Leaving Groups Are Nucleophiles Acting In Reverse

by James

in Alkyl Halides, Organic Chemistry 1, Understanding Electron Flow

We’ve covered “what makes a good leaving group” before – the weaker the base, the better the leaving group. In this short post, I just want to make an observation. Note how similar nucleophiles are to leaving groups:

  • A nucleophile donates a pair of electrons
  • A leaving group accepts a pair of electrons

Both contain a pair or electrons, and therefore both can act as Lewis bases. What determines whether one group is a nucleophile or a leaving group in a given reaction is often a question of basicity. Just like in acid base reactions, where we go from stronger base to weaker base, in nucleophilic substitution reactions, we generally go from stronger base (nucleophile) to weaker base (leaving group) as well.

So you can think of a leaving group as a nucleophile acting in reverse.

Next: now that we’ve talked about the components of substitution reactions, let’s look into what experiments tell us about how they might work.

Next Post: Two Types of Substitution Reactions

P.S. There are a few exceptions to substitution reactions being based solely on “base strength” considerations, of course. One interesting one is based on different solubilities. The Finkelstein reaction is a way of making alkyl iodides from alkyl chlorides. There is not a tremendous difference between the leaving group ability of chloride ion or iodide ion. However, there is a significant difference between the solubility of NaCl (or NaBr) and NaI in acetone. NaI is soluble in acetone; NaCl is not. Therefore as NaCl is formed, it precipitates out of solution, driving the reaction to the right.


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{ 4 comments… read them below or add one }


Can you explain more how solubility effects as driving force?



Imagine two large entrances to a building through which people are going through one after the other continuously, i.e. in and out of the building. Now visualize that police are on the outside and are capturing people and keeping them outside. In that case, there will be fewer people going into the building than are coming out. That’s what’s happening here. The insolubility of NaCl in acetone removes the Cl- species from solution and it can no longer participate in the reaction. I- however can remain in solution and Cl- is substituted on the alkyl by I- because there are less Cl- ions in solution than I- which disturbs the equilibrium.



Would the Finkelstein reaction be feasible in a polar protic solvent, since the iodide ion is a stronger nucleophile than the chloride ion (owing to solvation effect)?



Maybe, but why would you want to do that? In acetone, the reaction goes to completion because NaCl crashes out of solution. If both NaCl and NaI are present in solution, my guess is that you’ll likely get a mixture. The difference in nucleophilicity isn’t that big. That said, I should look this up in March before shooting off my mouth.


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