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most acidic hydrogen OCHEM question help

Discussion in 'DAT Discussions' started by thefifthbeatle, May 15, 2007.

  1. thefifthbeatle

    thefifthbeatle Junior Member 5+ Year Member

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    Jul 31, 2006
    i know the most acidic hydrogen is the one(s) on the alpha carbon of a carbonyl

    I have encountered ones such as
    ----O----O
    ----II----II
    CH3CHCH2CHCCH3

    The ones in middle are the most acidic--All the questions I have seen always have two carbonyls, with the H's in the middle

    BUT, if it were only one carbonyl:

    ----O
    ----II
    CH3CHCH2CH2CH3



    Are these five all the same??? Does the propyl group affect the acidity vs the methyl group?

    Thanks

    PS. I have taken the DAT and know friends that have---they guarantee ask you this type of question
     
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  3. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    Draw the carbanions with the respective alpha hydrogens removed. Then ask yourself which is more stable. One is more stable than the other...they're not all equal
     
  4. jdmsamurai

    jdmsamurai 2+ Year Member

    237
    1
    Feb 11, 2007
    carboanions are most stable as

    meth > 1 > 2 > 3

    therefore the most acidic should be the CH3 and not the CH2

    correct me if i am wrong
     
  5. PChemGrad

    PChemGrad I am a banana. 10+ Year Member

    946
    4
    Jun 29, 2006
    ----O
    ----II
    CH3CHCH2CH2CH3

    The Hydrogen colored red will be the most acidic, followed by the hydrogen in blue will be the second most acidic.

    this is due to the resonance structures possible. after abstraction of a hydrogen the remaining negative charge back form a double bond to the carbonyl carbon causing a single bond to the oxygen.
    hydrogens bonded to a alph carbon in a carbonyl will always be the most acidic.
     
  6. jdmsamurai

    jdmsamurai 2+ Year Member

    237
    1
    Feb 11, 2007
    i get your reasoning...but both of them can form double bonds to the carbonyl carbon...but what makes the red one more acidic though?...and isnt both the red and blue alpha carbons?
     
  7. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    You're right, therefore C1 will be most acidic. Because of the negative charge...carbanion on C3 will be destabilized due to the inductive effect
     
  8. jdmsamurai

    jdmsamurai 2+ Year Member

    237
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    Feb 11, 2007
    so therefore the "blue" one should be the most acidic...and not the red...
     
  9. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
  10. Mia305

    Mia305 5+ Year Member

    183
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    May 7, 2007

    A side question...If that is the order of carbanion stability then why is a carbanion with three phenyl groups attached more stable than a methyl carbanion... according to Kaplan this is due to all the resonance structures that the first one can form. but based on that order, the methyl carbanion would be more stable
     
  11. jdmsamurai

    jdmsamurai 2+ Year Member

    237
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    Feb 11, 2007
    resonance > methyl

    always
     
  12. Mia305

    Mia305 5+ Year Member

    183
    1
    May 7, 2007
    so even though the order is meth > 1 > 2 > 3, a tertiary carbanion that can form multiple resonance structures will be more stable than a methyl carbanion?
     
  13. phungy

    phungy Account Terminated 10+ Year Member

    377
    6
    May 8, 2006
    I just noticed that the carbonyl has 5 bonds :p
     
  14. thefifthbeatle

    thefifthbeatle Junior Member 5+ Year Member

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    Jul 31, 2006
    Hahahahhahah oops!!!!! I was more concerned with writing the chain and spacing it correctly!!! I will NOT mess things up like that on the test!
     
  15. jeninny44

    jeninny44 2+ Year Member

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    May 4, 2007
    Kryptonite.
     
  16. supraman

    supraman Boston Celtics 7+ Year Member

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    Oct 30, 2005
    Could have wrote it like this:

    CH3(C=O)CHCH2CH2CH3
     
  17. blueyes34

    blueyes34 5+ Year Member

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    Jan 12, 2007
    Ohio
    With the way I was taught...one of the hydrogens on C1 would be more acidic because it is more readily available for attack due to less steric hinderance. Is this right?
     

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