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Nucleophilicity with respect to the solvent

  1. Feb 28, 2016 #1
    I am currently studying nucleophilicity of molecules and am getting a set of conflicting information, so I wanted to clarify nucleophilicity trends with respect to solvent.

    The link below discusses how nucleophilicity increases down the periodic table with protic solvents and decreases down the periodic table with aprotic solvents; the main argument is that a solvating shell will form with the protic solvent, making the smaller atoms less likely to form a bond. When this effect is removed, the trend is reversed.http://www.masterorganicchemistry.com/2012/06/18/what-makes-a-good-nucleophile/

    However, during my lectures, my professor said that there is no such change in trend: the larger atoms such as I- will always be more nucleophilic than F- due to the fact that it is more polarizable and likely to donate an electron pair. The textbook that I am using, Organic Chemistry by Wade, also does not discuss this trend reversal depending on the solvent. The only discussion talks about how F- would be a poor nucleophile as it is small and holds its electrons "tightly."

    A practice problem provided during class also asked to compare the nucleophilic strength between H2O and H2S in polar aprotic solvent; similarly, the correct answer provided was that H2S would be a stronger nucleophile as it is larger and more polarizable.

    I would appreciate it if someone could clarify this matter.

    Thanks.
     
  2. jcsd
  3. Mar 1, 2016 #2

    TeethWhitener

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    The overall point is one that trips up a lot of chemistry students: thermodynamics (how deep is the well?) versus kinetics (how high is the wall?). Trends in nucleophilicity might reflect thermodynamic trends, but nucleophilicity itself is kinetic, and one must keep that in mind when thinking about these problems.

    For example, typically nucleophilicity tracks basicity: the more basic a species is, the better it is as a nucleophile. This makes sense from the point of view of Lewis basicity, where a base is simply an electron pair donor. (I believe this was probably your professor's point.) But remember that basicity is thermodynamic, whereas nucleophilicity is kinetic. This means that even though the product with the more basic species may ultimately be the most favored species thermodynamically (lowest overall energy), there might be a large kinetic effect (activation barrier) to prevent the reaction from happening. This is, for example, what you usually observe in the case of fluoride as a nucleophile. Fluoride ion is quite a strong base compared with the other halides (think about the strengths of their conjugate acids). In addition, the formation of an extremely strong C-F bond is usually highly energetically favorable. But fluoride can also form very strong intermolecular bonds (and equilibrium proton-sharing) with protic solvents. This means that even though, from a thermodynamic viewpoint, nucleophilic substitution should be favorable, in reality, fluoride doesn't react readily because it's locked up in its solvent cage, which has to be broken before the fluoride can react (a kinetic barrier to reaction). On the other hand, in an aprotic solvent, there's a much lower kinetic barrier because the solvent molecules are not as tightly bound to the fluoride, so fluoride tends to react faster.

    (Brief aside: the unsolvated fluoride ion is predicted to be an incredibly strong base/nucleophile and quite interesting from a theoretical standpoint. Trying to realize an essentially "naked" fluoride ion for use in reactions is an active area of research right now.)
     
  4. Mar 2, 2016 #3

    DrDu

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    I found a database with nucleophilicity indices:
    http://www.cup.lmu.de/oc/mayr/reaktionsdatenbank/

    There are also data for bromide and chloride in water, ethanol and MeCN.
    While Br- is more nucleophilic in water or ethanol than Cl-, this is only marginally so in MeCN.
     
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