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$3$ ⇆ Ni(NH$3$)$_6^{2+}$(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$2$O) acts as the base and initiates the reaction with carbonic acid (H$2$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$3$ and H$2$O

  • Reaction:

    • NH$3$(aq) + H$2$O(l) ⇌ NH$_4^{+}(aq) + OH^{-}(aq)

    • Classification:

    • Base (Nucleophile): NH$_3$

    • Acid (Electrophile): H$_2$O

Arrow-Pushing For NH$3$ and H$2$O Reaction

  • Follow the convention:

    • 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.