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worst electrophile: ester or ketone?

Discussion in 'MCAT Study Question Q&A' started by ICUatUCI, Jul 24, 2011.

  1. ICUatUCI

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    On TBR Test 4, it asks which one is a worst electrophile?
    Acid chloride, acid anhydride, ester and ketone. Answer is a ketone.

    From what i remember from ochem class, ester should be the answer.

    Most reactive acid chloride>anhydride>aldehyde>ketone>carboxylic acid>ester>amide.

    Also, which one is more reactive: ester or carboxylic acid? I did some searching and some sites give ester is more reactive than carboxylic acid. some vice versa. Some say they are about the same.

    Thank you so much
     
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  3. mitchlucker

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    An electrophile is something that is very attracted to electrons, right?

    So, think about the different answers...

    Acid chloride...it has an electronegative oxygen, and an electronegative chlorine attached to a single carbon, sucking the electron density away from the carbon. This makes the carbon an electrophile (by accepting an electron pair)

    Go through this with the other answers and ketone is the obvious choice.

    Ester or carboxylic acid would depend on the reaction.
     
  4. ICUatUCI

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    Thanks mitch, i know that an electrophile is something that is attracted to electrons, but i was thinking better electrophile = more reactive. may be since it is just asking for electrophilicity, i shouldn't be thinking in terms of reactivity. thanks again
     
  5. PingPongPro

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    Try to think which molecule creates the least positive dipole.

    Ketone has a smaller positive dipole on the carbonyl carbon. Ester has the carbonyl oxygen AND another oxygen attached to it making it more polar than the carbonyl which only has the carbonyl oxygen tugging on its electrons.
     
  6. loveoforganic

    loveoforganic -Account Deactivated-
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    I was taught that ester and ketone were roughly comparable electrophilicity, with the oxygen resonance and inductive effects roughly cancelling out
     
  7. turkmd

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    Either there is an error in the book, or....

    the reason for the ketone being more reactive than an ester is probably due to the hyperconjugation from the additional methyl group.
    in the resonance structure for a ketone, the carbon has a positive charge, and the methyl groups can donate their electrons through hyperconjugation, where the electrons from the C-H bonds will temporarily overlap with the empty p orbital of the carbon in the resonance structure.
    The ester, has one less methyl group, and so its one methyl group might not provide as much stabilization. The inductive effective of the oxygen can be considered, but as loveoforganic said, the ugly resonance structure with an oxonium ion will likely be more than enough to undo any effect that induction has.

    hope this helps.
     
  8. orgohacks

    orgohacks Organic chemist
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    Ester should be least reactive in the first example, due to the resonance form where oxygen donates a pair of electrons to the carbonyl. (when you donate electrons to an electrophile, you make it less reactive). Amides are less reactive than esters for the same reason.

    The reactivity of carboxylic acids is a little harder to nail down. Why? Because it will depend on the pH of your solution. Under basic conditions the carboxylic acid will be deprotonated to a carboxylate which is an extremely poor electrophile. (only LiAlH4 and a handful of other reagents will react with the carbonyl carbon under these conditions). Under acidic conditions, the reactivity of a carboxylic acid should be comparable to an ester.

    Hope this helps - James
     

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