Markovnikov's rule

(noun)

States that, with the addition of a protic acid HX to an alkene, the acid hydrogen (H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents.

Related Terms

Examples of Markovnikov's rule in the following topics:

  • Addition of Strong Brønsted Acids

    • He formulated this trend as an empirical rule we now call The Markovnikov Rule:
    • Empirical rules like the Markovnikov Rule are useful aids for remembering and predicting experimental results.
    • The Markovnikov Rule, for example, suggests there are common and important principles at work in these addition reactions, but it does not tell us what they are.
    • Evidently, alkyl substituents act to increase the rate of addition by lowering the activation energy, ΔE‡1 of the rate determining step, and it is here we should look for a rationalization of Markovnikov's rule.
    • From this information, applying the Hammond Postulate, we arrive at a plausible rationalization of Markovnikov's rule.

  • Reactions of Alkenes and Alkynes

    • If the alkene is asymmetric, the reaction will follow Markovnikov's rule—the halide will be added to the carbon with more alkyl substituents.
    • This rule dictates that the addition of a hydrogen halide (HX, in the case of HBr) to an alkene will lead to a product where the hydrogen is attached to the carbon with fewer alkyl substituents, while the halide group is attached to the carbon with more alkyl substituents.

  • Addition of Lewis Acids (Electrophilic Reagents)

    • The regioselectivity of the above reactions may be explained by the same mechanism we used to rationalize the Markovnikov rule.
    • The oxymercuration reaction gives the product predicted by Markovnikov's rule; hydroboration on the other hand gives the "anti-Markovnikov" product.
    • Nevertheless, the carbocation stability rule cited above remains a useful way to predict the products from hydroboration reactions.

  • Addition by Electrophilic Reagents

    • Although these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity.
    • As a rule, electrophilic addition reactions to alkenes and alkynes proceed by initial formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond.

  • Hydration of Alkynes and Tautomerism

    • For terminal alkynes the addition of water follows the Markovnikov rule, as in the second example below, and the final product ia a methyl ketone ( except for acetylene, shown in the first example ).

  • Intermolecular Addition Reactions

    • The anti-Markovnikov addition of HBr to alkenes is one such reaction, and the peroxide initiated addition of carbon tetrachloride to 1-hexene is another.

  • Rearrangement of Carbocations

    • The addition of HCl to 3,3-dimethyl-1-butene, for example, leads to an unexpected product, 2-chloro-2,3-dimethylbutane, in somewhat greater yield than 3-chloro-2,2-dimethylbutane, the expected Markovnikov product.

  • Addition Reactions Involving Other Cyclic Onium Intermediates

    • Since the hydroboration procedure is most commonly used to hydrate alkenes in an anti-Markovnikov fashion, we also need to know the stereoselectivity of the second oxidation reaction, which substitutes a hydroxyl group for the boron atom.

  • Hydroboration Reactions and Oxidations

    • As with alkenes, the B-H reagent group adds in an apparently anti-Markovnikov manner, due to the fact that the boron is the electrophile, not the hydrogen.

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