A nucleophile is a molecule or negatively charged ion with a lone pair of electrons that it can donate to a positively charged atom to form a covalent bond.
An example of nucleophilic substitution is the reaction between hydroxide and bromobutane. The bromobutane is heated under reflux, with ethanolic sodium hydroxide. Thus;
The \(OH^-\) nucleophile attacks the electron dificient carbon atom in the \(C\)-\(Br\) bond
The \(OH^-\) donates two electrons to form a dative covalent bond
The \(C\)-\(Br\) bond breaks heterolytically, and the bromine atom receives two electrons, forming a bromide ion, and is known as the leaving group
A free carbocation is not formed because the \(OH^-\) ion attacks as the \(C\)-\(Br\) bond breaks. A full headed curly arrow denotes the movement of a pair of electrons.