Introduction to Substitution Reactions

General Nucleophilic Substitution Reactions

• A substitution reaction occurs at an sp3 carbon center and displaces a leaving group with an incoming nucleophile.
• Nucleophiles are species which have lone pairs which they can use to form bonds with the electrophile. Examples include $\rm OH^-$$\rm H_2O$ , $\rm NH_2R$, etc.
• Electrophiles are organic molecules which contain an sp3 carbon bound to an electronegative element which can be displaced easily (the leaving group).
• Typical leaving groups include $\rm X^-$, where $\rm X$ is a halogen, $\rm H_2O$, $\rm ROH$ and sometimes $\rm OH^-$ and $\rm OR^-$. Good leaving groups are weak bases.

Substitution reaction

Steps for Identifying Species in a Substitution Reaction

1. Identify the leaving group on the products side
2. The other group on the products side must be the product
3. The species on the reactant side which contains the atoms of the leaving group is the electrophile
4. The other molecule on the reactant side must be the nucleophile