Acids and Bases

1. Acids and Bases or general chemistry, analytical and a whole lot more!

2. Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water

3. A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid acid base conjugatebase conjugateacid base acid

4. + H H O O [ ] + + - H H H O H O H H H2O (l) H+(aq) + OH-(aq) H2O + H2O H3O+ + OH- Acid-Base Properties of Water autoionization of water conjugateacid base conjugatebase acid

5. H+ + X- H X The stronger the bond Molecular Structure and Acid Strength The weaker the acid HF << HCl < HBr < HI

6. d- d+ O- Z + H+ O Z H Molecular Structure and Acid Strength • The O-H bond will be more polar and easier to break if: • Z is very electronegative or • Z is in a high oxidation state

7. O H O Cl O O H O Br O • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Molecular Structure and Acid Strength 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z Cl is more electronegative than Br HClO3 > HBrO3

8. Molecular Structure and Acid Strength 2. Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO4 > HClO3 > HClO2 > HClO

9. Definition of An Acid + OH- H H H O H + H+ H N H N H • • • • • • • • • • • • H H Arrhenius acid is a substance that produces H+ (H3O+) in water A Brønsted acid is a proton donor A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons H+ acid base acid base

10. H F F F B F B N H F F H H N H H • • Lewis Acids and Bases + acid (electrophile) base (nucleophile) No protons donated or accepted! Let’s look at this reaction, and others like it more closely. To see what’s really going on we need to look at the MOs.

11. 2 E 3 A1 HOMO 2 A1 1 E 1A1 LUMO 1 A1 : NH4+ H+ NH3 2 T2 2 A1 Energy 1 T2 1 A1

12. 2 E 3 A1 HOMO 2 A1 1 E 1A1 LUMO 1 A1 : NH4+ H+ NH3 2 T2 2 A1 Energy 1 T2 1 A1

13. Reaction Mechanisms How do the atoms of the reactant molecules rearrange to form the product molecules? What is the sequence of bond breaking and bond making? What are the energetics of the process? How do the electrons flow? What molecular orbitals are involved?

14. How do you analyze a mechanism? Follow the Electrons. Push your arrows. What orbitals are involved? What are the important MOs of HF and NH3?

15. acid – LUMOmost important σ* Looks most like H Lowest Unoccupied Molecular Orbital H 1s MO diagram of HF F 2p Highest Ooccupied Molecular Orbital σ Looks most like F

16. base – HOMOmost important HF acid – LUMOmost important NH3

17. S (symmetric) No nodes. Same Symmetry End OnGood Interaction HF LUMO NH3 HOMO Symmetric-Symmetric S-S Reaction is Symmetry Allowed Wrong SymmetryNo Good Interaction Does not work side on

18. Restating the Lewis Acid-Base Definition: • Base: has e- pair in HOMO of correct energy and symmetry • b)Acid: has LUMO of correct energy and symmetry

20. Carbon Monoxide as a Lewis Base 1) Electronegativity suggests O is the e- pair donor 2) In fact, C is always the donor a) Formal Charge MO Frontier Orbitals HOMO that is involved in bonding is mostly on C ii. C-like HOMO donated to the M Lewis acid

21. While we’re on the subject of directionality in reactions A little organic chemistry (not my favorite, but it does have MO’s)

22. Alkene Additions Regiochemistry Regiochemistry – At which atom center does a reaction take place

23. H2 + CH2=CH2 CH3CH3 Reaction Mechanisms How do the atoms of the reactant molecules rearrange to form the product molecules? ?

24. Yes Reaction Mechanisms But first. Do we expect the reaction to occur? Look at the thermodynamics. ΔGr°= Σproducts - Σreactants ΔGr°= -32 – 68 = -100 kJ/mol ΔGf°=0.0 ? ΔGf°=+68 kJ/mol ΔGf°=-32 kJ/mol

25. Energy Transition State ΔG‡ Ea Energy of activation reactants ΔGr products Reaction Coordinate

26. Energy ΔGr = -100 kJ/mol exergonic ΔG‡ ΔG‡ = ? Energy of activation reactants H2 + C2H40 + 68 kJ C2H6-32 kJ -100 kJ products Reaction Coordinate

27. Reaction Mechanisms How do the atoms of the reactant molecules rearrange to form the product molecules? No H2 + CH2=CH2 CH3CH3

28. It looks good. What is wrong with this mechanism? Examine the M.O.s H2 + CH2=CH2 CH3CH3

29. σ* LUMO π* A A LUMO H2 C2H4 π σ HOMO S HOMO S

30. S (symmetric with respect to reflection) A (antisymmetric with respect to reflection) It has a node. π* A LUMO A-S Wrong Symmetry No Interaction σ HOMO S

31. A-S Wrong Symmetry No Interaction σ* LUMO A This reaction path is forbidden by orbital symmetry π HOMO S

32. σ* LUMO π* A A LUMO The direct reaction of H2 and C2H4 is forbidden by orbital symmetry π σ HOMO S HOMO S

33. Energy ΔGr = -100 kJ/mol Forbidden Reaction ΔG‡ is too high ΔG‡ We need a catalyst! reactants H2 + C2H40 + 68 kJ C2H6-32 kJ -100 kJ products Reaction Coordinate

34. What is a catalyst? An added component that changes thereaction mechanism to one with a lower energy pathway. A lower ΔG‡ The catalyst is neither produced or destroyed during the reaction. It does not change ΔG of the reaction. It does not change Keq of the reaction. A catalyst can be simple like H+ or a metal ion or it can be complex like an enzyme.

35. The platinum metals are often used in catalysis They have filled d orbitals and empty s or p orbitals. This means they can act as either an acid or a base.

37. Classes of Lewis bases (Ligands) • Monodentate Ligands - A ligand that donates only one electron pair to a single metal • One of the best examples is NH3. • Bridging Ligands - A ligand that donates one or more electron pairs to two or more metals • Halides and hydroxide are good examples, since each possesses two or more electron pairs on the donor atom • As we considered before, one of the steps to forming insoluble hydroxide and oxo compounds is the bridging of two metals in the course of hydroysis.

38. Classes of Lewis bases – con’t • Ambidentate Ligands - Ligands that possess two or more donor atoms can act as a monodentate ligand through either donor atom, or bridge two metals.  • The pseudohalides such as cyanide, azide and thiocyanate are good examples.

39. Denticity • The number of potential donor interactions is called the denticity • ligands are classified as bidentate chelating ligands (as for ethylenediamine), hexadentate chelating ligands (as for ethylenediaminetetraacetic acid; EDTA) and so forth.

40. Chelating ligands • ethylenediamine (on the left) yields a five-membered ring • acetylacetonate (on the right) yields a six-membered ring

41. Structures of Common Chelating Ligands 2,2’-bipyridine (bpy) 1,10-phenathroline (phen) terpyridine (terpy) ethylenediamine (en) propylenediamine (prn) diethylenetriamine (dien) triethylenetetramine (trien) tri(ethylenediamine)amine (tren)

42. Macrocyclic ligands • Macrocyclic Ligands - A special class of generally tetradentate chelating ligands, where the four donor atoms are arranged in a circular ring array, as in the porphyrins and anes.

43. [Ni(H2O)6]2+ + n L ¾ [Ni(L)n(H2O)2]2+ + 4 H2O

44. Tetradentate Tetraaza Macrocycles [Cu(teta)]2+ Cu2+ + (teta)Hnn+ kd = 3.6 x 10-7 sec-1 in 6.1 M HCl at 25 oC [Cu(2,3,2)]2+ Cu2+ + (2,3,2)Hnn+ kd = 4.1 sec-1 in 6.1 M HCl at 25 oC 2,3,2 Teta=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane

45. Nature’s Macrocycle A porphyrin is “any of a group of compounds containing the porphin structure of four pyrrole rings connected by methine bridges in a cyclic configuration, to which a variety of side chains are attached; usually metalled, e.g., with iron to form heme.” Definition from Academic Press Dictionary of Science Technology Porphyrin The iron(II) protoporphyrin-IX complex is the prosthetic group in hemoglobins and myoglobins, which are responsible for oxygen transport and storage in living tissues. Heme can also be found in the enzyme peroxidase, which catalyzes the oxidation of substrates with hydrogen peroxide. The related enzyme catalase, also containing heme, catalyzes the breakdown of hydrogen peroxide to water and oxygen. Other heme-containing proteins include the cytochromes, which serve as one-electron carriers in the electron transport chain.

46. Reduction of one of the pyrrole units on the porphyrin ring leads to a class of porphyrin derivatives called chlorins. Chlorophylls (e.g. chlorophyll-a), found abundantly in green plants, belong to this category. They play very important roles in the process of photosynthesis. Porphyrins are also found in other systems such as the wing feathers of Turacus indicus , the oxygen-carrying pigment chlorocruorin from Sabella starte indica , oil shale, and some marine sponges. Vitamin B12 contains a porphyrin-like unit called corrin