Chapter 3 Acids and Bases. The Curved-Arrow Notation

1. Chapter 3Acids 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 + H2O H3O+ + Cl- H2O

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)

7. OH H C H  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-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)

9. Problems Use the curved arrow notation to derive a structure for your product in each of the following reactions CH3NH2 + 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

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

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

13. Problems H2O + HCl  H3O+ + Cl- Provide a curved arrow notation for each of the following reactions CH2CH2 + Br2 +

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

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

18. Problems • Using the curved arrow notation, derive resonance structures for the following compounds: • Benzene • Aniline • Diazomethane

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) BrØnsted Acid-Bases Reactions

20. Conjugate Acids and Bases • When a BrØnsted acid loses a proton, its conjugate base is formed • When a BrØnsted base gains a proton, its conjugate acid is formed 3.4 BrØnsted-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

23. H2O F- HCO3- SO42- H2O HCO3- HPO42- H2S Problems 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 BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry • For example: • …looks similar to: 3.4 BrØnsted-Lowry Acids and Bases

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

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

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

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

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+ HCl

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

34. As [H2O] effectively remains constant: Each acid has its own dissociation constant A large Ka = many H+ transferred Strong acid Weak conjugate base The Dissociation Constant 3.4 BrØnsted-Lowry Acids and Bases

37. The pKa Scale and pH • pKa = -log Ka • pKa values are more manageable than Ka values • Stronger acids have smaller pKa values • pH is a measure of [H+], a property of a solution (recall: pH = - log[H3O+]) • pKa is a measure of acid strength, a fixed property 3.4 BrØnsted-Lowry Acids and Bases

40. Strengths of BrØnsted Bases • Directly related to Ka/pKa of the conjugate acid • If a base is weak, its conjugate acid is strong • If a base is strong, its conjugate acid is weak 3.4 BrØnsted-Lowry Acids and Bases

41. Problems • Write out the dissociation constant expression for formic acid, HCO2H, in water • Identify the conjugate acid-base pairs in the equation for problem #1 • Using the Ka for formic acid, calculate the pKa • What is the Ka for acetic acid if its pKa = 4.74? Is acetic acid’s conjugate base weaker or stronger than the conjugate base of HF? HF’s Ka = 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:

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 3.6 Relationship of Structure to Acidity

46. The Charge Effect • Who is more acidic, H2O or H3O+? • Positively charged compounds attract electrons better than neutral ones • pKa of H2O = 15.7 • pKa of H3O+ = -1.7 3.6 Relationship of Structure to Acidity

47. Problems • Which of the following is the stronger acid? • PH3 orSH2 • H2O or SH2 • NH3, NH4+, or NH2-

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

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

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