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Electron Withdrawing/Donating groups

Discussion in 'MCAT Discussions' started by scotties123, Jun 10, 2007.

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  1. scotties123

    scotties123 GrandMacDaddy

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    it seems like there are a lot of both. Is there an easy way to know if something is electron withdrawing or donating, other than just memorizing the common electron withdrawing/donating groups? This may be a better way of asking it: What makes something electron withdrawing, versus electron donating? THANKS.
  2. jochi1543

    jochi1543 President, Gunner Central

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    If you have an o-chem book, your best bet is to flip to the benzene section, where you should be able to see a detailed description of e-withdrawing and e-donating properties.

    Basically, there are 2 ways e donation/withdrawal can be made - through a resonance effect and through an inductive effect. Both of these could withdraw or donate, or it could be that a functional group donates e through resonance and withdraws them inductively - then it will depend on which effect is stronger.

    Inductive effects occur due to sigma bond polarity - think C-Cl bond. Chlorine is highly electronegative and thus withdraws electrons through the sigma bond with carbon. It will pull the electrons away from the ring.

    Resonance effects occur due to pi orbital overlap between the substituent atom and the ring. Basically, the substituent will be forming a double bond with the ring as one or more of the resonance structures. For example, a substituent that has oxygen bonded to the ring (OCH3, for example) can have the oxygen form a double bond, thus transferring some of the electron density from the oxygen to the ring. Hence the e-donating property of OCH3.


    Hope this helps a bit...if you want, I can discuss this some more. However, I do have to say that while understanding some of the mechanics behind this helps, I thought it was fairly easy to memorize. Alkyl, alkoxy, hydroxy, oxy, phenyl, carboxy, and amine are ortho-para activators (electron donators), halogens are ortho-para deactivators (electron withdrawing b/c the resonance effect here is weaker than the inductive effect), and everything else is meta deactivating (electron-withdrawing).
  3. mmcnam

    mmcnam

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    Here are a couple of standards to go by:

    1. Alkyl groups are electron donating. You probably know this already because tertiary carbocations are stabilized by hyperconjugation.

    2. Highly electronegative elements are electron withdrawing. You probably know this already too - that's why lots of F's next to an OH makes the group a lot more acidic - it smears out the negative charge of the conjugate base across the molecule, thereby stabilizing it.

    3. Did you learn the directing effects on a benzene ring? This is a good way to remember certain electron withdrawing/donating groups that aren't so quick to reason out. NO2 is an amazing electron withdrawing group, along with carbonyls. They are also meta directing and deactivate a benzene ring. On the other hand, NH2 is electron donating (no oxygens to suck up the negative charge), along with alkoxy groups (-OR). I haven't been able to explain why alkoxy groups are electron donating to myself very well - it's just something that I remember. When in doubt, follow the rules of benzene directing.
  4. jochi1543

    jochi1543 President, Gunner Central

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    I think this is because 1) the alkyl part of the group is already donating electrons to the oxygen 2) the oxygen, as I mentioned earlier, can form a double bond to the ring, transferring some of its electron density there (same thing as happens with OH substituents).
  5. mrmilad

    mrmilad

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    A simplified way to think about it would be to consider what the significance of electron withdrawing or donating groups are when dealing with benzene, relevent to meta and para direction. When substituation reactions take place on a benzene one of the double bonds must be added to and the alkene opens up creating a carbocation. The carbocation is stabilized by resonance as the carbocation is dispersed over the carbons that create the ring. Thus the most stable and plentiful resonance structure the more stable the intermediate. So to explain why the -OR group and -NH2 are electron donating, you need to visualize the resonance structure where the carbocation is placed directly on the carbon that is attached to the -NH2 or -OR groups. In this step we basically have a large positive charge on the carbocation and low and behold a fairely large negative charge on the adjacent atom (OR or NH) due to the high EN nature of the O and N. Thus through induction the carbocation is stabilized, and furtherly this allows for an extra resonance structure which leads to further stabilization.

    In case of with withdrawing groups the carbocation prefers to skip the EWG because the EWG would either have a positive charge or a strong EN without resonance capability. This would make the carbocation even more unstable. Thus the EWG prefer meta direction as opposed to ortho/para which would cause the carbocation to land on the EWG.

    Guides to EW and ED:

    EW: Carbonyl groups, NO2, X(Halogens have high EN without resonance),Sulfites

    ED: -OR OH : NH2 (induction and resonance), Alkyl groups (inductive), Phenyl groups

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