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Thread: Electrophilic Addition

  1. #1 Electrophilic Addition 
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    In the mechanism for this reaction an H+ ion bonds with a carbon which is already in a double bond with another carbon.
    How does that bond work? Surely it having no electrons of its own means the carbon it is bonded to would have no interest in bonding to it, and if it borrows an electron of the other carbon as well as the one it is bonding to then why isn't the carbocation charged 2+?

    many thanks
    Joshua


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    It's a hell of long time since I've thought about this and I may be a little rusty but my take on it is the electrons in pi bond of a C=C double bond are quite loosely held and the can act as a very effective nucleophile. The two pi electrons form a new sigma bond to the H+ leaving one of the carbons short of one electron and positively charged. I.e. due to movement of electrons the positive charge on the proton has been transferred to one of the carbon atoms. As for the charge on the carbocation how do you think you can generate a 2+ charge from H+ and a neutral alkene?


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  4. #3  
    exchemist
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    Quote Originally Posted by 09jnewington View Post
    In the mechanism for this reaction an H+ ion bonds with a carbon which is already in a double bond with another carbon.
    How does that bond work? Surely it having no electrons of its own means the carbon it is bonded to would have no interest in bonding to it, and if it borrows an electron of the other carbon as well as the one it is bonding to then why isn't the carbocation charged 2+?

    many thanks
    Joshua
    I'm not sure why you say that having no electrons of its own would make H⁺ not of interest to the electrons from carbon. Unlike charges attract, after all. So a partial +ve charge brought close to an electron cloud will polarise it. If the polarisation goes far enough a new bond may start to form.

    But in electrophilic addition my (rather rusty) recollection is it is the partially +ve end of a polar molecule, rather than a cation, that normally attacks to pi bond. See this link for a fuller explanation: What is electrophilic addition?
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