Acid-Base Chemistry
A- + HB+
HA + :B
acid
conjugate conjugate
base
acid
base
HA and A- are a conjugate acid-base pair.
HB+ and B: are also a conjugate acid-base pair.
Ka is the equilibrium constant for the dissociation of an acid into H+ and its
conjugate base. Ka values are typically measure in water.
HA
We define pKa = -logKa
H+
+
A-
aH  a A
Ka 
a HA


just as pH = -log[H3O+]
Weak acids have small Ka and large pKa values
Strong acids have large Ka and small pKa values
Remember – when comparing acids, the weaker acid will have…the
stronger conjugate base.
1
A Brønsted acid is simply an electrophile that loses H+ to a base. The acidic site is
the hydrogen atom. A Brønsted base is simply a nucleophile that makes a bond to
the hydrogen atom of an acid. The basic site is the atom/bond donating the
electrons used to make this bond.
H
N
H
C
S
H
N
C
H
H
N
C
H
O
H
N
C
H
H
H
O
H
C
N
S
O
H
H
O
C
N
H
2
Acid-Base Chemistry
some typical pKa values
Acid
Ka
pKa
HClO4
1010
-10
HF
7.2 x 10-4
3.1
NH4+
6.3 x 10-10
9.2
H2O
2.0 x 10-16
15.7
H2
10-35
35
3
So, for an equilibrium between two conjugate acid/base pairs, the
equilibrium will always favour the side which has…the weakest acid.
What does this mean for ‘real life chemistry’?
OH
+ CH3CO2
Ka
pKa
1.0 x 10-10
10
O
+ CH3CO2H
1.8 x 10-5
4.72
So, if you wanted this equilibrium to be product favoured, you would use a
base…whose conjugate acid was weaker than phenol.
4
Table 5.1
(page 144)
pKa
Compound types
-10
mineral acids: H2SO4, HI, HBr, HCl, sulfonic acids RSO3H
0
H3O+, H3PO4
5
Carboxylic acids, HF, thiophenols ArSH, HN3
10
Weak inorganic acids (H2S, HCN, NH4+, amine salts RNH3+, phenols ArOH,
thiols (RSH), aromatic amides ArCONH2
15
H2O, alcohols ROH, amids RCONH2)
20
Ketones (the alpha proton H-CH2COR)
25
Esters (the alpha proton H-CH2CO2R), alkynes RCCH, nitriles (H-CH2CN)
30
Anilines ArNH2
40
Ammonia NH3, amines RNH2, benzylic protons ArCH3
45
Arenes ArH and alkenes RCH=CH2
50
Alkanes
In water, there can be no acid stronger than…H3O+
…and no base stronger than…-OH
5
Trends in Acidity
Across the period...
Conjugate Base
Acid
pKa
48
H3C
H
H2N
H
36
HO
H
15.7
F
H
3.1
Down a group...
HO
H
15.7
HS
H
11
6
Pi Bonds…
H3C
H
48
H
45
C
H
C
H
25
H
-2
O
H
-4 to -10
O
7
H
Resonance...
pKa
H3CH2C
17
O
H
O
5
O
H
O
H
10
8
Resonance eff ects greatly enhance the acidity of "activated" C—H bonds...
48
CH4
CH3
N
C
O
36
CH2
H
25
O
H
19
H 9
9
Inductive and Field effects
X
H2
C
O
C
+ H2O
O
X
CH3
H
I
Br
Cl
F
CN
NO2
H
X
H2
C
O
+ H3O+
C
O
pKa
4.87
4.76
3.17
2.90
2.86
2.59
2.46
1.68
10
These effects are…additive.
O
ClH2C
C
O
Cl2HC
O
C
Cl3C
C
OH
OH
OH
2.85
1.48
0.70
These effects…drop off rapidly with increasing number of bonds.
O
C
OH
4.82
O
Cl
C
OH
Cl
2.86
O
O
Cl
C
C
OH
OH
4.05
4.52
11
Bases
pKa + pKb for a conjugate acid/base pair = 14
Although potentially confusing, when evaluating base strength we often
look at the pKa of its conjugate acid. The higher the pKa of the conjugate
acid, the stronger the base.
12
Base
pKa of conjugate acid
Conjugate acid
N:
N
H
5
R3N:
R3NH
11
H
O
H2O
15
R
O
ROH
H2NH
H2N
N
NH
16-18
38
40
13
The same things that affect acidity, also affect basicity...
Across the periodic table...
H3N > H2O > HF
Down a column...
H2N: > H2P:
HO > HS
H3N > H3P
H2O > H2S
Hybridization...
NH
N:
O
O
H
Conj. pKa:
12.20
5.3
-2
-4 to -10
14
Resonance…
H
H
N
N
H
H
O
pKa of conjugate is 4.62
N
O
pKa of conjugate is 1.00
Sterics…
pKa
CA
base
Et2NH
Et2NH2
NH
11.0
NH2
12.2
15
Inductive effects...
NH3
pKa of conj.
9.25
CH3NH2
10.65
(CH3)2NH
10.73
(CH3)3N
9.78
Amphiprotic species
CH3OH + H2SO4
CH3OH2+ + HSO4-
CH3OH + NH2
CH3O + NH3
16
Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify which nitrogen atom is more basic. Why?
O
C
H2N
NH2
C
HH
17
Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify the acidic and basic sites and rank them in
terms of reactivity (by category; don’t compare acid to base). Rationalize your
rankings.
O
C
O
O
18
Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify the acidic, basic, electrophilic, and
nucleophilic sites and rank them in terms of reactivity (by category). Rationalize
your rankings.
O
H
C
C
H3C
C
C
HH
19
Acids, bases, electrophiles, and nucleophiles.
When solving a mechanism, we can use the different kinds of sites (A, B, E, Nu)
to help us.
e.g. How can we rationalize the following reaction?
D
O
H
C
H
D
H
C
C
Cl
H
+
H3C
O
H
O
C
H3C
O
C
H
D
C
+
Cl
D
20
Acids, bases, electrophiles, and nucleophiles.
•It is incorrect to mix-and-match the terms acid and electrophile, or base and
nucleophile. They are not interchangeable.
e.g. I – is a good nucleophile but not a good base
•It is, however, entirely possible for a molecule to be both an acid and an
electrophile – or both a base and a nucleophile. It is even possible for the same
molecule to be an acid, a base, a nucleophile and an electrophile all at the same
time. In that case, how it reacts will depend on the other species in the reaction
flask (since usually the most nucleophilic site reacts with the most electrophilic
site – assuming it can reach).
21