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Chemistry LibreTexts

6.18: Comparison of E1 and E2 Reactions

  • Page ID
    182917
  • Learning Objective

    • distinguish 1st or 2nd order elimination reactions

    Elimination reactions of alkyl halides can occur via the bimolecular E2 mechanism or unimolecular E1 mechanism as shown in the diagram below.

    Fileimage034.png

    Comparing E1 and E2 mechanisms

    When considering whether an elimination reaction is likely to occur via an E1 or E2 mechanism, we really need to consider three factors:

    1) The base: strong bases favor the E2 mechanism, whereas, E1 mechanisms only require a weak base.

    2) The solvent: good ionizing xolvents (polar protic) favor the E1 mechanism by stabilizing the carbocation intermediate.

    3) The alkyl halide: primary alkyl halides have the only structure useful in distinguishing between the E2 and E1 pathways. Since primary carbocations do not form, only the E2 mechanism is possible.

    Reaction Parameter E2 E1
    alkyl halide structure tertiary > secondary > primary tertiary > secondary >>>> primary
    nucleophile high concentration of a strong base weak base
    mechanism 1-step 2-step
    rate limiting step anti-coplanar bimolecular transition state carbocation formation
    rate law rate = k[R-X][Base] rate = k[R-X]
    stereochemisty retained configuration mixed configuration
    solvent not important polar protic

    Exercises

    1. Predict the dominant elimination mechanism (E1 or E2) for each reaction below. Explain your reasoning.

    ch 7 sect 18 exer 1 question.png

    2. Which one of the following groups of compounds would eliminate \(\ce{HCl}\) most readily on reaction with potassium hydroxide? Explain your reasoning, draw the bond-line structure and give the IUPAC name of the product.

    a) \(\ce{(CH_3)_3CCl}\) \(\ce{CH_3CH_2CH_2CH_2Cl}\) \(\ce{CH_3CH(Cl)CH_2CH_3}\)

    b) \(\ce{(CH_3)_3CCH_2Cl}\) \(\ce{(CH_3)_2CHCH_2Cl}\)
    c) alt

    3. Specify the reaction conditions to favor the indicated elimination mechanism.

    ch 7 sect 18 exer 3 question.png

    Answer

    1.

    ch 7 sect 18 exer 1 solution.png

    2.

    ch 7 sect 18 exer 2 solution.png

    3. a) strong base, such as hydroxide, an alkoxide, or equivalent

    b) water or alcohol or equivalent weak base with heat

    c) strong base, such as hydroxide, an alkoxide, or equivalent

    Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)