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8.10: The E1 Reaction

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  • Scope and Mechanism

    Many secondary and tertiary halides undergo \(\text{E}1\) elimination in competition with the \(\text{S}_\text{N}1\) reaction in neutral or acidic solutions. For example, when tert-butyl chloride solvolyzes in \(80\%\) aqueous ethanol at \(25^\text{o}\), it gives \(83\%\) tert-butyl alcohol by substitution and \(17\%\) 2-methylpropene by elimination:

    The ratio of substitution and elimination remains constant throughout the reaction, which means that each process has the kinetic order with respect to the concentration of tert-butyl halide. The \(\text{S}_\text{N}1\) and \(\text{E}1\) reactions have a common rate-determining step, namely, slow ionization of the halide. The solvent then has the choice of attacking the intermediate carbocation at the positive carbon to effect substitution, or at a \(\beta\) hydrogen to effect elimination:

    Factors influencing the \(\text{E}1\) reactions are expected to be similar to those for the \(\text{S}_\text{N}1\) reactions. An ionizing solvent is necessary, and for easy reaction the \(RX\) compound must have a good leaving group and form a relatively stable \(R^\oplus\) cation. Therefore the \(\text{E}1\) orders of reaction rates are \(X = I\) \(>\) \(Br\) \(>\) \(Cl\) \(>\) \(F\) and tertiary \(R\) \(>\) secondary \(R\) \(>\) primary \(R\).

    With halides such as 2-chloro-2-methylbutane, which can give different alkenes depending on the direction of elimination, the \(\text{E}1\) reaction is like the \(\text{E}2\) reaction in tending to favor the most stable or highly substituted alkene:

    Rearrangement of Carbon Cations

    Another feature of \(\text{E}1\) reactions (and also of \(\text{S}_\text{N}1\) reactions) is the tendency of the initially formed carbocation to rearrange, especially if a more stable carbocation is formed thereby. For example, the very slow \(\text{S}_\text{N}1\) solvolysis of neopentyl iodide in methanoic acid leads predominantly to 2-methyl-2-butene:

    In this reaction, ionization results in migration of a methyl group with its bonding pair of electrons from the \(\beta\) to the \(\alpha\) carbon, thereby transforming an unstable primary carbocation to a relatively stable tertiary carbocation. Elimination of a proton completes the reaction.

    Rearrangements involving shifts of hydrogen (as \(H:^\ominus\)) occur with comparable ease if a more stable carbocation can be formed thereby:

    Rearrangements of carbocations are among the fastest organic reactions known and must be reckoned with as a possibility whenever carbocation intermediates are involved.

    Acid-Catalyzed Elimination Reactions

    Alcohols and ethers rarely undergo substitution or elimination unless strong acid is present. As we noted in Section 8-7D the acid is necessary to convert a relatively poor leaving group (\(HO^\ominus\), \(CH_3O^\ominus\)) into a relatively good one (\(H_2O\), \(CH_3OH\)). Thus the dehydration of alcohols to alkenes is an acid-catalyzed reaction requiring strong acids such as sulfuric or phosphoric acid:

    These are synthetically useful reactions for the preparation of alkenes when the alkene is less available than the alcohol. They can occur by either the \(\text{E}1\) or \(\text{E}2\) mechanism depending on the alcohol, the acid catalyst, the solvent, and the temperature.

    Contributors and Attributions

    • John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."