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Organic Chemistry Nucleophile Reactivity Rate

  1. Nov 29, 2015 #1
    CH3Br + Nu- → CH3Nu + Br-

    The decreasing order of the rate of the above reaction with the following nucleophiles is:
    [ Nu = (I) PhO- (II) AcO- (III) HO- (IV) CH3O- ]

    My answer is IV > III > II > I
    However the answer is IV > III > I > II

    I'm having a hard time understanding why PhO- is a better nucleophile than AcO-.
    Isn't PhO- more stable than AcO- due to the negative charge being delocalized over the entire phenyl ring while for AcO- there is comparatively less delocalization? Also, isn't PhO- bulkier than AcO-?
    So, it looks to me like there are two factors working against the actual answer.
    Please help.
     
  2. jcsd
  3. Nov 29, 2015 #2
    AcOH is a stronger acid than PhOH, so the acetate anion is more stable than the phenoxide anion (in water, at least).
    However, thermodynamic stability of anions may or may not play a role here, as we're talking about reaction _rates_, i.e. kinetics. Bulk is indeed more important.
    In general I would comment that test questions like the one you cited necessarily simplify the problem for didactic purposes, but in reality solvent, counterion, temperature, etc. may all affect the experimentally observed rate.
    So is acetate more or less nucleophilic than phenoxide? I would say less, purely from experience as an organic chemist.
    As for the proof, I'm sure physical chemists know how to compute rigorously the electron density on these organic anions, and that would provide a big part of the answer.
    From my naive perspective, I see on one hand a molecule where a negative charge is spread over two oxygen atoms. Both pull with the same strength, because they are both the same. On the other hand, OK, we have a ring with many atoms trying to pull the negative charge off the oxygen, but they are all carbons. The oxygen wins.
    I seem to recall that the propensity of the electron 'cloud' to be stretched by an approaching electrophile also plays a role (what was it called? polarizability?), which explains for instance why many sulphur nucleophiles are quite good. E.g. I think we could say PhS- is a better nucleophile than PhO-.
    Guys pls correct me if I'm wrong.
     
  4. Nov 29, 2015 #3
    I asked a friend an hour ago, he said that resonance stabilisation in AcO- is much better because it forms equivalent resonance structures. The more equivalent the resonance structures are, the more stable the ion is.
     
  5. Nov 29, 2015 #4
    I'm not sure if your friend's statement is as general as that. Acetate has 2 resonance structures, phenoxide has 4, two of which are also 'equivalent'. So...
    But that's not the point. If you check the second sentence in my reply above, I'm arguing that thermodynamic stability alone may not always be the major determinant of nucleophilicity (kinetics).
    There are plenty of cases of more 'stable' anions that are more nucleophilic than less 'stable' ones. I mentioned sulphur derivatives. PhSH is about 2000 times more acidic in water than PhOH, but PhS- is more nucleophilic than PhO-.
    I advise you to read more on the subject (excellent online resources exist): unfortunately it's not as black and white as that, as one finds out when doing 'real' organic chemistry in a lab.
     
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