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Hyperconjugation and electromeric effect

  1. Mar 1, 2016 #1
    As we know in unsymmetric alkenes as a reagent approaches towards it a pi bond is shifted towards an constituent atom so these decisions are governed with the help of inductive effects...correct? So my question is can we govern this shifting on the basis of hyperconjugation....like a constituent containing less no. Of hyperconjugates will be negatively charged as reagent approaches. Please correct me if am wrong .Thankyou
     
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  3. Mar 2, 2016 #2

    TeethWhitener

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    Hyperconjugation is basically an inductive effect, so it works just like any other inductive effects in that it pushes the electron density on an alkene around. In fact, this is one way to think about Markovnikov addition. For example, if HBr attacks isobutylene, the H+ preferentially forms a bond with the less substituted carbon because the resulting tertiary carbocation is stabilized by hyperconjugation (sigma orbitals on the methyl groups donating electron density into the empty carbon p orbital).
     
  4. Mar 2, 2016 #3
    Ohk i got it......but I'm confused in a case that when an alkene named as4,4-dimethyl pent-2-ene is treated with HCl(markovnikov's addition) then where will H+ ion attacks more that means on which carbon atom has the most probability of getting negative charge. As there are three electron donating methyl groups and the opposing it are three hyperconjugates H atoms. Please refer to structure and answer. Thankyou
     
  5. Mar 2, 2016 #4

    TeethWhitener

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    Sorry, I'm having a little bit of trouble understanding what you're asking. You'll have to bear with me. Are you asking what the major Markovnikov product of HCl with 4,4-dimethyl-2-pentene is?

    If so, then I would guess the favored product would put the H at the 2 position and the Cl at the 3 position (3-chloro-4,4-dimethylpentane) because larger alkyl groups tend to be more electron donating than smaller ones. But in reality, upon addition of H+ at either the 2 or 3 position, the resulting intermediate will have a secondary carbocation, and I doubt the two possible species will have vastly different stabilities. So it probably won't make that much of a difference (in other words, expect the difference in yield between major and minor product to be smaller than in a case like isobutylene).

    As far as the hyperconjugation question is concerned: Hyperconjugation happens when a sigma bond partially aligns with an adjacent pi bond, thereby stabilizing it. (This can also occur to some extent with σ* antibonds.) Because of the geometry of the methyl group, you can only ever line up one C-H sigma bond at a time with an adjacent pi bond. So both the 2 and the 3 position only really each have 1 hyperconjugated C-H bond.
     
  6. Mar 2, 2016 #5
    So you say here the presence of CH3- groups are reason for the addition if H+ ion on 2 position hence here it dominates over the hyperconjugation effect. Am i correct?
     
  7. Mar 2, 2016 #6

    TeethWhitener

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    I'm saying that
    1) There's probably not that much difference between addition at the 2 position and addition at the 3 position,
    2) but if I had to guess, I'd say that the 3 position was the more stable carbocation because of electron-donating inductive effects from the larger alkyl group.
    But I don't have the data to tell you conclusively which is the major product.
     
  8. Mar 2, 2016 #7
    Ohk....thank you so much for clearing my doubt
     
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