SN2 Intramolecular Reactions

Key Concepts

Intermolecular vs. Intramolecular Reactions

• Intermolecular Reaction: Reaction between two different molecules.
  • Example: Hydroxide reacting with methyl bromide.
• Intramolecular Reaction: Reaction where both nucleophile and leaving group are in the same substrate.
  • Often results in the formation of rings.

Intramolecular SN2 Reaction Example

1. Initial Reaction

   • Strong base (e.g., hydride ion) removes acidic proton from OH group.
   • Forms an alkoxide ion (nucleophile) within the molecule.

2. Reaction Mechanism

   • Nucleophile attacks carbon, displacing the leaving group.
   • Formation of a cyclic product.
   • Example discussed forms a six-membered ring, a cyclic ether.
   • Steps:
     1. Deprotonation by hydride.
     2. Nucleophilic attack forms a ring structure.

Example Problem

1. Initial Setup

   • Reactant with sodium hydride.
   • Hydride ion removes proton forming alkoxide ion.

2. Stereochemistry Considerations

   • Nucleophile (e.g., negatively charged oxygen) must approach from the back.
   • Bromide must be on the opposite side (anti conformation) for reaction to proceed.

3. Outcome

   • Formation of a three-membered ether known as epoxide.
   • Requires anti positioning of nucleophile and leaving group.

Additional Example

1. Reactant: 1,5-bromo pentane with ammonia.
2. Product Formation
   • Ammonia acts as a nucleophile (weak base).
   • Primary alkyl halide undergoes an SN2 reaction.
   • Process overview:
     1. Nucleophilic attack by ammonia displaces bromine.
     2. Formation of a positively charged nitrogen intermediate.
     3. Deprotonation to form a nucleophilic NH2 group.
     4. Intramolecular attack to form a six-membered ring with nitrogen.
   • Final product: Cyclic amine.