Factors that contribute to nucleophilicity

There are four key factors that contribute to a species' nucleophilicity:

  • Charge A nucleophile reacts by donating electrons. This means, that the higher the electron density on a species, the more nucleophilic it is, all other things being equal. In general, a negatively charged species will be more nucleophilic than its neutral counterpart.

  • Size A nucleophile donates electrons to an electrophile, but to do so it needs to be in close proximity to the electrophile. This can be hard if the nucleophile is a large and bulky molecule. In general, a smaller nucleophile is a stronger nucleophile!

  • Electronegativity Highly electronegative atoms have a high electron density, but they also have a high electron affinity, meaning that they attract the electrons strongly. A nucleophile reacts by donating electrons, which a highly electronegative atom is less willing to do. Therefore, a less electronegative atom is more nucleophilic, all other things being equal.

  • Solvent Solvents can be either protic or aprotic. A protic solvent can participate in hydrogen bonding with the nucleophile, which causes the nucleophile to be cushioned by the solvent. This makes the nucleophile less reactive, than if it was dissolved in an aprotic solvent.