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Homework Help: Hydroboration of alkenes

  1. Mar 22, 2009 #1
    I'm a bit confused on why the BH2 adds to the less substituted carbon in this reaction. I have found this explanation online,

    "The boron atom is highly electrophilic because of its empty p orbital (ie. it wants electrons), and forms a slight bonding interaction with the pi bond. Since some electron density from the double bond is going towards bonding with the boron, the carbon opposite the boron is slightly electron deficient, left with a slightly positive charge. Positive charges are best stabilized by more highly substituted carbons, so the carbon opposite the boron tends to be the most highly substituted."

    To me, this explains why the boron would add to the more substituted carbon, not the other way around. If the boron is electron deficient, wouldn't it add to the more, not less, substituted carbon?
  2. jcsd
  3. Mar 27, 2009 #2
    Hydroboration-oxidation is believed to be mainly a steric effect, although there is regioselectivity (hydrogen is the negative half of the B-H bond and boron is the positive half). The steric effect stems from the fact that boron (and its attached substituents) is MUCH larger than a hydrogen atom, so it bonds to the "less crowded" carbon of the double bond.

    Regioselectivity, although less of a factor, supports the passage you quoted, since the more negative hydrogen will be attracted to the slight positive charge of the more substituted carbon, and boron to the more negative.
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