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-   -   What the difference between anti markovnikov rule and markovnikov rule (https://www.askmehelpdesk.com/showthread.php?t=573584)

  • Apr 30, 2011, 01:56 AM
    susus
    What the difference between anti markovnikov rule and markovnikov rule
    What the difference between anti markovnikov rule and markovnikov rule, and when the anti markovnikov happens. Is it only when , there is BH3,THF/H2O2, OH- ?
    I can understand if u explain it deeply, I'm studying organic chemistry course..
  • Apr 30, 2011, 07:59 AM
    DrBob1
    Markovnikov'v rule explains the regiochemistry in the addition of unsymmetrical reactants to alkenes. Most commonly this is seen in the addition of water (H-OH) and hydrogen bromide (H-Br). For Markovnikov's rule to hold sway, the reaction must proceed through a carbocation mechanism, The Markovnikov product is, of course, formed from the more stable carbocation. As an example of this, look at the addition of water to propene under acid catalysis: CH3CH=CH2 + H-OH ---H+--> CH3CHOHCH3. 2-Propanol is the "Markovnikov Product"

    The hydroboration/oxidation sequence gives, as you tell, the other isomeric propanol (1-propanol). This is the "AntiMarkovnikov Product" We also see this reversal of regiochemistry with the addition of HBr in the absence (Markovnikov) and presence (AntiMarkovnikov) of peroxides.

    I think it should always be pointed out that in antiMarkovnikov additions the RULE still holds true - it is the reactants that are altered. When H-OH is added with acid catalysis the major product is formed from the more stable carbocation. Stoichiometrically, you add H+ then OH-. If you examine the mechanism of hydroboration/oxidation you see that the sequence results in the addition of H- then OH+ -- you have reversed the polarity of water!! (BH3 is s borohydrIDE) In HBr addition, the difference is the ionic vs the radical mechanisms.

    I hope this works for you. A picture is worth a thousand words but I have a new computer and no Chemdraw yet.
  • Apr 30, 2011, 08:57 AM
    susus
    Comment on DrBob1's post
    I got it thanks :)
    I was confused because I related it , with the syn and the anti !
    Haha I thoght the antimark.. is an addition to some side rather than other side.. something like that..
  • Apr 30, 2011, 10:42 AM
    DrBob1
    Comment on DrBob1's post
    You were confusing regiochemistry with stereochemistry.

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