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Understanding Lewis Acids and Bases

Apr 29, 2025

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

  • For more information, explore the BYJUS website and mobile app.