E2
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Definition:

  1. E2 stands for Elimination Bimolecular.
  2. This means that the rate of these reactions depends not only on the substrate (usually an alkyl halide) concentration but also on the concentration of a base that promotes the elimination.
  3. The rate of the disappearance of substrate = d[ substrate ]/dt = -k[ substrate ][ base ], negative slope asymptotic to both axis.

Mechanism:

  1. The E2 mechanism starts out looking like the SN2 mechanism in that a nucleophile (the base) approaches an electron deficient carbon.
  2. The nucleophile is too basic or the electrophilic carbon is too sterically hindered for the base to approach it.
  3. The next best thing is for the base to deprotonate a carbon vicinal to the most electrophilic carbon forming a carbanion, the carbanion in turn releases its electrons to the electron-deficient carbon (an intramolecular SN2 reaction!), which in turn releases its electrons to the leaving group, forming an alkene.
  4. The vicinal hydrogen must be anti to the leaving group so that the reaction resembles an intramolecular SN2 reaction in which the nucleophile is the vicinal carbanion that is being formed by deprotonation with base.
  5. The steps of the E2 mechanism are thought to occur in a concerted fashion or otherwise the following control would not be possible.

Factors:

  1. The E2 reaction is favored by strong bases, steric congestion, and heat.
  2. The E2 reaction can occur with primary, secondary, and tertiary alkyl halides but not alcohols, at least with base.

Regiochemistry:

  1. With small bases, the most sterically hindered vicinal hydrogen is removed because:
    1. that carbon hydrogen bond is the weakest,
    2. the alkene product formed is the most substituted and most stable,
      1. Inductive effect
      2. Hyper-conjugation
      3. Least steric hindrance
  2. Saytzeff's rule is to remove a hydrogen from the vicinal carbon that has the least hydrogens, "the poor get poorer".
  3. With large bases, the least sterically hindered vicinal hydrogen is removed.
  4. Hofmann's rule is to remove a hydrogen from the vicinal carbon that has the most hydrogens, "the rich get poorer".
  5. The E2 reaction is also observed in the Hofmann fashion for ammonium groups even with small bases.
  6. You should always use a three-dimensional model to check what product is possible in the E2 reaction!

Stereochemistry:

  1. The alkene diastereomer (E or Z) that comes from the correct anti elimination is the major product. 

E2 Reactions of Alkyl Halides

  Strong Base
(-OH, -OR, acetylides)
Big/Strong Base
(K+-OC(CH3)3, LDA, RLi, RMgX)
methyl SN2 SN2
1 SN2 E2
1 hindered E2 E2
2 E2, Saytzeff if possible E2, Hofmann if possible
3 E2, Saytzeff if possible E2, Hoffmann if possible

 

 

 

 

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