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TBR bond energy confusion

Discussion in 'MCAT Study Question Q&A' started by andy96, 03.12.12.

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  1. andy96

    andy96

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    On page 11 of berkeley review organic chemistry book, example 1.4, the correct answer is C "bond C is stronger than bond A, despite both sharing an sp2 and sp3 carbon, because bond C contains more highly substituted carbon"

    However, on page 19,example 1.7 asks for the most stable bond, the correct answer is A,"an sp2-carbon to a primary sp3-carbon". But I think according to answer to example 1.4, "a sp2-carbon to a secondary sp3-carbon" seems more reasonable.

    Any help?
    Thanks
     
    Last edited: 03.13.12
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  3. MedPR

    MedPR Banned

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    The question on page 19 tells you to refer to a table, not to past knowledge or knowledge of a specific case. Based on the table alone, an sp2-primary sp3 bond is the most stable. On page 11 the problem is not involving a vinyl group (CH2=CH-R), it is involving a tetra-substituted alkene. The reason bond c is weaker than bond a is because if you cleaved bond a you would end up with a methyl carbocation, which is much less stable than the isopropyl carbocation you would get if you cleaved bond c.
     
  4. Golaso

    Golaso

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    but bond c is stronger than bond a that's what they have as answer, it said

    "Bond c is stronger than bond a, despite both sharinf an sp2 and an sp3 carbon, because bond c contains the more highly substituted carbon."

    My question is by looking at

    H3C-C=C-R the hydrogen have a very low electron pulling tendency compare to

    (H3C)2HC-C=C-R in which the two C3H have the tendency to pull electrons more which makes the C-C bond weaker and easy to dissociate (theefore less energy required to break the bond =weakerbond)
    But this doesn't match my predictions. Any Help?


     
  5. SH3

    SH3 2+ Year Member

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    Refer back to the question. It states the correct answer is B - "Bond A is stronger than bond C, despite both sharing an sp2 hybridized and an sp3 hybridized carbon, because bond c contains the more highly substituted carbon."
     
  6. SH3

    SH3 2+ Year Member

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    My book is 2012...check your version i guess
     
  7. Golaso

    Golaso

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    That make sense...thanks
     
  8. SH3

    SH3 2+ Year Member

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    np :)
     
  9. Golaso

    Golaso

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    yah I guess alkyl groups donate electrons which makes it easy to disociate (less energy)
     
  10. hellocubed

    hellocubed

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    It should be noted that a substituted carbocation is more stable than a primary carbocation.
    And considering we base a lot of our relative bond strengths on the stability of the reaction products, it would be easier to break a more substituted C-C bond.

    I'm not exactly sure why this is in atomic terms, but perhaps extra alkyl groups donate electrons and make the bond easier to break?

    The exception to this is a C=C double or triple bond, where hyperconjugation from extra alkyl groups stabilizes the pi bonds.
     

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