MCQ Detailed View

The E2 reaction (elimination bimolecular) is one of the fundamental types of elimination reactions in organic chemistry. In this mechanism, a base abstracts a proton from the β-carbon while the leaving group departs from the α-carbon simultaneously in a single concerted step. Because bond-breaking and bond-forming occur at the same time, no carbocation intermediate is formed.

The rate law of the E2 reaction is:
Rate = k [substrate] [base]

This clearly shows that the reaction follows second-order kinetics, since the rate depends on the concentration of both the alkyl halide (substrate) and the base. Increasing the concentration of either species increases the reaction rate.

Some important features of E2 reactions are:

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  Concerted mechanism: Both proton abstraction and leaving group departure occur in one step.

  
  


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  Anti-periplanar geometry: For maximum overlap and stability, the β-hydrogen and leaving group must be anti-periplanar (in opposite planes).

  
  


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  Strong base requirement: E2 reactions usually require a strong base such as OH⁻, OR⁻, or tert-butoxide.

  
  


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  Substrate preference: More substituted alkyl halides generally undergo E2 reactions faster because they form more stable alkenes, following Zaitsev’s rule.

  
  

Comparison with E1:

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  In E1 reactions, the rate depends only on the substrate concentration (first-order kinetics) since carbocation formation is the rate-determining step.

  
  


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  In E2 reactions, both substrate and base are involved in the rate-determining step, giving second-order kinetics.

  
  

Therefore, E2 elimination is classified as a second-order reaction, which differentiates it from the unimolecular E1 pathway.