ELIMINATION REACTIONS

Definition: Elimination reaction involves the removal of two substituents, from a pair of adjacent atoms in a molecule, without their being replaced by other atoms or groups. As a consequence of the removal of atoms or groups from the adjacent atoms of molecule, unsaturation is introduced. The most common multiple bonds formed are those of alkene, alkyne and their hetroatom variations such as carbonyl and cyano groups. There are three pathways that differ in the timing of when the proton is pulled off and when the Leaving group L falls off.

E1-Elimination reaction occurs when the Leaving group departs first to produce a reasonably stable cation; the proton is then lost to form the alkene.

E2-Elimination reaction occurs when there is a simultaneous loss of proton and the leaving group L.

E1cB-Elimination reaction involves proton transfer from an anion stabilized by an electronegative atom, and then the Leaving group departs by b -elimination from an Anion or Lone Pair.

Mechanism of 1,2-(b -)Elimination(alkene formation from as electron sink)

The factors that decide which elimination path occur are listed in Figure 2. The E1 and E1cB should be considered the two extremes of a spectrum of elimination mechanism in which E2 is in the center. Figure 2: The E1/E2/E1cB spectrum

Orientation (Saytzev vs. Hofmann) of Regiochemistry in Elimination

(a) Hofmann 1851, working on RNÅ Me₃ (alkyl onium compounds) stated that the alkene will predominate which has least alkyl substituents on the double bond carbons.

(b) Saytzev 1875, working on RBr (alkyl bromides) stated that alkene will predominate which has most alkyl substituents on the double bond carbons.

The different elimination paths often produce different alkene constitutional isomers as products (Regiochemistry). The E1 process favours the formation of the more substituted alkene because reversible protonation of the double bond occurs and creates an equilibrium mixture that favours the more stable product. The E2 regiochemistry is controlled by the need to minimize steric interactions in the transition state; the size of the base is important because one proton may be more accessible than another. The E1cB regiochemistry is determined by the loss of the most acidic proton.

In addition to the formation of different regioisomers, elimination reactions can produce different stereoisomers, for example, cis and trans alkenes. Since the trans isomer is usually of lower energy because of steric reasons, it usually predominates over the cis isomer in the product mixture. The same factors that determined the regiochemistry also influence stereochemistry.

E1 example (good L and cation, forms most substituted alkene):

E2 example (concerted):

E1cB example (ewg makes C-H acidic; anion needed to kick out poor L)

Hetero E1cB example: Protons on heteroatom are usually acidic due electronegativity of the heteroatom, so the first step is deprotonation, and can occur with a rather weak base. The second step is beta elimination from an anion, Eb , which is the reverse of nucleophilic addition to a polarized multiple bond. 

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