A substitution reaction is a type of chemical reaction in which one atom or group of atoms in a molecule is replaced by another atom or group of atoms. Essentially, something in the molecule is “substituted” with something else.
There are two main types of substitution reactions:
- Nucleophilic Substitution (common in organic chemistry)
- A nucleophile (electron-rich species) replaces a leaving group in a molecule.
- Example: In alkyl halides (like CH₃Br), a hydroxide ion (OH⁻) can replace the bromide ion (Br⁻) to form an alcohol (CH₃OH).
- Mechanisms:
- SN1 (unimolecular) – involves a two-step process with a carbocation intermediate.
- SN2 (bimolecular) – involves a single-step process where the nucleophile attacks as the leaving group leaves.
- Electrophilic Substitution (common in aromatic chemistry)
- An electrophile (electron-deficient species) replaces a hydrogen atom on an aromatic ring.
- Example: In benzene, a nitronium ion (NO₂⁺) can replace a hydrogen atom to form nitrobenzene (C₆H₅NO₂).
Key points:
- One group leaves; another group takes its place.
- Often involves a leaving group (the part that is replaced).
- Common in organic reactions but also occurs in inorganic chemistry (like halogen exchange reactions).