What does this mean for nucleophiles and electrophiles? Well, in general, fi lled orbitals tend to be low in energy—that is after all why they are fi lled! Conversely, empty orbitals tend to be high in energy. So, the best interaction (the one that gains the new molecule the most energy) is likely to be between the highest in energy of all the fi lled orbitals—an orbital we can term the ‘highest occupied molecular orbital’ or HOMO for short—and the lowest in energy of all of the unfilled orbitals—the ‘lowest unoccupied molecular orbital’ or LUMO for short. This diagram may help clarify this idea—it’s a repeat of the best interaction above (the one on the left), but with other orbitals sketched in.

Remember, we can ignore all interactions between pairs of fi lled orbitals (bonding and antibonding cancel out, see p. 94) and pairs of unfilled orbitals (they don’t contain electrons so don’t contribute to the stability of the molecule). Of the interactions that are left, the one that gains the molecule the most energy is between the LUMO of the electro phile. and the HOMO of the nucleophile. To make these orbitals as close as possible in energy, we want the nucleophile to have a high-energy HOMO and the electrophile to have a low-energy LUMO.

• The best nucleophiles have high-energy occupied molecular orbitals (HOMOs).

• The best electrophiles have low-energy unoccupied molecular orbitals (LUMOs).

The very fi rst stage in understanding any reaction is to work out which of the reacting mol ecules is the nucleophile and which is the electrophile. It is impossible to stress too much how important it is to be able to identify nucleophiles and electrophiles correctly. For this reason, we’ll now conduct an identity parade of each class. We’ll show you some of the top performing nucleophiles and top performing electrophiles, with a few comments on why they are so good at what they do, before we move on to see them in action.