Substitution reaction

Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical, and whether the substrate is aliphatic or aromatic.

One of them breaks a C–H covalent bond in CH4 and grabs the hydrogen atom to form the electrically neutral HCl.

The electron pair (:) from the nucleophile (Nuc:) attacks the substrate (R−LG), forming a new covalent bond Nuc−R−LG.

The prior state of charge is restored when the leaving group (LG) departs with an electron pair.

In the second step, the nucleophilic reagent (Nuc:) attaches to the carbocation and forms a covalent sigma bond.

If the substrate has a chiral carbon, this mechanism can result in either inversion of the stereochemistry or retention of configuration.

If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon center, the substitution will involve an SN1 rather than an SN2.

Aromatic substitution occurs on compounds with systems of double bonds connected in rings.

[4][5] Associative substitution, for example, is typically applied to organometallic and coordination complexes, but resembles the Sn2 mechanism in organic chemistry.

Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry.

The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate-determining step.

Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution.

The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected.