To initiate a substitution reaction, a minimum of a nucleophile and a substrate are necessary. Collision orientation between reacting species is crucial for the formation of new bonds in the product. The substitution can proceed via unimolecular or bimolecular mechanisms.
In order for a substitution reaction to take place, at a minimum, a nucleophile and a substrate are needed. A substitution reaction is a type of chemical reaction in which an atom or a group of atoms in a molecule is replaced by another atom or group of atoms. An example of a substitution reaction is when alkyl halides are formed by the substitution of a halogen atom for a hydrogen atom in an organic compound.
For a substitution reaction to proceed, it requires that the reacting species must collide in an orientation that facilitates the correct interaction. This alignment must allow contact between the atoms that will form new bonds in the product. Substitution reactions can either proceed as unimolecular (S1), involving a single reactive species in the rate-determining step, or bimolecular (S2), involving two reactants in the rate-determining step. The elementary step of an S2 reaction is bimolecular, involving both the incoming nucleophile and the substrate.