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

2. 2.1 An Introduction to Acids and Bases
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, its conjugate base is formed
When a base gains a proton, its conjugate acid is formed

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

6. Brønsted–Lowry Acids and Bases
Acidity: a measure of the tendency of a compound to give up a proton.
The stronger the acid, the more readily it gives up a proton.
Basicity: a measure of a compound’s affinity for a proton.
The stronger the acid, the weaker is its conjugate base.

7. Acids Have Different Strengths
2.2 pKa and pH
Acids Have Different Strengths
stronger acid
weaker acid

8. 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.

9. The Acid Dissociation Constant, Ka
The Ka implies the concentrations of the acid and the ions
The larger the Ka, the stronger the acid.
Ka > 1 Ionized products greater than intact acid.
Ka < 1 Ionized products less than intact acid.
Ka >> 1 Ionization goes to completion (strong acid).
(e.g., > 103)
Ka << 1 Ionization does not occur to an appreciable amount.
(e.g., < 10–3)

10. The Acid Dissociation Constant, Ka
Since the Ka values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use
pKa = – log (Ka)

11. pKa = – log Ka for Ka = 10–20, pKa = 20 for Ka = 10–5, pKa = 5

12. Compare pKa and Ka Values
strong acids
weak acids
pKa -2 2 4 6 8 10 12 14
102
10-2
10-6
10-10
10-14 Ka
The smaller the value of the pKa, the stronger the acid.

13. pKa is a Covenient Scale for
Acid Strength
The stronger the acid, the larger the Ka value
The stronger the acid, the smaller the pKa value

14. pH The concentration of hydrogen ions is used as a measure of acidity
This concentration is expressed as pH
pH = – log[H+]
The higher the concentration, the more acidic the solution and the lower the pH

15. pH Indicates the Acidity of a Solution
Neutral water: [H3O+] = 1.0 × 10–7 M
pH = – log[H3O+] = 7
pH < Acidic solution
pH = Neutral solution
pH > Basic solution

16. pH vs. pKa Do not confuse pH and pKa!
The pH scale is used to describe the acidity of a solution.
The pKa is characteristic of a particular compound, indicating the tendency of the compound to give up its proton.

17. 2.3 Organic Acids and Bases
Carboxylic acids
Most common organic acids!
pKa: 3~5 Weak acids!

18. Alcohols
Compounds having an OH group!
pKa~ 16 Much weaker acids!

19. Amines Compounds with NH2 groups!
pKa~ 40 Extremely weak, rarely behave as acids.
In fact, amines are the most common organic bases!

20. Protonated Amines
Protonated Alcohols and
Protonated Carboxylic Acids

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

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

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

25. 2.4 How to Predict the Outcome of an Acid-Base Reaction
To determine which of the two reactants of the reaction will be the acid, we need to compare their pKa values!
The stronger acid will donate a proton to the weaker acid.
Here water is behaving as a base Here water is behaving as an acid.

26. 2.5 How to Determine the Position of Equilibrium
Compare the pKa value of the acid on the left and right of the arrow!
The equilibrium favors: Strong reacts to form weak!

27. 2.6 How the Structure of an acid Affects its pKa
The stronger the acid, the weaker is its conjugate base!
The stronger the acid, the more stable is its conjugate base!
Factors that influence stability of the conjugate base include:
Resonance
Electronegativity
Atomic Size
Hybridization
Inductive Effects

28. Electronegativity
Elements in the same row are all about the same size, but they have very different electronegativity!
When atoms are similar in size, the strongest acid will have its hydrogen attached to the most electronegative atom!

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

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

31. Electronegativity pKa Values 20 15 5 CH4 NH3 H2O HF ~60 36 15.7 3.2
increasing
electronegativity
pKa Values 20 15 5
CH4
NH3
H2O HF
~60 36
15.7
3.2
RCH3
RNH2
ROH 45 35 18
Consider the conjugate bases

32. Hybridization Affects Electronegativity
I would make the blue structural formulas on the right much larger.
The weakest acid has the
strongest conjugate base.

33. Atomic Size
When atoms are very different in size, the strongest acid will have its hydrogen attached to the largest atom!

34. Atomic Size pKa Values HF HCl HBr HI 3.5 –7 –9 –10 F– 1.36 Å 1.81 Å
increasing
size
pKa Values HF
HCl
HBr HI
3.5 –7 –9
–10 F–
1.36 Å
1.81 Å
1.95 Å
2.16 Å
H2O
H2S
H2Se
H2Te 16 7 4 3
Cl–
Br– I–
Consider the ionic radii

35. Some pKa Values

36. 2.7 How substituents affect the strength of an acid
inductive electron withdrawal

37. A Substituent’s Effect on pKa Depends on Distance

38. Why is a Carboxylic Acid a Stronger Acid Than an Alcohol?
1. inductive electron withdrawal

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

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

41. That Affect Acid Strength
Summary of Factors
That Affect Acid Strength
hybridization

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

43. That Affect Acid Strength
Summary of Factors
That Affect Acid Strength
electron delocalization

44. 2.10 How pH Affects the Structure of an Organic Compound
A compound will exist primarily in its acidic form if the pH of the solution is less than its pKa.
A compound will exist primarily in its basic form if the pH of the solution is greater than its pKa.

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

46. 2.11 Buffer Solutions
A solution of a weak acid (HA) and its conjugate base (A-) is called a buffer solution.
A buffer solution will maintain nearly constant pH when small amounts of acid or base are added to it!