As mentioned earlier, a good electrophile must be able to accommodate a new sigma bond between its electrophilic center and the nucleophile. When the electrophilic center is an atom with an incomplete octet, this is no problem.
For electrophiles containing polarized pi bonds such as carbonyl groups, at least one resonance form shows an atom with an incomplete octet.
resonance forms of acetone
You can use either resonance structure to write the reaction between the nucleophile and the electrophile. The following are acceptable representations of a nucleophilic attack of hydroxide ion on acetone, but the second one makes it more apparent that the central carbon can take the extra bond.
Two acceptable representations of the nucleophilic attack of hydroxide ion on acetone using different resonance structures.
Sometimes the substrate has an electrophilic atom which is sp3-hybridized and already has a complete octet. In this case there are no pi electrons to displace as the new σ-bond forms. The nucleophile must displace another group as it bonds to the electrophile. The displaced group is called a leaving group. The leaving group can be displaced only if it leaves as a weak base, because weak bases are stable molecules that can take the electrons with them. In the following example, hydroxide ion is the attacking nucleophile. As it bonds to the sp3 electrophilic carbon, it must displace another group. The leaving group in this case is the bromine atom. It is a good leaving group because it leaves as bromide ion, which is a weak base and can take the electrons with it.
The reverse reaction, however, could not happen. Although bromide is a good nucleophile and methyl alcohol contains an electron deficient center (the carbon bonded to oxygen), the molecule does not contain a good leaving group. Hydroxide ion is a strong base, therefore it cannot be displaced by bromide.
Although the hydroxide ion is not a good leaving group, it is possible to do nucleophilic displacements on alcohols by protonating them with acid first. The protonated hydroxyl group is a potential water molecule, which is a weak base and therefore a good leaving group.
This approach has limitations. The most important is that the nucleophile must be a weak base, or it will prefer to react with the acidic protons. For all practical purposes, the only nucleophiles that can be used in this way are chloride and bromide ions. But this provides a good way to convert alcohols into primary, secondary, or tertiary chlorides and bromides.
Notice that in these case the reverse reaction can happen. Water is a good nucleophile and chloride and bromide are good leaving groups. One must isolate the product as it forms to keep it from reacting with water and go back to alcohol.