SN1 - First-order Nucleophilic Substitution
Introduction to Reactions
Fluorine, chlorine, and bromine (X) possess a higher electronegativity than carbon does. As a result, the bonding electrons of C-X bonds are unevenly distributed. The carbon atom is partially positively charged, while the halogen atom carries a partial negative charge. In other words, the carbon atom is positively polarized (δ+), whereas the halogen atom is negatively polarized (δ-).
The polarity of carbon-halogen bonds forms the basis of two frequently found reaction types of this compound family, namely that of substitution reactions and eliminations. With respect to kinetics, substitutions, as well as eliminations, may be classified into two categories:
- Second-order reactions. Reactions of primary and secondary alkyl halides are usually second-order reactions.
- First-order reactions. Tertiary alkyl halides, for instance, normally react in first-order reactions.
Substitution reactions are further explained in the following.
In both kinetic cases of substitutions a ligand of a substrate (electrophile) is substituted by a nucleophile. Therefore, these chemical reactions are called nucleophilic substitutions (SN). The reaction mechanisms of first-order and second-order nucleophilic substitutions are quite different:
Second-order nucleophilic substitution: A nucleophile attacks a positively polarized carbon atom. The attack of the nucleophile results in the heterolytic cleavage of the carbon-ligand bond, whereat the bonding electron pair is completely passed on to the ligand (X). The substrate, along with the nucleophile, participates in the rate-determining step. Thus, the reaction rate depends on both the substrate's and the nucleophile's concentration. Therefore, this reaction type is called bimolecular nucleophilic substitution, or reaction.
First-order nucleophilic substitution: The carbon-ligand bond is cleaved independently, forming an anion (nucleofuge) and a carbocation (a)). The attack of the nucleophile on the carbocation then yields the substitution product (b)). The rate-determining step is the spontaneous cleavage of the carbon-ligand bond. Thus, only the substrate participates in the rate-determining step. The reaction rate depends solely on the substrate's concentration. Therefore, this reaction type is known as the monomolecular nucleophilic substitution, or reaction.
The question of which mechanism occurs in a given case mainly depends on four parameters. Thus, the mechanism may be controlled by varying them:
- The structure of the substrate.
- The reactivity and structure of the nucleophile.
- The chemical qualities of the leaving group (the ligand that is substituted).
- The solvent applied.