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Difference between inductive effect and resonance

Discussion in 'MCAT Discussions' started by axp107, Jul 12, 2007.

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  1. axp107

    axp107 UCLA 09': Italian Pryde

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    Does the inductive effect result only in the presence of an electron withdrawing group...

    what about resonance stabilization
  2. ahmedqman

    ahmedqman

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    I think inductive effects are based on electronegativity of an atom but resonance is the ability to donate electrons to a benzene ring...

    For example, chlorine is an ortho/para deactivator. This is because its inductive effects outweigh its resonance effects.

    The reason it deactivates is because halogens are electronegative, but the reason it's ortho para is because it stabilizes intermediates by donating an electron pair to the benzene ring in an electrophillic aromatic substitution reaction.
  3. apoptos

    apoptos purulent & supple

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    Think ahmedqman is right...found this...

    "The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects: The first is the inductive effect of the substituent. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring (left-hand diagram below).

    The second effect is the result of conjugation of a substituent function with the aromatic ring. This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift. If the atom bonded to the ring has one or more non-bonding valence shell electron pairs, as do nitrogen, oxygen and the halogens, electrons may flow into the aromatic ring by p-pi conjugation (resonance), as in the middle diagram. Finally, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, as in the right-hand diagram. Note that in the resonance examples all the contributors are not shown. In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.
    In the case of nitrogen and oxygen activating groups, electron donation by resonance dominates the inductive effect and these compounds show exceptional reactivity in electrophilic substitution reactions. Although halogen atoms have non-bonding valence electron pairs that participate in p-pi conjugation, their strong inductive effect predominates, and compounds such as chlorobenzene are less reactive than benzene. The three examples on the left of the bottom row (in the same diagram) are examples of electron withdrawal by conjugation to polar double or triple bonds, and in these cases the inductive effect further enhances the deactivation of the benzene ring. Alkyl substituents such as methyl increase the nucleophilicity of aromatic rings in the same fashion as they act on double bonds."

    [​IMG]
  4. wryandwatchful

    wryandwatchful

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    The most concise way to put it:

    Inductive effects occur through the sigma bond network.
    Resonance effects occur through the pi bond network.

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