Nucleophilicity
A chemical species, or a region of a molecule, is NUCLEOPHILIC when it has an excess of electron density and thus tends to attack chemical species or regions of a molecule that are poor in electron density.
▶ Watch the video on YouTube
Reactions between molecules can be understood in terms of their electron distribution. Electron density is to molecules what atmospheric pressure is to the Earth: weather is explained by low-pressure (storm) and high-pressure (anticyclone) zones. Electronegative atoms concentrate electron density and leave their surroundings depleted of it.
A NUCLEOPHILIC species or region of a molecule will attack species or molecular regions that are poor in electron density.
NUCLEOPHILE literally means 'nucleus-loving', i.e. attracted to positive charges.
Does this reaction make sense to you?
Electron density distribution in methyl chloride (red = high; blue = low)
The hydroxide anion (HO−) comes from water by loss of a proton and has a high electron density. It therefore acts as the nucleophile (red). It reacts with methyl chloride by attacking the methyl end of the molecule, which, because of the highly electronegative Cl atom, has a significant lack of electron density. The C atom therefore acts as the electrophile (blue). As a result, the C-Cl bond breaks and the C-O bond forms. The C-O bond is stronger than the C-Cl bond, so the energy balance is favourable.