Substitution and Elimination
⇒ Mechanism Concerted reaction (occurs in a single step). This single step is also the Rate Determining Step. The reaction occurs in two steps. The Rate Determining Step is the formation of the carbocation. Major Obstacle Steric hindrance blocking nucleophilic attack No major obstacle. Steric hindrance is not an issue, as the base is not incorporated into the structure. Reactions only occur when a stable carbocation can be formed. Ionisation (formation of carbocation) is disfavoured which results in a high activation energy barrier. Stereochemist ry Results in inversion. Nucleophile attacks at 180 degrees to the leaving group. Results in either cis (Z) or trans (E) conformation. Can be determined through the use of Newman projections. Results in a racemic mixture. Nucleophile can attack from both sides of the planar carbocation. Both cis (Z) and trans (E) isomers will be produced due to no requirement for antiperiplanar beta hydrogen (because LG is no longer present) Regiochemistr y E2: The major product is most often the most substituted alkene (most stable). However, with a bulky base, the major product may be a less substituted alkene. E1: Zaitsev’s Rule states that the major product is the most substituted (most stable) alkene, whose transition state and intermediate will also be most stable. (Unless a large/hindered base is used) Nucleophile & Base Quality Requires a very good nucleophile to stimulate the release of the leaving group and overcome the Ea barrier. Nucleophile involved in RDS. Requires a very strong base to stimulate the release of the leaving group and overcome the Ea barrier. Base involved in RDS. Can react with a poor nucleophile. There is no Ea barrier to overcome, or leaving group to release. Nucleophile does not participate in RDS. Can react with a poor base. There is no Ea barrier to overcome, or leaving group to release. Nucleophile does not participate in RDS. Leaving Group Quality Requires a good leaving group. Generally the conjugate base of a strong acid. Leaving group is involved in the RDS. Requires an excellent leaving group. Leaving group is involved in the RDS.