Acid-Base and Donor-Acceptor Chemistry Chapter 6

Acids and Bases The Major Acid-Base Definitions:

Acids and Bases Arrhenius: Acid is proton source in water, base is hydroxide source Acid + Base = Salt + Water; HCl + NaOH  NaCl + H2O Works in aqueous solutions only Brønsted-Lowry: Acid is proton donor, base is proton acceptor Acid(1) + Base(1)  Acid(2) + Base(2) Can this happen without water? Conjugate acid-base pairs Reaction favors the weaker acid/base pair Non-aqueous solvents, gas and solid phase reactions

Acids and Bases Solvent system definition: If a species can autoionize (autodissociate, form a cation and an anion), the cation is an acid and the anion is a base In this system, an acid increases [cation] and a base increases [anion] Water: 2 H2O  H3O+ + OH– HCl is an acid in this solvent system (increases [cation]) HCl + H2O  H3O+ + Cl–

Protic Acids and Bases

Protic Acids and Bases Kion is K for the autoionization of the pure solvent Solvent Acid Base pKion H2SO4 H3SO4+ HSO4– 3.4 at 10 ºC H2O H3O+ OH– 14 NH3 NH4+ NH2– 27 Acetic acid is acid in water, but base in H2SO4 CH3CO2H + H2SO4  HSO4– + CH3CO2H2+ Urea is a base in water, but an acid in liquid NH3 (NH2)2CO + NH3  NH4+ + (NH2)(NH)CO–

Aprotic Acids and Bases

Aprotic Acids and Bases Aprotic solvent systems: BrF3, PCl5 2 NH3  NH4+ + NH2- 2 BrF3  BrF2+ + BrF4– 2 PCl5  PCl4+ + PCl6– In BrF3, SbF5 is an acid and KF is a base SbF5 + BrF3  SbF6– + BrF2+ KF + BrF3  K+ + BrF4– In this definition, Acid + Base = Solvent BrF2+ + BrF4–  2 BrF3 Acid Base Solvent Broader definition - acid/base character is related to the solvent

Lewis Acid-Base Theory Frontier Orbitals: HOMO and LUMO

Lewis Acid vs. Lewis Base vs. Oxidation vs. Reduction

Lewis Acid vs. Lewis Base vs. Oxidation vs. Reduction

Lewis Acid vs. Lewis Base vs. Oxidation vs. Reduction

Lewis Acid vs. Lewis Base vs. Oxidation vs. Reduction

Measuring Lewis Acid-Base Interaction Strength via Electronic Spectra

Hard and Soft Acids and Bases Examples: Ionic Compound Solubility AgF(s) + H2O(l)  Ag+(aq) + F-(aq) Ksp = 205 AgCl(s) + H2O(l)  Ag+(aq) + Cl-(aq) Ksp = 1.8 x 10-10 AgBr(s) + H2O(l)  Ag+(aq) + Br-(aq) Ksp = 5.2 x 10-13 AgI(s) + H2O(l)  Ag+(aq) + I-(aq) Ksp = 8.3 x 10-17 Similarly, solubility: Hg2F2 > Hg2Cl2 > Hg2Br2 > Hg2I2 BUT, LiF < LiI MgF2 is less soluble than MgCl2 AlF3 is less soluble than AgCl3

Hard and Soft Acids and Bases Examples: Complexation

Hard and Soft Acids and Bases Examples: Thiocyanate Bonding to Metals SCN- can bond through S or N S-binding with M = large, polarizable metals like Hg2+, Au+, Pd2+ N-binding with M = smaller, less polarizable metals: Zn2+, Ni2+

Hard and Soft Acids and Bases Definitions Hard base: small, not polarizable Soft base: large, highly polarizable

Hard and Soft Acids and Bases Definitions Hard acid: small, not polarizable Soft acid: large, highly polarizable

Relate to oxoanions.