When basicity and/or steric effects outweigh nucleophilicity, and a good leaving group (LG) is present, we observe elimination chemistry leading to olefins (alkenes) instead of substitution. The E2 reaction (elimination bimolecular) takes place via a concerted mechanism, and involves an antiperiplanar alignment of orbitals – proton and LG lined up across from one another however in the same plane.
The rate of the reaction involving a base and an alkyl halide (RX) can be expressed in the following manner,
Rate = k [RX] [Base]
Here, k is the rate constant for the bimolecular reaction. As can be derived from the rate law, kinetics are dependent upon both the concentration of RX and Base. Increasing the concentration of either component will increase the rate of reaction. An example reaction is,
Observed mostly is Zaitsev elimination (55%) involving abstraction of an adjacent 2° proton (leading to formation of the most substituted olefin). Note the possible formation of both trans and cis stereoisomers. This is contrasted with Hofmann elimination (25%) involving abstraction of the adjacent 1° proton (leading to formation of the least substituted olefin).
So what types of factors increase the likelihood of an E2 reaction? (1) 1° LG and an anion that is more basic than nucleophilic, i.e. strong base; (2) 2° LG and a dipolar protic or polar protic solvent with a strong base; (3) and 3° and a strong base. Additionally, (4) heat augments the proportion of elimination observed in a product mixture. Increased branching at alpha and beta carbons also increases the amount of E2 over SN2.
The requirement for antiperiplanar alignment of orbitals cannot be overstated. In the example of trans-4-tert-butylbromocyclohexane, elimination be effected from the more stable conformer having the bulky tert-butyl group in the equatorial position because the proper alignment of orbitals cannot be achieved. Rather, elimination takes place from the least stable conformer where antiperiplanar alignment is available.
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