Elimination reactions remove two substituents from a molecule, often forming a carbon–carbon double bond. This lesson introduces the main elimination pathways and compares the concerted E2 mechanism with the stepwise E1 mechanism.
An elimination reaction removes two groups from a molecule. In a common case, a hydrogen atom and a leaving group are removed from neighboring carbon atoms, producing an alkene. This is called β-elimination. The two original sigma bonds are lost as a pi bond forms.
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What is an elimination reaction?
A reaction in which two substituents are removed from a molecule. In a common β-elimination, a hydrogen and a leaving group are removed from neighboring carbons to form an alkene.
What does antiperiplanar mean in the context of E2?
The bonds to the hydrogen being removed and the leaving group lie in the same plane and point in opposite directions. This geometry supports the orbital alignment needed for pi-bond formation.
How does an E2 reaction proceed?
E2 is a concerted, one-step process: the base removes a neighboring hydrogen as the leaving group departs and the double bond forms. Its rate depends on both substrate and base.
How does an E1 reaction proceed?
E1 occurs in two steps. The leaving group first departs to form a carbocation; then a base removes a neighboring hydrogen to form the alkene.
Why can E1 reactions produce rearranged products?
E1 forms a carbocation intermediate. That intermediate can rearrange before a base removes a hydrogen and forms the double bond.
Quiz questions
What product commonly forms when a hydrogen and a leaving group are removed from neighboring carbon atoms?Answer: An alkene, formed as a carbon–carbon double bond develops.
Why is E2 described as bimolecular in its rate law?Answer: The rate depends on both substrate and base concentrations.
Which arrangement is generally favored for the hydrogen and leaving group in a typical E2 reaction?Answer: Their bonds lie in one plane and point in opposite directions.
Which elimination mechanism allows a carbocation to rearrange before the alkene forms?Answer: E1, because it forms a carbocation before the hydrogen is removed.
What is the expected rate dependence for a typical E1 reaction?Answer: The rate depends on substrate concentration, not base concentration.