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H-X Addition to Alkenes: Hydrohalogenation

H-X Addition to Alkenes

Mechanism of Hydrohalogenation

Summary of Key Points

  • Two-step mechanism with carbocation intermediate.
  • Regiochenistry - Markovnikov addition
    • Regiochemistry is controlled by carbocation stability.
      • 3o > 2o > 1o > Methyl (Markovnikov's rule)
  • Stereochemistry -  none
  • Carbocation rearrangements are possible (e.g hydride or alky shifts).
  • Evidence for this mechanism
    • regiochemistry and
      rearrangements.

Regioselectivity of Hydrohalogenation

Hydrohalogenation reactions are regioselective, meaning that the addition of the hydrogen halide (HX) to an unsymmetrical alkene favors the formation of one constitutional isomer over the other. This regioselectivity is explained by Markovnikov's rule, which states that the hydrogen atom from HX adds to the carbon of the double bond that already has more hydrogen atoms, while the halide (X⁻) adds to the carbon with fewer hydrogen atoms.

Mechanistically, this preference is due to the stability of the carbocation intermediate formed during the reaction. When HX adds to the alkene, the protonation step generates the most stable carbocation possible. This is because more highly substituted carbocations (such as secondary or tertiary) are stabilized by electron-donating alkyl groups through inductive and hyperconjugative effects.

For example, in the hydrohalogenation of propene (CH₂=CH-CH2-CH3), the proton (H⁺) preferentially adds to the less substituted carbon (CH₂), forming a more stable secondary carbocation (CH₃-CH⁺-CH₃). The halide (X⁻) then attacks this carbocation, resulting in the Markovnikov product, where the halogen is bonded to the more substituted carbon.  This is referred to as a Markovnikov addition.  If instead the CH carbon accepts the proton (H+) it is referred to as anti-Markovnikov.  I think that students get the "anti" confused with anti stereochemistry, so I tend to use the term non-Markovnikov instead.

Stereochemistry of Hydrohalogenation

Hydrohalogenation of an alkene forms a planar carbocation intermediate, this carbocation has two distinct faces.  It is planar since the carbocation is sp2 hybrid (i.e. trigonal planar).

This carbocation has two faces.  Recall from our discussion of prochirality, that sp2 centers have two faces called Re and Si.

These terms describe the spatial orientation of the substituents around the planar sp²-hybridized carbon atom in the carbocation:

  • Re face: If the substituents around the carbocation are viewed such that they appear to rotate clockwise from highest to lowest priority (following the Cahn-Ingold-Prelog rules), this is called the Re face.
  • Si face: If the substituents appear to rotate counterclockwise from highest to lowest priority, it is the Si face.

Since the carbocation intermediate is planar, the nucleophilic halide ion (X⁻) can attack from either the Re or Si face. This non-stereoselective attack results in the formation of a racemic mixture if the final product has a chiral center, because the halide can approach the carbocation from either side with equal probability. Thus, both enantiomers are produced in equal amounts (i.e. racemic mixture).