1. CHE2060 Lecture 5: Acid-base chemistry
5.1 Acids & bases: overview & basics
5.2 Acid & base strength
5.3 Equilibrium acid-base reactions
5.4 The leveling effect of solvents
5.5 Estimation of acidity by conceptual knowledge
5.6 Classes of organic acids & bases
5.7 Functional groups: acid-base nature
Daley & Daley
Chapter 5:
Acid-base theory

2. Common organic acids & bases Predicting direction of equilibrium
Acid & base strength
Ionization & pH
Strength: Ka & pKa
Common organic acids & bases
Predicting direction of equilibrium

3. Ionization of water & pH
The strength of a Brønsted-Lowry acid is determined by the extent to which it ionizes in water: autoionization.
hydronium
ion
Because both the “acid” and “base” are water, the degree of dissociation or auto-ionization is very slight.
For water, the concentration of the dissociated products are constant, Kw.
Kw = [H3O+1][OH-1] = Kw = x 10-14 M
While Kw is constant, the concentration of hydronium ion can change greatly. It is this [H3O+1] that chemists measure as pH!
pH = -log[H3O+1] … often expressed as pH = -log[H+1]
M&B p.6-8

4. Acid strength is measured as Ka or pKa
An acid’s strength is measured as its dissociation constant, Ka. So, imagine an acid, Ka:
hydronium
ion
The larger the Ka value stronger the acid’s strength.
Strong acids have pKa values between at zero or below. M
Most organic acids are quite weak and have Ka values between 10-2 and To make it easier to compare this very wide range of values, Ka is often converted to pKa, a logarithmic scale:
pKa = -log(Ka)
Since this scale is inverse logarithmic, but lower the pKa the stronger the acid.
D&D, p.215-7

5. Common pKa values
weakest “acid” M
Strongest acid
D&D, p.217

6. pKa’s of common organic acids
M&B p.6-8

7. Examples: using pKa to gauge acid strength
Which of each pair is the stronger acid?
And what are the molecules named?
p-methylphenol
p-chlorophenol M
How much stronger is the p-chlorophenol?
The difference in pKa values = – 9.43 = 0.83.
The difference in acid strength = = 6.8.
So p-chlorophenol is 6.8 times as strong an acid.
Karty p.300

8. Examples: pKa for most acidic proton?
For each molecule, identify the most acidic proton (hydrogen).
methyl 48
carboxylic acid 4.75
ethanol 16
thiol 10.3
phenol 10
benzoic acid 4.2 M
methyl ammonium
10.63
amine 38
hydronium -1.7
Karty p.300

9. Strong bases are weak acids.
You can predict the strength of a base if you know the Ka or pKa values of the base’s conjugate acid.
So bases are stronger when their conjugate acids have:
lower Ka values; or
higher pKa values.
So which is the stronger base, hydroxide (OH-1) or ammonia (NH3)?
The conjugate acid of hydroxide (OH-1) is water: pKa 15.7
The conjugate acid of ammonia is ammonium: pKa 9.4 M
So, hydroxide is the stronger base.
How much stronger?
Difference in pka: – 9.4 = 6.3
Difference in strength: = 2.0E6 = 2 million times stronger!
Karty p.301

10. Example: predict the strength of a base
Which is the stronger base, chloride ion or phenoxide ion (C6H5O-1)?
The conjugate acid of chloride ion is hydrochloric acid: pKa ~ - 7
The conjugate acid of phenoxide ion is phenol: pKa 10
So, phenoxide ion is the stronger base.
How much stronger?
Difference in pka: 10 – (-7) = 17
Difference in strength: 1017 = 1.0 E17 times stronger! M
Karty p.301

11. pKa’s of common organic bases
Stength of bases is measured by the acidity of their conjugate acid.
pKa + pKb = > pKb = 14 - pKa
So stronger bases have higher pKa values. M
M&B p.8-9