E1 and E2 Reactions
Alkyl halides are very good leaving groups. In substitution reactions, a leaving group is replaced by a nucleophile. Another reaction involving a leaving group is an elimination reaction. An elimination reaction (or 1,2-elimination) is a chemical reaction in which the removal of two atoms (usually a hydrogen and a leaving group) forms a pi bond, a bond formed when two orbitals overlap side by side on the same plane.Nucleophilic reagents, chemicals added to a reactant, yield substitution reactions, reactions where one functional group is replaced by another. Elimination reactions predominate when strong bases are used as the reagent. Elimination reactions are also known as 1,2-eliminations or beta eliminations. The electrophilic carbon with the halogen is known as the alpha carbon. The beta carbon is the carbon adjacent to the alpha carbon. In an elimination reaction, the bonds between the alpha carbon and leaving group plus the bond between the beta carbon and hydrogen are broken and replaced with a pi bond between the alpha carbon and the beta carbon. Most often, there is a single bond between the alpha and the beta carbon in the original molecule, which leads to the formation of a double bond when the new pi bond forms.
Regiochemistry of E1 and E2 Reactions
- Substrate, which is a reactant molecule with an electrophilic carbon attached to a halogen
- -hydrogen on the substrate
- Leaving group, which is the halogen attached to the electrophilic carbon
- Base, which is an electron-rich reagent that is usually an ionic compound that is more basic than nucleophilic
Examples of bases include alkoxide bases (hydroxide, methoxide, ethoxide, and so on) and nitrogen bases (sodium amide, ammonia, and so on).
Elimination Reaction Outcomes
There are two mechanistic pathways to form an elimination product—the E1 and E2 mechanisms. E2 mechanisms are concerted (all in one step) and bimolecular (same as SN2). The rate of the reaction is determined by two substances—the base and the substrate. E1 mechanisms are stepwise (more than one step) and unimolecular (same as SN1), where the substrate determines the rate of the reaction. E1 reaction rates are not based on the concentration of the base, just like SN1 reaction rates are not based on the concentration of the nucleophile.
For an E2 reaction, the base removes the beta hydrogen from the alkyl halide, and the leaving group leaves. The two steps happen at the same time, which is called a concerted mechanism. In an E1 reaction, the mechanism steps are:
1. Leaving group leaves, forming a carbocation intermediate2. Base removes a beta hydrogen, forming an alkene
Comparing E1 and E2 Mechanisms
With all bases (NaOH, NaOMe, NaOEt, KOH, and so on) except for sterically hindered bases (big and bulky bases), such as potassium tert-butoxide (KOtBu), the elimination favors the more substituted product. It is called the Zaitsev product and comes from Zaitsev's rule, a rule that states that the alkene formed is more highly substituted because it is more stable. The rule can be used to predict the favored alkene product of an elimination reaction.If a sterically hindered base, such as potassium tert-butoxide (KOtBu), is used, the less substituted product will form. It is called the Hofmann product and comes from Hofmann's rule, a rule that states that steric effects have the most influence on outcomes, specifically the loss of a beta hydrogen from the least substituted position. Since sterically hindered bases are too big and bulky to easily remove a hydrogen atom from a more substituted beta carbon, the hydrogen atom is selectively removed from a less statically hindered beta carbon and a Hofmann product is formed.
Comparing Hofmann and Zaitsev Products
Comparing E1 and E2 Reactions
|Conditions||E1 Reaction||E2 Reaction|
|Substrate||3° and 2°||1°, 2°, and 3°|
|Base||Very weak base or no base (solvent only)||Strong base|
|Temperature||Heat favors elimination over substitution.||Heat favors elimination over substitution.|
|Regiochemistry||Zaitsev with sterically hindered base (KOtBu)
Hofmann with all other bases
|Zaitsev with sterically hindered base (KOtBu)
Hofmann with all other bases
|Stereochemistry||No effect||No effect|
|Solvent||Polar protic||Polar protic|