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carbocation stability

Discussion in 'DAT Discussions' started by sweetpea2, Jun 22, 2008.

  1. sweetpea2

    sweetpea2 2+ Year Member

    Apr 21, 2008
    Ok, ive asked this a few times, but different practice tests keep saying different things. Im looking at the free kaplan DAT test and the question is which carbonium ion will be most stable.

    Answer says Ph3C+ which is stable but......

    Another choice is Ch2=CHCh2+ Thought this should be the answer due to special stability of allylic carbocations.

    -Isnt stability in this order Benzylic carbocation, allylic, tert, secondary, etc. And phenyl groups are better at stabilizing than methyl.

    Anyone? Thanks
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  3. DentalDeac

    DentalDeac 2+ Year Member

    May 28, 2008
    Alexandria, VA
    In the first choice, the carbocation is attached to 3 phenyl groups, making it a tertiary carbocation, which is preferrable. In the second choice, the carbocation is only directly attached to one other carbon, making it primary, and not very stable.
  4. drgreen

    drgreen 5+ Year Member

    Feb 20, 2008
    Ph3C+ is extremely stable because it is a tertiary c+ but mainly because adjacent to the c+ are 3 phenyl groups (which stabilize the c+ through resonance of their double bonds). as you mentioned benzylic carbocations are the most stable, here we have 3.
  5. sweetpea2

    sweetpea2 2+ Year Member

    Apr 21, 2008
    But isnt allylic still even more stable than the Ph3C+
  6. Mstoothlady2012

    Mstoothlady2012 5+ Year Member

    Apr 7, 2007
    no just like you stated the order...benyzllic > allylic.

    In the question there are 3 phenyl groups attach, which makes it the most stable carbocation.
  7. harrygt

    harrygt 2+ Year Member

    Jun 16, 2008
    carbocation stability is Benzylic > tert > allylic > sec > prim
    radical stability is Benzylic > allylic > tert > sec > prim

    I hope this helps.

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