Unimolecular Elimination Reactions (E1Reaction)
The elimination reaction in which the rate of reaction is dependent on the concentration of the substrate only, i.e. kinetically first order. Since, the reaction follows first order kinetics so, takes place in two steps. In the first slowest step, the ionization of substrate occurs followed by fast removal of proton from β-carbon by reagent in the second step.
When more than one alkene formed, that alkene will predominate which has larger number of alkyl groups on the double bonded carbons as per Saytzeff's rule.
Dehydration of alcohols follows E1 mechanism and takes place in acidic medium at high temperature. Alcohol is first protonated and produce carbocation by lossing water followed by loss of proton to form stable alkene.
Conditions for Unimolecular Elimination Reactions
- Branching at α- and β- carbons of the substrate for stability of the olefin.
- Strong polar solvent for ionization.
- Low concentration of base.
E1Reaction in Bicyclic Compounds
Eliminations in bridged cyclic compounds follow the Bredt's rule. A double bond never goes to the bridgehead carbon in rings of smaller size (usually less than eight). For E1 elimination, usually the formation of E-alkene is favoured over the Z-alkene.
Stereochemistry of Unimolecular Elimination Reactions
In general, the unimolecular elimination reactions are stereoselective, as they favor the formation of the E or trans alkene over the Z or cis isomer. However, they are not stereospecific like bimolecular elimination reactions and do not factor in the planarity of the hydrogen and leaving group.
Also read E2 Reaction | E1cB Reaction | Ei Reaction
