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benzene by itself

Discussion in 'MCAT Study Question Q&A' started by Premedico, Jun 4, 2008.

  1. Premedico

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    If the benzene is a substituent by itself, does it have electron withdrawing effects or does it donate?

    What about pi bonds?

    Thank you!
     
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  3. Kaustikos

    Kaustikos Archerize It
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    Do you mean if benzene has no substituents or if benzene has a benzyl group attached? Cause if it's the latter, then I think that it's looked at as an electron donating group because of the resonant pi bonds. But I'm not exactly sure what you mean.
     
  4. sleepy425

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    a phenyl group is electron withdrawing because aromatic rings are electron deficient. a benzyl group is usually electron donating because you have an electron-rich methylene group attached directly to the carbon you're referencing.

    I don't know what you mean by "What about pi bonds?" though, sorry...
     
  5. bluemonkey

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    A phenyl group is electron DONATING because of its conjugated p electrons. Draw out the resonance forms when a substituent is added o/p on the original ring. Notice that the conjugation extends into the phenyl substituent.

    There are several means through which a substituent can be electron donating:
    lone pairs adjacent to the system, hyperconjugation, & p electrons.
     
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  6. hal9000

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    Whoa! Phenyl group donates electrons, because aromatic rings are slightly electron rich.

    EDIT: ha, just noticed bluemonkey posted it already.
     
  7. sleepy425

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    no, this is a misconception, phenyl groups are well-known to be slightly electron withdrawing, but they can stabilize carbocations benzyllic to them because of resonance. in other words, they stabilize a carbocation because the phenyl group, despite being electron deficient, is electron rich when compared to a carbocation. in other words, a carbocation is more electron withdrawing than a phenyl group so the net effect is that the phenyl group donates electron density into the carbocation.

    so what I really should have said is that a phenyl group can be electron donating if it is in a configuration where it can undergo resonance with a more electron-poor species. but for a neutral or an anionic species, the phenyl group is almost always electron withdrawing. for example, aniline, which is neutral, is much much much much much much less nucleophilic than an aliphatic amine like methylamine or ethylamine, etc. Why? because the phenyl group of aniline is electron withdrawing. So, in summary, electron-withdrawing ability is relative, but in most cases, a phenyl group will actually function as an electron withdrawing group. There are notable exceptions, like benzylic carbocations, but for the most part they function as electron withdrawing groups.
     
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  8. bluemonkey

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    I was also assuming we were discussing a phenyl group as a substituent on a benzene ring. There are a number of other subsituents which are inductively electron withdrawing, but are electron donating via resonance (eg Cl, OH). Comparing the relative donating vs withdrawing effects will allow you to figure out whether the group acts as an electron donator or withdrawing group. As someone above said, these properties are largely dependent on the reaction you are interested in.
     
  9. 161927

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    Not quite. A phenyl group is almost always slightly electron donating by induction. Aniline makes the benzene ring a better nucleophile, because its lone pair is in conjugation with the ring. The nitrogen of aniline is a relatively poor nucleophile due to this conjugative stabilization through resonance, not because benzene is exerting an electron-withdrawing inductive effect.
     

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