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Acids and Bases: Central to Understanding Organic Chemistry

Chapter 2. Acids and Bases: Central to Understanding Organic Chemistry. Paula Yurkanis Bruice University of California, Santa Barbara. An Acid Loses a Proton A Base Gains a Proton. Most Acid–Base Reactions are Reversible. An Acid and its Conjugate Base A Base and its Conjugate Acid.

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Acids and Bases: Central to Understanding Organic Chemistry

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  1. Chapter 2 Acids and Bases: Central to Understanding Organic Chemistry Paula Yurkanis Bruice University of California, Santa Barbara

  2. An Acid Loses a Proton A Base Gains a Proton

  3. Most Acid–Base Reactions are Reversible

  4. An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid.

  5. An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid. The stronger the acid, the weaker its conjugate base.

  6. Acids Have Different Strengths

  7. The Acid Dissociation Constant (Ka) is a Measure of the Extent of Dissociation of an Acid Put the constants on the same side of the equation.

  8. Acid Strength The stronger the acid, the larger theKa. The stronger the acid, the smaller the pKa.

  9. pH The pH indicates the concentration of protons is a solution.

  10. Carboxylic Acids are the Most Common Organic Acids

  11. Alcohols

  12. Amines

  13. Protonated Amines

  14. Protonated Alcohols and Protonated Carboxylic Acids

  15. An Alcohol Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor.

  16. A Carboxylic Acid Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor.

  17. An Amine Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor.

  18. Which Reactant is the Acid? The stronger acid behaves as the acid. Water is the base. Water is the acid.

  19. The Position of Equilibrium The equilibrium favors formation of the weaker acid.

  20. Electronegativity Affects pKa Values When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom.

  21. The Stronger the Acid, the Weaker Its Conjugate Base stable basesare weak bases

  22. Why are Alcohols Stronger Acids Than Amines? Oxygen is more electronegative than nitrogen.

  23. Why Are Protonated Alcohols Stronger Acids Than Protonated Amines? Oxygen is more electronegative than nitrogen.

  24. Hybridization Affects Electronegativity The weakest acid has the strongest conjugate base.

  25. Size Affects pKa Values When atoms differ in size, the strongest acid has its hydrogen boded to the largest atom.

  26. Some pKa Values

  27. Substituents Affect the Strength of the Acid inductive electron withdrawal

  28. A Substituent’s Effect on pKaDepends on Distance

  29. Why is a Carboxylic Acid a Stronger AcidThan an Alcohol? 1. inductive electron withdrawal

  30. Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 2. delocalized electrons (resonance)

  31. Summary of Factors That Affect Acid Strength electronegativity and size

  32. Summary of Factors That Affect Acid Strength hybridization

  33. Summary of Factors That Affect Acid Strength inductive electron withdrawal

  34. Summary of Factors That Affect Acid Strength electron delocalization

  35. The Henderson–Hasselbalch Equation The Henderson–Hasselbalch equation tells us whether (at a given pH) a compound will be in its acidic form (with its proton) or in is basic form (without its proton).

  36. A Compound with pKa = 5.2 blue = acidic form green = basic form

  37. Water and Diethyl Ether Form Two Layers Diethyl ether is less dense than water, so diethyl ether is on top.

  38. Is a Compound Charged or Neutral? A carboxylic acid is neutral in its acidic form and charged in its basic form. An amine is charged in its acidic form and neutral in its basic form.

  39. A Buffer Solution A buffer solution contains an acid and its conjugate base.

  40. Lewis Acids and Bases Lewis definitions: acid: a species that accepts a share in an electron pair base: a species that donates a share in an electron pair All Brønsted acids are Lewis acids. All Brønsted bases are Lewis bases.

  41. Lewis Acids and Bases

  42. How Chemists Use the Terms “acid” = proton-donating acid “Lewis acid” = non-proton-donating acid “base” = a compound that shares its electrons with a proton “nucleophile” = a compound that shares its electrons with an atom other than a proton

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