6.2: Nucleophilic Substitution Reactions
In 1896, the German chemist Paul Walden discovered that he could interconvert pure enantiomeric (+) and (-) malic acids through a series of reactions. This conversion suggested the involvement of optical inversion during the substitution reaction. Further, in 1930, Sir Christopher Ingold described for the first time two different forms of nucleophilic substitution reactions, which are known as SN1 (nucleophilic substitution unimolecular) and SN2 (nucleophilic substitution bimolecular) reaction.
Nucleophilic substitution reaction
The word “substitution” is derived from the Latin word “substituō,” which means “to take the same place”. Nucleophilic substitution reactions are reactions in which a nucleophile, a Lewis base, reacts with an electrophile, a Lewis acid. The nucleophile substitutes the halogen atom bonded to the carbon of the molecule, releasing a stable ion called the leaving group. These reaction motifs are very similar to the Lewis acid/base reactions and involve very similar species:
• The electron-rich species analogous to the Lewis base is the nucleophile.
• The electron-deficient species analogous to the Lewis acid are electrophile.
Factors affecting the nucleophilic substitution reaction
Various factors govern the pathway of the nucleophilic substitution reaction:
- Nature of the substrate (primary, secondary, and tertiary alkyl halides)
- Strength of the nucleophile
- Strength of the electrophile
- Nature of the leaving group
- Solvent (protic vs. aprotic solvent)