Big-picture perspective: Acid-base chemistry is highly diverse, encompassing not only the traditional H + /OH – chemistry that characterizes aqueous solutions but also proton transfer in non-aqueous systems, and aprotic solvent systems and metal cations. In this unit, we dive into acid-base chemistry, emphasizing the connections especially to inorganic chemistry. Learning goals: Understand the Bronsted and Lewis definitions of acids and bases. Understand the periodic trends of acidic, basic, and amphoteric compounds Predict, describe, and rationalize acid/base chemistry in non-aqueous systems, including acidic and basic solvents, aprotic solvents, and molten salts. Apply the principles of acid-base chemistry to the design of molecules and Lewis acids with target functions. Understand the connection between acid-base chemistry and the stabilization of oxidation states. Predict favorable and stable compounds using hard-soft acid-base (HSAB) theory. Acid-Base Chemistry

Theories of acid-base chemistry Arrhenius concept: Acids (bases) release H + (OH – ) in water (Limited to aqueous systems) Brønsted concept: Acids (bases) lose (gain) a proton (Focus on proton transfer and expand to non-aqueous systems) Upon proton transfer, HCl becomes “deprotonated” to form Cl – and H 2 O becomes “protonated” to form H 3 O + Strong acid: fully deprotonated Weak acid: partially deprotonated (dynamic equilibrium) Lewis concept: Bases donate an electron pair Acids accept an electron pair (encompasses Brønsted & Arrhenius definitions)

Theories of acid-base chemistry Solvent system concept: Solvent auto-dissociation produces an acid (cation) and base (anion). (Extends to aprotic non-aqueous solutions.) H 2 O  H + (“H 3 O + ”) + OH – 2 BrF 3  BrF BrF 4 – Lewis concept: Acids (bases) accept (donate) electron pairs (Expands to metal ions and non-aqueous systems and is most general, including in its scope the other acid/base concepts) H + + NH 3  NH 4 + BF 3 + NH 3  BF 3 NH 3

Periodic table showing basic (blue), amphoteric (green) and acidic (red) oxides. The metal-nonmetal boundary is indicated by the gray staircase line. Basic, acidic, and amphoteric oxides

Periodic Trends Rank HF, HCl, HBr, and HI in order of increasing H-X bond strength. Rank HF, HCl, HBr, and HI in order of increasing H + donating ability. Rank HF, HCl, HBr, and HI in order of increasing acidity in water.

Non-aqueous solvents Exercise: Write a balanced chemical equation corresponding to the reaction of HCl with H 2 O(l). Exercise: Write a balanced chemical equation corresponding to the reaction of HOAc with H 2 O(l). [HOAc = acetic acid, HC 2 H 3 O 2 ] Exercise: Write a balanced chemical equation corresponding to the reaction of HCl with HOAc(l). [Note: No water is present]

Non-aqueous solvents A Brønsted acid that is strong in water (e.g. HCl) may be weak in a solvent that is a less effective proton acceptor than water (e.g. HOAc). Non-aqueous solvents that are good proton acceptors (e.g. NH 3 ) encourage acids to ionize in them, and all acids can appear to be strong (i.e., all are completely ionized: their acidity is leveled) In a basic solvent (e.g. liq. NH 3, bp = -33°C), all acids are strong. In an acidic solvent (e.g. HOAc, H 2 SO 4 ), ionization of bases is facilitated, and most acids are therefore weak. HOAC solvent discriminates HCl, HBr, HI (which are leveled in H 2 O)

Leveling and discrimination effects Describe the acid-base behavior of HClO 4 dissolved in H 2 SO 4 HClO 4, pK a = – 8 H 2 SO 4, pK a = – 3 Which is the stronger acid, HClO 4 or H 2 SO 4 ? What are the predominant species that are present? Compare to a solution of HClO 4 and H 2 SO 4 in water

Molten salts as solvents Crystals of metal oxides (e.g., K x Ba 1-x BiO 3 superconductor) are often grown in solutions of molten NaOH & KOH (mp ~ 250 °C) autodissociation equilibrium: 2 OH - = H 2 O + O 2- What is the strongest base in this medium? Strongest acid? How does added water affect the solubility of K x Ba 1-x BiO 3 crystals?

Bases Stabilize High Oxidation States First make a stable Mn(IV) fluoride: 4 MnO 4 - (aq) + 10 H 2 O(l) + 24 F - (aq) → 4 MnF 6 2- (aq) + 3O 2 (g) + 20 OH - (aq) 2 K + (aq) + MnF 6 2- (aq) → K 2 MnF 6 (s) Then add Lewis acid: K 2 MnF 6 (s) + 2 SbF 5 (l) → 2 KSbF 6 (s)+ "MnF 4 "(s) "MnF 4 "(s) → MnF 3 (s) + 1/2 F 2 (g) Why is Mn(IV) unstable in acid? Can we reach higher metal oxidation states with oxides or fluorides? Karl Christe Henri Moissan Application: Chemical Synthesis of F 2 (K. O. Christe, 1986)

Lewis acids and bases Lewis acid: Electron-pair acceptor Lewis base: Electron-pair donor Lewis acid + Lewis base  Lewis acid-base adduct Examples: AlCl 3 + Cl - (in CHCl 3 ) Al(NO 3 ) 3 in H 2 O

Identifying Lewis acids and bases Lewis acid: Positive charge or unfilled octet (H +, BX 3 ) Highly charged transition metal cations (Fe 3+ Fe 2+ Co 3+ ) Group III cations and compounds (Al 3+ Ga 3+ AlCl 3 ) Smaller group II cations (Be 2+ Mg 2+ ) Lewis base: Neutral or anionic, lone pair electrons OH –, NH 3, Bronsted bases

Designing better Lewis acids Fe 3+ is a good Lewis acid. What related species are similar, but better (or worse) Lewis acids?

Consider these experimental observations… What does this mean? Can you rationalize it?

Hard-soft acid-base theory Consider these experimental observations… SCN – bonds to Hg 2+ through its sulfur atom to form [Hg(SCN) 4 ] 2– SCN – bonds to Zn 2+ through its nitrogen atom to form [Zn(NCS) 4 ] 2– What does this mean? Can you rationalize it?

Polarizability

Hard-soft acid-base theory Hard acids and bases Soft acids and bases The hard-soft acid-base (HSAB) concept provides a useful guide for predicting and explaining acid-base chemistry and other chemical phenomena. Key concepts: Electrostatics vs. Covalency Like likes like

Can you identify trends and/or predictors of hard & soft acid & base behavior?

Hard & soft – periodic trends AcidsBases

Applying the HSAB concept Which are stable: CuF, CuF 2, CuI, CuI 2 ? What do you predict, and how would you verify your prediction? Which base would you use to stabilize Au(I)? OH -, NO 3 -, NH 3, CN - Gold mining: Let’s also look at it energetically…