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Elimination Reactions

Written by tutor Mark M.

An organic elimination reaction is one where a starting product breaks into two new products that contain all the atoms of the original. Two categories of elimination reaction are identified: the E1 and E2.

E2 elimination reactions involve the combined steps of one carbon atom losing a hydrogen atom to a base, and a second carbon atom releasing a leaving group and pi bonding to the first carbon atom, all of this occurring almost simultaneously. Structural and reactivity factors determine the reaction mechanism. E2 reactions occur with periplanar geometry, such that all four reacting atoms lie in the same plane. In the case of cyclohexane rings, the hydrogen and the leaving group must be trans diaxial for the E2 reaction to occur. We also require a good nucleophile (strong base) for E2 to occur. Finally, an aprotic polar solvent favors the E2 if either reactant is charged.

E1 elimination reactions proceed stepwise. The first step, which is the rate-determining step for the reaction, involves a leaving group producing a cation. Then a second carbon atom is deprotonated and the broken bond forms a pi bond with the carbon cation produced in the first step. E1 initiates in the same manner as the SN1 reaction, followed by deprotonation from the adjacent carbon rather than by the substitution. Since E1 and SN1 have identical rate determining steps, they generally occur simultaneously and have the same properties. Mixes of substitution and elimination products are formed when we have a poor nucleophile, or a weak base, a polar protic solvent ( if the reactant is not charged) and 2- or 3-degree substitution. The SN1 usually predominates this mix under the conditions mentioned.

An example of elimination reactions is the preparation of alkenes. Alcohols can eliminate water to form alkenes. This is accomplished by adding a strong acid to the alcohol solution. Alternately, dehydrohalogenation is accomplished by treatment with a strong base. Dehydrations occur in vivo when the OH is located two carbons away from a carbonyl group.

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