Lewis Acids and Bases

Overview

• Lewis Theory: Describes acids as electron-pair acceptors and bases as electron-pair donors.
• Lewis Adduct: The product of a Lewis acid and base reaction, forming a coordinate covalent bond.
• Named after chemist Gilbert Newton Lewis.

Lewis Acid

• Definition: Chemical species with empty orbitals that accept electron pairs.
• Structure: Often trigonal planar with an empty p-orbital.
• Examples:
  • H+ ions and onium ions like H3O+
  • Cations of d block elements with high oxidation states (e.g., Fe3+)
  • Metal cations such as Mg2+ and Li+
  • Carbocations (e.g., H3C+)
  • Pentahalides of group 15 elements (e.g., Antimony, Arsenic, Phosphorus)
  • Electron-deficient systems (e.g., enones)

Lewis Base

• Definition: Chemical species with a highly localized HOMO capable of donating an electron pair.
• Examples:
  • Ammonia, alkyl amines, and conventional amines
  • Pyridine and its derivatives
  • Compounds with Oxygen, Sulphur, Selenium, Tellurium at oxidation state -2
  • Simple anions (e.g., H-, F-) and complex anions (e.g., sulfate anion SO4^2-)
  • Electron-rich π-systems (e.g., benzene, ethyne, ethene)

Chemical Reactions Between Lewis Acids and Bases

• H+ and H2O Reaction: Forms hydronium ion (H3O+), with oxygen donating an electron pair to the proton.
• H+ and NH3 Reaction: Forms ammonium ion (NH4+), with nitrogen donating an electron pair.
• Ag+ and NH3 Reaction: Forms Ag(NH3)2+, with nitrogen donating electron pairs to silver ion.
• Fluoride Ion and Boron Trifluoride: Forms BF4-, with fluoride donating an electron pair to boron trifluoride.

Applications

• Catalysts:
  • Lewis acids in Friedel-Crafts reactions (e.g., AlCl3)
  • Encourage cationic or pseudo-cationic reactions in organic chemistry
• Metallic Catalysts:
  • Lewis bases modify selectivity and activity
  • Used in asymmetric catalysis for enantioselective synthesis
  • Multidentate Lewis bases or chelating agents have wide industrial and agricultural applications.

Frequently Asked Questions

• Definition: Acids accept electron pairs, bases donate them.
• Hydrochloric Acid: Not a Lewis acid; does not accept electron pairs.
• Metal Lewis Acids: Li+, Mg2+ can accept electron pairs, forming coordination compounds.
• Examples of Lewis Bases: Amines, fluoride ion, ammonia, water, acetone.
• Ethyl Acetate as Lewis Base: Can donate an electron pair, thus acts as a Lewis base.

Additional Learning

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