SN1 - First-order Nucleophilic Substitution

Introduction to $SN1$ Reactions

Fig.1
Charge distribution in alkyl halides.

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 , substitutions, as well as eliminations, may be classified into two categories:

1. Second-order reactions. Reactions of primary and secondary are usually second-order reactions.
2. 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 () is substituted by a . 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 $SN2$ reaction.

Fig.2
$SN2$ 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 $SN1$ reaction.

Fig.3
$SN1$ 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.

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