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.
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.