halides shown below eliminate to form a cyclohexene derivative; however, one species eliminates more rapidly than the other. Using chair conformations and reaction mechanisms, rationalize this observation.
