Carbocation & Markovnikov's Rule

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Carbocation Stability Part 1
14:54 — by Leah F.

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1 of 3 videos by Leah F.

Key Questions

  • Alkyl groups stabilize carbocations by partially neutralizing the cation with electrons of their bonds with the positively charged carbon

  • Let's consider a comparison between the two transition states (alkene vs. alkyne) of a typical electrophilic addition reaction. When you do these, one way to catalyze them is with an acid, so let's look at the first few steps of the acid-catalyzed hydration of an alkene vs. an alkyne:


    (form of the transition state from Organic Chemistry, Paula Yurkanis Bruice)

    You can see that for the transition state of the alkyne, the hydrogen is not entirely bonded; it is "complexing" with the double bond, forming a #\mathbfpi# complex; "idle", until something breaks the interaction (the nucleophilic attack of the water) to get the molecule out of its unstable state.

    The complex is shaped like a cyclopropane analog, which is highly strained. Also, the high electron density in the double bond makes for some immensely disruptive repulsions that destabilize the transition state.

    This combination of a highly-strained ring structure and high electron density in the intermediate (transition state) makes alkynes less reactive than alkenes in electrophilic addition reactions. Pictorially, the energy of the transition state is higher on the reaction coordinate diagram.

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  • Answer:

    It is dependant on the stability of the carbocation intermediates.

    Explanation:

    When doing an addition across a double bond of compounds such as hydrogen halides (HX), Markovnikov's rule tells us that the Hydrogen will go to the carbon with the most hydrogens bonded to it.

    Consider the following example:

    Propene + HBr -> 2-Bromopropane

    enter image source here
    This occurs because a secondary carbocation is far more stable than a primary carbocation.

    The same is true for examples where there is a branch bonded next to a double bond, for example in 2-methylbut-2-ene. In this case, a tertiary carbocation is more stable than a secondary carbocation so in a reaction with HBr, the bromine would add to the second position giving 2-bromo-2-methylbutane.

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