Elimination Reactions

• The degree of substitution, determined by the number of R groups around the carbon with the leaving group, is the most important factor in alkyl halide's reactivity.
• Because elimination reactions create pi bonds, alkyl halides only react with sp3 hybridized carbons.
• If a leaving group is attached to a sp2 or sp hybridized carbon, that group will not participate in an elimination reaction.
• Alkyl halides are weakly polar, with that being the key to its reactivity.
• Elimination reactions involve the removal of a hydrogen atom by a base, resulting in the collapse of the molecular structure and the departure of the leaving group. This process leads to the formation of a double bond, which stabilizes the structure.
• Beta- Eliminations result in the loss of elements on adjacent carbon atoms.
• Dehydrohalogentation is the loss of hydrogen and a halogen leaving group.
• The more substituted alkenes are more stable. The greater the number of R groups attached to the alkene, the greater the stability, making them a more major product.
• There are no rotations about the pi bonds due to the overlap of p orbitals which lock the structure.
• Because alkenes possess double bonds, they can exhibit stereochemistry. In this case, elimination reactions prefer alkenes where the bulky groups are trans to each other.
• E2 reactions are concerted bimolecular elimination reactions, with the rate dependent on both the concentration of the base and the reactant. The reaction takes place in one simultaneous step.
• Unlike in Sn2 reaction the reaction rate increases with the amount of r groups around the carbon.
• The rate of E2 reactions is increased by polar aprotic solvents.
• Zaitsev Rule: The major product in beta-elimination has a more substituted double bond.
• E2 reactions are stereoselective, often forming one stereoisomer over others. This favors the anti-periplanar stereoisomer; all 4 atoms involved must be in the same plane.
• E1 reactions are non-concerted unimolecular elimination reactions in which the leaving group determines the rate at which a carbonation is formed.
• Unlike an E2 reaction, an E1 reaction will not occur at primary carbons, as the product of such a reaction is highly unstable.
• E1 reactions do not have anti-periplanar restrictions and follow Zaitsev's rule, favoring the more substituted alkyne.
• Like SN1 reactions, E1 reactions are favored in polar protic solvents.
• An SN2 and E2 reaction occurs when a primary alkyl halide reacts with a strong, non-bulky nucleophile.
• A primary halide reacts via the E2 mechanism with a strong bulky base such as DBU and DBN.