Lecture on Lewis Acids and Bases

Definitions

• Lewis Acid: An electron pair acceptor.
• Lewis Base: An electron pair donor.

Examples

Lewis Acids (Electron Pair Acceptors)

• Ions with positive charges:
  • H$^+$
  • Fe$^{2+}$
  • Al$^{3+}$
• Other examples:
  • ZnCl$_2$
  • BH$_3$
  • FeBr$_3$
  • AlCl$_3$
  • Methyl carbocation

Lewis Bases (Electron Pair Donors)

• Ions with negative charges:
  • Chloride (Cl$^-$)
  • Bromide (Br$^-$)
• Other examples:
  • Hydroxide (OH$^-$)
  • Iodide (I$^-$)
  • Water (H$_2$O)
  • Ammonia (NH$_3$)
  • Carb anion
• Characteristic: Nucleophiles
  • Electron rich

Amphoteric Substances

• Water (H$_2$O)
  • Can act as both a Lewis acid and a Lewis base.

Reactions Involving Lewis Acids and Bases

General Reaction Mechanism

• The electron pair is donated from the Lewis Base to the Lewis Acid.
• Curved arrows in reaction mechanisms indicate the movement of electron pairs.

Example Reactions

1. Ammonia (NH$_3$) and Boron Trifluoride (BF$_3$)

   • NH$_3$: Lewis Base (donates electron pair)
   • BF$_3$: Lewis Acid (accepts electron pair)
   • Product: Covalent bond formed, nitrogen gets a positive charge, and boron gets a negative charge.

2. Iron(III) Chloride (FeCl$_3$) and Chloride Ion (Cl$^-$)

   • Reaction forms FeCl$_4^-$.
   • Iron atom ends up with a negative formal charge.

3. Zinc Chloride (ZnCl$_2$) and Water (H$_2$O)

   • Oxygen from water attacks zinc.
   • Product: Trigonal planar shape around zinc, oxygen gets a positive charge, and zinc gets a negative charge.
   • Oxygen typically gets a positive charge if it forms three bonds.

4. Aluminum Tribromide (AlBr$_3$) and Bromide Ion (Br$^-$)

   • Forms AlBr$_4^-$.
   • Tetrahedral molecular geometry, aluminum atom carries a negative charge.

Key Takeaways

• Electrophiles

  • Lewis acids: Electron poor.

• Nucleophiles

  • Lewis bases: Electron rich.

• Understanding Lewis Acid-Base Reactions

  • Reactions are based on the transfer of electron pairs.
  • Typically result in a change in the formal charges of the atoms involved.