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Clarification of HBr + UV + Butene

Discussion in 'DAT Discussions' started by ET87, Jun 19, 2008.

  1. ET87

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    The reaction of 1-butene with Hydrogen Bromide in the presence of UV light results in the formation of
    A 2-butene
    B butane
    C 1,2-dibromobutane
    D 2-bromobutane
    E 1-bromobutane

    My thoughts were that this would be a typical markovnikov addition of HBr, where H is added to the 1 carbon creating a secondary carbocation where the Br would add, thus creating D, 2-bromobutane. I am I correct that if UV were not part of the said reaction that 2-bromobutane would be the product? I do not want to post the correct answer as I would like to hear everyone's thoughts purely =] Also if you could mention some sort of mechanism that would make it easier to understand rather than just stating an answer :) thank you very much! Btw I am guessing that since UV is mentioned that there is some sort of radical within the mechanism somewhere..
     
  2. harrygt

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    Nice question, and you are guessing right. It would also be nice if they would mix a peroxide in the mixuture to emphasis the on the free radical mechanism, although UV still indicates that.
    The correct answer is 1-bromobutane. What is the point of putting UV there? it tells you that radicals are involved in the mechanism.
    A free radical mechanism will result in an ANTI-markovnikov addition, giving you 1 bromo-butane.
    Check this website for the mechanism. I just googled it.
    http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/radikale/radi_subst_add_alkane_alkene.vlu/Page/vsc/en/ch/12/oc/radikale/radikale_an_alkene/radikale_an_alkene.vscml.html
    I hope this helps
     
  3. Simiam

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    2-Bromobutane would form with a carbocation through Markovnikov addition...

    BUT Watch the setup because any time you see UV, ROOR, or peroxides you should instantly key in on the fact that it is going to add ANTI Markovnikov... thus the correct answer would be 1-Bromobutane.
     
  4. sweetpea2

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    I know this is a separate topic, but its kinda similar. So when adding HBr by itself it is markovnikov and when adding HBr with either peroxides or UV it is anti mark.

    So heres my question... it is on free radicals adding to ALKANES! in free radical bromination of an alkane with Br2 with UV the bromine adds onto the most stable radical, i know. Are there any conditions when Br2 will add to the primary radical. Also, does Chlorination add to the primary more than most stable radical? And lastly, Kaplan doesnt really explain this well at all.

    And lastly, does NBS always add to primary radical. Thanks a lot
     
  5. harrygt

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    To sweetpea2,
    1) there are no conditions when Br2 will add to the primary, if you got a more stable radical present.

    2) Chlorination still prefers the Benzylic, allylic and III more than II, and more than I. HOwever, chlorination is faster than bromination, that is why it is less selective, I mean that is why you get more primary and seconday halo alkane compared to bromination. Imagine 100 people having only 10 seconds to find a mate in a room. Obviously they are not gonna have enough time to find the person they want. NOw if those people have 10 minutes, they will find the one whom they like.
    . Bromination is a slower reaction, which means there is enough time to choose the better candidate [Benzylic, allylic and III]. In chlorination, more stable radicals are still gonna be the winner, but the ratio of for example III to I is gonna be less than the same ration in bromination.
    I hope this helps.
     
  6. Glycogen

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    To add to whatever everybody said already and were al right,UV will make radical as you guys said and radicals are more stable at primary positions.E is the answer.
     
  7. harrygt

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    Glycogen, What do you mean by "radicals are more stable at primary positions"? That is exactly opposite of the fact.
     
  8. Glycogen

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    Yes you are right.I just looked at note and it says 3>2>1>methyl for radical stability!
    wow,I can not believe it and I just reviewed ochem yesterday!!!!
    Making such a huge mistake on a basic stuff!!!!!
    This is not a good new.:scared:
     

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