Solvents in Nucleophilic Substitution Reactions

Classes of Solvents

1. Protic Solvents

• Definition: Contain acidic protons (protons attached to an electronegative element).
• Examples: Ammonia, water, alcohols.
• Characteristics:
  • Polar solvents.
  • Good for nucleophilic substitution due to polarity matching reactants (polar/charged substrates and nucleophiles).
  • Form hydrogen bonds, aiding solvation of nucleophile and substrate.
  • Effect on Nucleophilicity: Solvation can weaken nucleophilicity as solvated nucleophiles cannot react.
    • Solvation involves coating the nucleophile with solvent molecules.
    • Stronger solvation means weaker nucleophile.
    • Smaller nucleophilic ions or atoms are more strongly solvated due to closer approach of solvent and stronger hydrogen bonds.
    • Trends: Larger ions are better nucleophiles, especially in protic solvents.

2. Polar Aprotic Solvents

• Definition: Polar but do not contain acidic protons.
• Examples: Acetonitrile, dimethylformamide, acetone.
• Characteristics:
  • Good for nucleophilic substitution as they maintain solubility of polar substrates and nucleophiles.
  • Do not form hydrogen bonds with nucleophiles, so do not affect nucleophilicity.

3. Nonpolar Aprotic Solvents

• Definition: Nonpolar and do not have acidic protons.
• Examples: Hexanes, benzene, toluene.
• Characteristics:
  • Do not affect nucleophilicity.
  • Poor solvents for nucleophilic substitution due to low solubility of reactants in nonpolar solvents.