Lewis Acid-Base Reactions Study Notes

CHAPTER 7: LEWIS ACID-BASE REACTIONS

Electron Pair Transfer

Lewis Acids and Bases

Definitions

• LEWIS ACID: An electron pair acceptor, characterized as an electrophile.

• LEWIS BASE: An electron pair donor, characterized as a nucleophile.

• Electron deficient substance: Typically a Lewis acid.

• Electron rich substance: Typically a Lewis base.

Classifications of Lewis Acids and Bases

Lewis Acids

• Electron deficient substances include:

  • Cations: Examples include Al$^{3+}$.

  • Central atom with an incomplete octet: For instance, BF$_3$.

  • Central atom capable of having more than 8 valence electrons: Caused by empty d orbitals, e.g., SiF$_4$.

  • Molecules with multiple bonds between atoms of different electronegativities: Example includes CO$_2$.

Lewis Bases

• Electron rich substances include:

  • Anions: Example is Cl$^{-}$.

  • Neutral species that possess lone pair electrons: Generally, neutral species such as NH$_3$ are classified here.

Identifying Lewis Acids and Bases

Examples of Lewis Acid-Base Reactions

1. F$^{-}$(aq) + x ⇆ BF$_4^{-}$(aq)

   • Lewis acid: BF$_4^{-}$

   • Lewis base: F$^{-}$

2. Ni$^{2+}$(aq) + 6 NH$<em>3$ ⇆ Ni(NH$</em>3$)$_6^{2+}$(aq)

   • Lewis acid: Ni$^{2+}$

   • Lewis base: NH$_3$ (six molecules form the coordination complex)

Characteristics of Lewis Acids and Bases

• Lewis acids are characterized as:

  • [+] charged

  • Containing an electron deficient atom.

• Lewis bases are characterized as:

  • [-] charged

  • Containing lone pairs of electrons.

Relationship Between Various Acid-Base Theories

Lewis Acid-Base Theory

• Hierarchy of From Lewis Acid-Base Theory:

  • Lewis Acid-Base theory encompasses both Bronsted-Lowry and Arrhenius theories.

  • Any Bronsted-Lowry acid can be classified as a Lewis acid.

  • Any Bronsted-Lowry base can be classified as a Lewis base.

Comparison: Lewis vs. Bronsted-Lowry

• Lewis Acid-Base Theory:

  • Views the base (Lewis base) as the initiator of the reaction.

• Bronsted-Lowry Acid-Base Theory:

  • Views the acid (Bronsted-Lowry acid) as the initiator of the reaction.

Mechanism of Acid-Base Reactions According to Lewis

• The nucleophile attacks the electrophile.

• In an example reaction, water (H$<em>2$O) acts as the base and initiates the reaction with carbonic acid (H$</em>2$CO$_3$).

Arrow-Pushing Conventions in Acid-Base Reactions

• General rules for arrow-pushing include:

  • A negative charge attacks a positive charge (base attacks acid).

  • Electron-rich species attack electron-poor species, illustrating the nucleophile (BASE) attacking the electrophile (ACID).

Example Reaction Analysis of NH$<em>3$ and H$</em>2$O

• Reaction:

  • NH$<em>3$(aq) + H$</em>2$O(l) ⇌ NH$_4^{+}(aq) + OH^{-}(aq)</p></li><li><p><strong>Classification</strong>:</p></li><li><p>Base (Nucleophile): NH$_3$</p></li><li><p>Acid (Electrophile): H$_2$O</p></li></ul></li></ul><h4 id="1a1e1e3e-5cca-4b20-80ba-f888e6e3de4e" data-toc-id="1a1e1e3e-5cca-4b20-80ba-f888e6e3de4e" collapsed="false" seolevelmigrated="true">Arrow-Pushing For NH$3$ and H$2$O Reaction</h4><ul><li><p>Follow the convention:</p><ul><li><p>Negative (from NH$3$) attacks positive (from H$2$O).

• Summary of Arrow-Pushing Convention:

  • Negative attacks positive (Base to Acid)

  • Nucleophile attacks electrophile.

• Implications in this convention highlight that electron pairs attack empty orbitals providing mechanistic insight into Lewis acid-base reactions.

Conclusion and Practical Application

Worksheet Time

• Interactive engagement to enhance understanding of Lewis acid-base reactions through practical exercises based on the content discussed above.