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7.11: Evidence for the Mechanism of Electrophilic Additions- Carbocation Rearrangements

  • Page ID
    67132
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    Objective

    After completing this section, you should be able to explain the “unusual” products formed in certain reactions in terms of the rearrangement of an intermediate carbocation.

    Key Terms

    Make certain that you can define, and use in context, the key terms below.

    • alkyl shift
    • hydride shift
    Study Notes

    Whenever possible, carbocations will rearrange from a less stable isomer to a more stable isomer. This rearrangement can be achieved by either a hydride shift, where a hydrogen atom migrates from one carbon atom to the next, taking a pair of electrons with it; or an alkyl shift, in which an alkyl group undergoes a similar migration, again taking a bonding pair of electrons with it. These migrations usually occur between neighbouring carbon atoms, and hence are termed 1,2-hydride shifts or 1,2-alkyl shifts.

    [A hydride ion consists of a proton and two electrons, that is, [H:]. Hydride ions exist in compounds such as sodium hydride, NaH, and calcium hydride, CaH2.]

    An electrophilic reaction such as HX with an alkene will often yield more than one product. This is strong evidence that the mechanism includes intermediate rearrangement steps of the cation.

    1,2-Hydride Shift

    A 1,2-hydride shift is a carbocation rearrangement in which a hydrogen atom in a carbocation migrates to the carbon atom bearing the formal charge of +1 (carbon 2) from an adjacent carbon (carbon 1).

    eg:

    see also 1,2-aryl shift

    1,2-Alkyl Shift

    A 1,2-alkyl shift is a carbocation rearrangement in which an alkyl group migrates to the carbon atom bearing the formal charge of +1 (carbon 2) from an adjacent carbon atom (carbon 1), e.g.

    see also 1,2-aryl shift, hydride shift, alkyl shift

    Exercises

    Exercise \(\PageIndex{1}\)

    Draw the expectred produts of the following reaction.

     

    Answer

    Worked Example 1.svg

    Exercise \(\PageIndex{2}\)

    The second phase of the cyclase reaction mechanism involves multiple rearrangement steps and a deprotonation. Please supply the missing mechanistic arrows.

    clipboard_ec86bc30507cefeedb91d5332dacdea9b.png

     

    Answer

    clipboard_e3d239f716a440528aac53c223efebee4.png

    Exercise \(\PageIndex{3}\)

    The following reaction shows a rearrangement within the mechanism. Propose a mechanism that shows this.

    7-11qu.png

    Answer

    7-11sol.png

    Exercise \(\PageIndex{4}\)

    Propose a mechanism for the following reaction. It involves an electrophilic addition and the shift of a C-C and a C-H bond.

     

     

     

     
    Answer

    In most examples of carbocation rearrangements that you are likely to encounter, the shifting species is a hydride or methyl group. However, pretty much any alkyl group is capable of shifting. Sometimes, the entire side of a ring will shift over in a ring-expanding rearrangement.

    The first 1,2-alkyl shift is driven by the expansion of a five-membered ring to a six-membered ring, which has slightly less ring strain. A hydride shift then converts a secondary carbocation to a tertiary carbocation, which is the electrophile ultimately attacked by the bromide nucleophile.

     

     

    Contributors


    7.11: Evidence for the Mechanism of Electrophilic Additions- Carbocation Rearrangements is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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