Acid-Base and Donor-Acceptor Chemistry

Chapter 6

Acids and BasesThe Major Acid-Base Definitions:

Acids and Bases• Arrhenius:

Acid is proton source in water, base is hydroxide sourceAcid + Base = Salt + Water; HCl + NaOH NaCl + H2OWorks in aqueous solutions only

• Brønsted-Lowry:Acid is proton donor, base is proton acceptorAcid(1) + Base(1) Acid(2) + Base(2) Can this happen without water?

• Conjugate acid-base pairsReaction favors the weaker acid/base pairNon-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

• Kion is K for the autoionization of the pure solvent

Solvent Acid Base pKion

H2SO4 H3SO4+ HSO4

–3.4 at 10 ºCH2O H3O+ OH– 14NH3 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–

Protic 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 baseSbF5 + BrF3 SbF6

– + BrF2+

KF + BrF3 K+ + BrF4–

In this definition, Acid + Base = Solvent

• BrF2+ + BrF4

– 2 BrF3Acid Base Solvent

• Broader definition - acid/base character is related to the solvent

Aprotic Acids and Bases

Lewis Acid-Base TheoryFrontier 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 BasesExamples: 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 BasesExamples: Complexation

Hard and Soft Acids and BasesExamples: 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 BasesDefinitions

Hard base: small, not polarizable

Soft base: large, highly polarizable

Hard and Soft Acids and BasesDefinitions

Hard acid: small, not polarizable

Soft acid: large, highly polarizable

Relate to oxoanions.