Organic Chemistry: Types of Reactions

Overview of Reactions

• Four main types of reactions in organic chemistry:
  1. Addition reactions
  2. Elimination reactions
  3. Substitution reactions
  4. Rearrangement reactions

Key Reaction Types

1. Addition Reactions

• Definition: Adding atoms/groups to a molecule.
• Example: Converting an alkyne (triple bond) to an alkene (double bond) or alkene to alkane (single bond).
• Mechanism:
  • E.g., Hydrocarbon + H₂ (with metal catalyst) → Alkane.

2. Elimination Reactions

• Definition: Removing atoms/groups from a molecule, resulting in a double bond formation.
• Example: 2-butanol + concentrated H₂SO₄ (heat) → Alkene.
• Mechanism:
  • Remove H and OH → Creates water (dehydration reaction).

3. Substitution Reactions

• Definition: Replacing one atom/group with another.
• Example: 2-chloropentane + H₂O → 2-petanol.
• Mechanism:
  • Replace Cl with OH.

4. Rearrangement Reactions

• Definition: Movement of atoms/groups within a molecule to form a more stable structure.
• Example: Secondary carbocation adjacent to tertiary carbon will rearrange for stability.
• Mechanism:
  • Hydride or methyl shifts to create a more stable carbocation.

Practice Problems

Example: Reaction of 1-butene with HBr

• Type: Addition Reaction
• Mechanism: H adds to primary carbon, Br adds to secondary carbon → Product: 2-bromobutane.

Example: Reaction of Cyclopentanone with NaBH₄

• Type: Nucleophilic Addition Reaction
• Mechanism: NaBH₄ donates H⁻ → Alkoxide → Alcohol.

Example: Secondary Alkyl Halide with Water

• Possible products: Alkene (elimination, E1) or Alcohol (substitution).
• Mechanism (E1): Leaving group → Carbocation → Base abstracts proton → Double bond formation.

Example: Tert-butyl Bromide with Methanol

• Type: SN1 reaction
• Mechanism: Leaving group departs → Carbocation formation → Nucleophile attacks → Ether formation.

Specific Reaction Types

Electrophilic Aromatic Substitution (EAS)

• Example: Benzene + HNO₃ + H₂SO₄ → Nitrobenzene.
• Mechanism: Addition of electrophile → Elimination of H → Overall substitution reaction.

Nucleophilic Aromatic Substitution (NAS)

• Example: Chlorobenzene with NaOH → Phenol.
• Mechanism: Addition of OH group → Elimination of Cl.

Free Radical Substitution

• Example: Alkane with Br₂ (uv light) → Alkyl bromide.
• Mechanism: Initiation (bond cleavage) → Propagation (radical reactions).

Conclusion

• Main Types of Reactions:
  • Addition, Elimination, Substitution, Rearrangement.
• Understanding variations (e.g., electrophilic addition, nucleophilic substitution) is key for organic chemistry.