1. Chapter 3 Acids and Bases. The Curved-Arrow Notation

2. Arrhenius Acids and Bases Acid: a substance that, when dissolved in water, increases the concentration of H + (protons) HCl H + + Cl - HCl + H 2 O H 3 O + + Cl - H2OH2O

3. Base: a substance that, when put in water, increases the concentration of OH - ions or a substance that accepts H + ions NaOH(aq)  Na + (aq) + OH - (aq)

4. Bronsted-Lowry Acids and Bases Acid: proton (H + ) donor Base: proton (H + ) acceptor

5. Lewis Acids and Bases Lewis Acid – Electron deficient/poor – Electron acceptor – Electrophile – Tend to have less than an octet Lewis Base – Electron rich – Electron donor – Nucleophile – Must have a lone pair of electrons product called an adduct Fluorine is electron rich

6. Lewis acids tend to react so as to fulfill their valence-shell octet Note the conservation of charge Recall: FC = # valence e - ’s – ( # LP e - ’s + ½ # of bonding e - ’s) 6

7. Problems Complete the following Lewis acid-base reactions. Draw in any missing electrons, label the nucleophiles and electrophiles, identify the adduct, and calculate any formal charges needed for the adduct OH HCH  + OH -1 

8. Curved-Arrow Notation A tool for tracking electrons in a chemical reaction Electrons flow from the electron donor (Lewis base) to the electron acceptor (Lewis acid) 8

9. Problems Use the curved arrow notation to derive a structure for your product in each of the following reactions CH 3 NH 2 + H + 

10. Electron Pair Displacement Reactions Not all acceptors are electron-deficient When an atom is NOT electron deficient, an electron pair must depart from the atom before it receives another electron pair This preserves the octet rule 10

11. Curved-Arrow Notation for Displacement Displacement reactions require two arrows Watch for conservation of total charge! Donated electron pairs can also originate from a lone pair or a bond 11

12. The Wrong Way Curved-arrows show the movement of electron pairs not nuclei Electrons are responsible for chemistry! 12

13. Problems Provide a curved arrow notation for each of the following reactions H 2 O + HCl  H 3 O + + Cl - + CH 2 CH 2 + Br 2

14. Problems For each of the following reactions, give the product that results

15. Two Reactions Represented by Curved Arrows Most reactions in O-chem involve moving electrons – Every reaction involving electron pairs fits into one of these two categories: 1)Lewis base + Lewis acid 2)Electron-pair displacement reactions Reactions may be a combination of the two types above 15 3.3 Review of the Curved-Arrow Notation

16. Problems For the following reactions, indicate whether you have a Lewis acid-base reaction or an electron pair displacement reaction

17. Curved-Arrow Notation for Resonance Resonance structures differ only by movement of electrons (and usually electron pairs) Curved-arrow notation is ideal to help derive resonance contributors Note: the interconversion of resonance structure by movement of electron is NOT a reaction 17 3.3 Review of the Curved-Arrow Notation

18. Problems Using the curved arrow notation, derive resonance structures for the following compounds: 1)Benzene 2)Aniline 3)Diazomethane

19. Br Ø nsted Acid-Bases Reactions 19 A Bronsted acid-base reaction involves an electron-pair displacement on a proton Bronsted Acid: A species that donates a H + – Keeps the electrons that were bonding to H Bronsted Bases: A Lewis base that donates its electron pair to a proton (in order to grab it)

20. Conjugate Acids and Bases When a Br Ø nsted acid loses a proton, its conjugate base is formed When a Br Ø nsted base gains a proton, its conjugate acid is formed 20 3.4 Br Ø nsted-Lowry Acids and Bases

22. Amphoteric Compounds Compounds that can act as either an acid or a base are called amphoteric Observe the behavior of a compound in a reaction to classify it as an acid or base Water is amphoteric 22

23. Problems H 2 O F- HCO 3 - SO 4 2- H 2 O HCO 3 - HPO 4 2- H 2 S Identify the conjugate acids for the compounds on the left and the conjugate bases for the compounds on the right. Also, identify all amphoteric compounds

24. Organic Reactions The Br Ø nsted-Lowry acid-base concept is central to many reactions in organic chemistry For example: …looks similar to: 24 3.4 Br Ø nsted-Lowry Acids and Bases

25. Nucleophiles and Electrophiles Nucleophile = Lewis base (“nucleus loving”) 25 3.4 Br Ø nsted-Lowry Acids and Bases

26. Nucleophiles and Electrophiles Electrophile = Lewis acid (“electron loving”) The atom that receives the electron pair 26 3.4 Br Ø nsted-Lowry Acids and Bases

27. Leaving Groups The group or atom that receives electrons from the breaking bond is a leaving group 27 3.4 Br Ø nsted-Lowry Acids and Bases

28. Leaving Groups Can also be applied to Lewis acid-base dissociation reactions 28 3.4 Br Ø nsted-Lowry Acids and Bases

31. Problems Classify each of the following reactions as a Bronsted acid-base reaction or a Lewis acid-base association/dissociation. Identify each species in the following reactions as a Bronsted acid, Bronsted base, Lewis acid, Lewis base, nucleophile, electrophile, and/or leaving group. Draw in the appropriate curved arrow notation where appropriate.

32. H3O+H3O+ HCl

33. Strengths of Br Ø nsted Acids A measure of the extent of proton release to a Br Ø nsted base The standard base traditionally used is water The equilibrium constant is: 33 3.4 Br Ø nsted-Lowry Acids and Bases

34. The Dissociation Constant As [H 2 O] effectively remains constant: Each acid has its own dissociation constant A large K a = many H + transferred – Strong acid – Weak conjugate base 34 3.4 Br Ø nsted-Lowry Acids and Bases

37. The pK a Scale and pH pK a = -log K a pK a values are more manageable than K a values Stronger acids have smaller pK a values pH is a measure of [H + ], a property of a solution (recall: pH = - log[H 3 O + ]) pK a is a measure of acid strength, a fixed property 37 3.4 Br Ø nsted-Lowry Acids and Bases

39. 39

40. Strengths of Br Ø nsted Bases Directly related to K a /pK a of the conjugate acid If a base is weak, its conjugate acid is strong If a base is strong, its conjugate acid is weak 40 3.4 Br Ø nsted-Lowry Acids and Bases

41. Problems 1)Write out the dissociation constant expression for formic acid, HCO 2 H, in water 2)Identify the conjugate acid-base pairs in the equation for problem #1 3)Using the K a for formic acid, calculate the pK a 4)What is the K a for acetic acid if its pK a = 4.74? Is acetic acid’s conjugate base weaker or stronger than the conjugate base of HF? HF’s K a = 7.2 x 10 -4

42. Relationship of Structure to Acidity Which of the following molecules is the weakest acid? Which is the strongest? HF HCl HBr HI What about these: 42

43. The Element Effect Evaluate the atom attached to the proton The acidities of Bronsted acids (H-A) increase down a group Acidities increase as the atomic # of A increases Due to decrease in bond strength The acidities of Bronsted acids (H-A) increase across a period from left to right Due to increasing electronegativity of A 43 3.6 Relationship of Structure to Acidity

45. 45

46. The Charge Effect Who is more acidic, H 2 O or H 3 O + ? Positively charged compounds attract electrons better than neutral ones pK a of H 2 O = 15.7 pK a of H 3 O + = -1.7 46 3.6 Relationship of Structure to Acidity

47. Problems Which of the following is the stronger acid? 1)PH 3 or SH 2 2)H 2 O or SH 2 3)NH 3, NH 4 +, or NH 2 - 47

48. The Polar Effect Which of the following is more acidic? 48

49. The Polar Effect The presence of electronegative substituents has an acid strengthening effect = polar effect or inductive effect – Such substituents are said to be electron withdrawing Acids with stable conjugate bases tend to be more acidic – Resonance = stabilization 49

50. Consider the following series: Which molecule is the most acidic and why? 50

51. Problems Rank the following molecules, in each series according to order of increasing acidity and explain your reasoning 51

52. 52

53. Homework Problems 3.1 – 3.13, 3.19 – 3.21, 3.24 – 3.37, 3.39, 3.44, 3.49, 3.50 53