Reactions at ${\text{sp}}^3$-Hybridized Carbon: Substitution and Elimination

Fundamentally, organic compounds that contain a carbon with a more electronegative substituent display two types of reactions: nucleophilic substitution reactions and elimination reactions. In nucleophilic substitutions, the electronegative substituent (leaving group) is exchanged (substituted) for another substitutent (nucleophile). In eliminations, however, the electronegative substituent is released from the organic molecule along with another atom or group which is usually a hydrogen from the vicinal carbon atom.

Alkyl halides (R-X) are representative examples of such organic compounds. The leaving groups in nucleophilic substitutions and eliminations with alkyl halides are the halide anions (${\text{X}}^{-}$) ${\text{Cl}}^{-}$, ${\text{Br}}^{-}$ and ${\text{I}}^{-}$. ${\text{F}}^{-}$ virtually never functions as a leaving group.

Substitution reactions are particularly useful in organic chemistry because they enable synthetically easily available alkyl halides to be converted into multiple other organic compounds.

Exercise