A nucleophile was defined as a species that donates two electrons to carbon to form a new covalent σ-bond. Using a Lewis base analogy, a nucleophile is a two-electron donor, but the product is not an “ate” complex. Nucleophiles react with a variety of molecules that have electrophilic carbon atoms in a reaction where one atom or group replaces another. When iodide and bromomethane collide, the energy transferred must be equal to the energy of activation in order to initiate a SN2 reaction. The polar and protic water can be contrasted with an aprotic solvent. The SN2 reaction is one of the more important transformations in organic chemistry since one functional group can be transformed into a different functional group. Halide ions are important nucleophiles to interconvert the halogen of alkyl halides.