Download Acids and Bases

Chapter 2
Acids and Bases:
Central to
Understanding
Organic Chemistry
Paula Yurkanis Bruice
University of California,
Santa Barbara
2.1 An Introduction to Acids and Bases
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
When an acid loses a proton, its conjugate base is formed
When a base gains a proton, its conjugate acid is formed
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
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.
2.2 pKa and pH
Acids Have Different Strengths
stronger acid
weaker acid
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.
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
(e.g., < 10–3)
Ionization does not occur to an appreciable amount.
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)
pKa = – log Ka
for Ka = 10–20, pKa = 20
for Ka = 10–5, pKa = 5
for Ka = 107, pKa = –7
Compare pKa and Ka Values
pKa
Ka
strong acids
-2
102
0
weak acids
2
10-2
4
6
10-6
8
10
10-10
12
14
10-14
The smaller the value of the pKa, the stronger the acid.
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
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
pH Indicates the Acidity of a Solution
H2O + H2O
Neutral water:
pH < 7.00
pH = 7.00
pH > 7.00
OH
+ H3O
[H3O+] = 1.0 × 10–7 M
pH = – log[H3O+] = 7
Acidic solution
Neutral solution
Basic solution
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.
2.3 Organic Acids and Bases
Carboxylic acids
 Most common organic acids!
 pKa: 3~5 Weak acids!
Alcohols
 Compounds having an OH group!
 pKa~ 16 Much weaker acids!
Amines
 Compounds with NH2 groups!
 pKa~ 40 Extremely weak, rarely
behave as acids.
 In fact, amines are the most common
organic bases!
Protonated Amines
Protonated Alcohols and
Protonated Carboxylic Acids
An Alcohol Can Behave as
an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor
A Carboxylic Acid Can Behave
as an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor
An Amine Can Behave
as an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor
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.
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!
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
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!
Why are Alcohols Stronger Acids Than Amines?
Oxygen is more electronegative than nitrogen.
Why are Protonated Alcohols Stronger Acids Than
Protonated Amines?
Oxygen is more electronegative than nitrogen.
Electronegativity
increasing
electronegativity
pKa Values
CH4
~60
NH3
36
H2O
15.7
HF
H
H C
3.2
H
H N
O
RCH3
45
RNH2
35
ROH
18
R C CH3 20
O
R C NH2 15
H
O
H O
F
Consider the conjugate bases
R C OH
5
Hybridization Affects Electronegativity
The weakest acid has the
strongest conjugate base.
Atomic Size
 When atoms are very different in size, the strongest
acid will have its hydrogen attached to the largest atom!
Atomic Size
increasing
size
pKa Values
O
HF
3.5
F–
1.36 Å
H2O
16
HCl
–7
Cl–
1.81 Å
H2S
7
H2Se
4
H2Te
3
HBr
HI
–9
–10
Br– 1.95 Å
I–
Consider the ionic radii
2.16 Å
R C OH
O
R C SH
S
R C SH
Some pKa Values
2.7 How substituents affect the strength
of an acid
inductive electron withdrawal
A Substituent’s Effect on pKa
Depends on Distance
Why is a Carboxylic Acid a Stronger Acid
Than an Alcohol?
1. inductive electron withdrawal
Why is a Carboxylic Acid a Stronger Acid
Than an Alcohol?
2. delocalized electrons (resonance)
Summary of Factors
That Affect Acid Strength
electronegativity and size
Summary of Factors
That Affect Acid Strength
hybridization
Summary of Factors
That Affect Acid Strength
inductive electron withdrawal
Summary of Factors
That Affect Acid Strength
electron delocalization
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.
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.
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!