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Addition reactions: Alkenes vs Alkynes

  1. Aug 9, 2013 #1
    1. The problem statement, all variables and given/known data

    Alkenes are more reactive than alkynes toward addition of electrophilic reagents like HCl. Yet when alkynes are treated with one molar equivalent of these reagents, it is easy to stop the reaction at the alkene stage. This appears to be paradox. Explain.



    3. The attempt at a solution

    No idea.
     
  2. jcsd
  3. Aug 12, 2013 #2

    chemisttree

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    Any ideas about the mechanisms for HCl addition a ross the double and triple bond? What intermediates are formed? Are they stable?
     
  4. Aug 12, 2013 #3
    Across a double bond I understand the mechanism as-

    1)First the pi bond is cleaved heterolytically due to the hydrogen atom (electromeric effect) and there is a positive and negative charge on the carbons.
    2)An electron pair is donated to the hydrogen by the negatively charged carbon. The carbocation formed will be stabilised by hyperconjuation and I effect.
    3)The chlorine anion donates an electron pair to the positively charged carbon, forming the compound.

    Across a triple bond I consider ethyne with HCl

    1)First a pi bond is cleaved heterolytically due to the hydrogen atom (electromeric effect) and there is a positive and negative charge on the carbons.
    2)An electron pair is donated to the hydrogen by the negatively charged carbon.

    The carbocation will be less stable than in the first case because it has an electronegative sp2 hybridised carbon attached to it which increases its positive charge. Also in general sp2 hybridised carbocations aren't stable. Maybe this is why it is said that "Alkenes are more reactive than alkynes toward addition of electrophilic reagents like HCl."

    3)The chlorine anion donates an electron pair to the positively charged carbon, forming Chloroethene.

    Please correct me if I was wrong anywhere.

    The question asks why Chloroethene doesnt form 1,2 dichloroethane while above reaction happens.
    Intermediate with Chloroethene is H3ClC---CH2+ which seems more stable. I am stuck!
     
  5. Aug 14, 2013 #4

    chemisttree

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    The proton approaches the pi system and forms a three center intermediate. The propensity for the substrate to do this is a function, in part, of the pi system's ability to give up some electron density to the proton. Another way of saying electronegativity, yes? Which is more electronegative, an sp2 or sp hybridized carbon?

    Think about this some... have you considered the stability of a vinyl cation?

    What does an electron withdrawing group like chlorine do to the stability of the growing electropositive charge on the carbon? Stabilize it or not?
     
  6. Aug 14, 2013 #5
    An sp hybridised carbon is more electronegative due to the increased s character in its bonds. Also it is less suited to stabilise a positive charge on it.

    Maybe when we have a compound with both double and triple bonds in it like pent-4yne-1ene and add one equivalent of an electrophlilic reagent like Br2 to it then addition will take place on the double bond only?

    I think that the vinyl cation ie H2C==CH+ is very unstable. In my previous post I totally forgot that the carbon with the positive charge is sp hybridised and so it will be more electronegative than the carbon attached to it. The main thing is that the sp hybridised carbon is not suited to stabilise the positive charge on it.


    I think I got it. The chlorine will increase the positive charge on the carbon thus reducing the stability of the compound. But will this intermediate show hyperconjugation? This structure has more hyperconjugative structures so it should be more stable?
     
  7. Aug 15, 2013 #6

    chemisttree

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    So these reasons are responsible for a hundredfold increase in reaction rate for HCl with double bonds vs triple bonds even though the reaction with the triple bond is substantially favored thermodynamically.
     
  8. Aug 18, 2013 #7
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