5._Lewis_Acid_Base_-_Filled-In

Chapter 6: Acid Base Chemistry

Overview

• Focus on sections 6.4 and 6.6 of "M,F&T".

• Review sections 6.1 to 6.3 on Brønsted Acids and Bases.

Brønsted Acids and Bases

Definition:

• Acid: A species that donates a proton (H+), for example, hydrofluoric acid (HF).

• Base: A species that accepts a proton, such as hydroxide ion (OH⁻).

HA Acidity Trends

General Trends:

• The acidity of Brønsted acids increases with the increasing electronegativity (X) of element A.

  • Example Trend: Ammonia (NH3) is less acidic than hydrofluoric acid (HF).

Influence of Size:

• Larger atoms can hold lower charge density due to their larger orbitals.

  • Trend: Water (H2O) < Hydrogen sulfide (H2S) < Hydrogen selenide (H2Se); as the atomic size increases, the acidity increases.

Acidity of Oxyacids

Conjugate Base Stability:

• The stability of the conjugate base correlates with the number of resonance structures; more resonance structures lead to a more stable conjugate base and therefore stronger acids.

  • Stronger acids have a more delocalized negative charge distributed over multiple atoms.

Lewis Acid Base Chemistry

Terminology:

• Lewis Acid (LA): An electron-pair acceptor.

• Lewis Base (LB): An electron-pair donor, typically neutral.

• Adduct: The product formed from the interaction between a Lewis Acid and a Lewis Base.

• Dative Bond: A bond formed when both electrons in a shared pair originate from the same atom (LB). Breaking a dative bond results in returning the electrons to the Lewis Base.

Direction of Electron Flow:

• The flow of electrons in Lewis acid-base reactions is illustrated with arrows to denote the transfer between LA and LB.

Examples of Dative Bonds

Lewis Acid-Base Examples:

• Ammonia reacts with Ruthenium to form a dative complex: NH3 + Ru → H3N-NH3.

• Neutral silver ions (Ag) can also form complexes with ligands like ammonia (NH3).

MO Diagrams of LAB Bonding

Energy Levels:

• Energy diagrams are utilized to show the mixing of orbitals between LA and LB.

• In the Lewis model, adduct formation is represented using molecular orbital (MO) diagrams.

Ammonia Borane Adduct Formation

Chemical Reaction:

• The reaction between borane (BH3) and ammonia (NH3) leads to the formation of a dative bond.

• The Lewis structure of this adduct shows reactivity linked to empty p-orbitals and shared electrons.

Lumo MO Diagrams of BH3 and NH3

• Diagrams depict bonding interactions within Lewis adducts by comparing atomic orbitals.

Quantifying Lewis Basicity

Methods:

• One can calculate the equilibrium constant (KBA) using log(KBA) values.

• Measure the affinity of Lewis Acids, such as BF3 in non-polar solvents.

• Theories discuss aspects of exothermic reactions and the enthalpy of adduct formation.

Influence of Substitution on Acidity/Basicity

Inductive Effects:

• Electron Withdrawing Groups (EWG): Reduce acidity of Lewis Acids and decrease basicity of Lewis Bases.

• Electron Donating Groups (EDG): Increase acidity of Lewis Acids and enhance basicity of Lewis Bases.

  • Example: B(CH3)3 is less acidic than BHy.

Steric Effects of Substituents:

• Steric hindrance from larger substituents can destabilize dative bonds, which in turn affects the formation of adducts.

• Easier adduct formation occurs in scenarios with minimal steric clash, like when comparing CH3 with H groups.

Frustrated Lewis Pairs (FLPs)

Steric Restrictions:

• An example is the interaction between B(C6F5)3 and P(tBu)3, where adduct formation is impeded due to steric hindrance.

Hard Soft Acid Base (HSAB) Theory

Concept:

• This theory provides a decision-making framework to predict reactivity based on experimental observations regarding acids and bases.

Definitions:

• Hard Acids/Bases: Characterized by low polarizability and smaller atomic size.

• Soft Acids/Bases: Characterized by high polarizability and larger atomic size.

Polarizability

Definition:

• This involves the interaction of electron orbitals with charged entities that impacts bonding properties.

  • Size Dependency: Larger atoms tend to have higher polarizability.

Summary: HSAB Theory

• The assessment of stability and reactivity between various Lewis Acids and Bases categorizes them into hard and soft classifications based on their polarizability and size.