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    Hi,

    What's the reason for the hydrogen going onto the less stable carbocation? I know the rule states it does but I would like to know why.

    Many thanks

    JT80
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    Firstly Markovnikovs rule was drawn up before we understood why it works - and this science behind it is the relative stability of the carbocations formed.

    I assume you're adding something like HCl across a double bond? The Hydrogen adds so that the more stable carbocation is formed preferentially - the reason why one carbocation is more stable than another is to do with the positive inductive effects (electron pushing/releasing) of the other alkyl groups such as CH3, CH2CH3 etc. The more substituted the carbon carrying the positive charge the more stable the carbocation
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    (Original post by JTeighty)
    Hi,

    What's the reason for the hydrogen going onto the less stable carbocation? I know the rule states it does but I would like to know why.

    Many thanks

    JT80
    It is more stable that way.
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    Say you have CH3CH=CH2 and you stick HBr in there, the first step is attack of the C=C bond on H+. The H+ can either go on carbon 1 so you get this intermediate:

    CH3-C(+)H-CH3

    Or it can go on carbon 2, giving you this intermediate:

    CH3-CH2-C(+)H2

    The first intermediate I drew has a secondary carbocation whilst the second one has a primary carbocation. You might already know that the stability of carbocations goes tertiary > secondary > primary. In this case, the secondary carbocation is most stable so you will predominantly get the first intermediate I drew. From here the Br- can only attack carbon 1, giving you the markovnikov product.
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    (Original post by Kyri)
    Say you have CH3CH=CH2 and you stick HBr in there, the first step is attack of the C=C bond on H+. The H+ can either go on carbon 1 so you get this intermediate:

    CH3-C(+)H-CH3

    Or it can go on carbon 2, giving you this intermediate:

    CH3-CH2-C(+)H2

    The first intermediate I drew has a secondary carbocation whilst the second one has a primary carbocation. You might already know that the stability of carbocations goes tertiary > secondary > primary. In this case, the secondary carbocation is most stable so you will predominantly get the first intermediate I drew. From here the Br- can only attack carbon 1, giving you the markovnikov product.
    Thanks for your response but what I meant to ask was why does the hydrogen go to the less stable carbocation (primary in this case)? The hydrogen is an electrophile?
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    (Original post by JTeighty)
    Thanks for your response but what I meant to ask was why does the hydrogen go to the less stable carbocation (primary in this case)? The hydrogen is an electrophile?
    There is no carbocation before the hydrogen is added to the molecule :nah:
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    (Original post by EierVonSatan)
    There is no carbocation before the hydrogen is added to the molecule :nah:
    Sorry but now I'm really confused. The hydrogen is an electrophile that attacks the electron rich double bond. It joins onto the less stable carbocation (don't know why, but apparently it does) and then the halide attaches to the other carbocation which is more stable as the alkyl groups give over some of their electron density.

    If this is correct then what is the reason for the hydrogen attaching to the carbon it attaches to?
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    Well, the H+ is attacking a C=C bond which is an area of high electron density (negative charge) so yes, I guess it is acting as an electrophile.

    Before the hydrogen has attacked a carbon, there is no carbocation yet so talking about a less stable carbocations has no meaning. But as you can see, once the hydrogen has added onto a carbon, you get a carbocation next to it and it's the stability of this carbocation that matters. You could say, the hydrogen wants to go somewhere which would then give you the most stable intermediate. Putting the hydrogen on the terminal carbon gives you a secondary carbocation which is much preferred because it is lower in energy than the primary. As a result, the energy barrier for the reaction is much lower via the secondary carbocation than via the primary. Therefore the product where the hydrogen goes onto the terminal carbon is much faster and this gives you your major product.
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    (Original post by JTeighty)
    Sorry but now I'm really confused. The hydrogen is an electrophile that attacks the electron rich double bond. It joins onto the less stable carbocation (don't know why, but apparently it does) and then the halide attaches to the other carbocation which is more stable as the alkyl groups give over some of their electron density.
    The bold bit is where you are getting confused. The hydrogen (ion) adds to one of the carbons such that the most stable carbocation is then formed. It basically has a choice to which carbon it adds to but it is energetically more favourable to form the more stable carbocation.
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    (Original post by EierVonSatan)
    The bold bit is where you are getting confused. The hydrogen (ion) adds to one of the carbons such that the most stable carbocation is then formed. It basically has a choice to which carbon it adds to but it is energetically more favourable to form the more stable carbocation.
    Ah now I understand. Thanks a lot.:-)
 
 
 
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