ACIDITY AND pK a VALUES
123
Table 4.1 pK a values of H–X acids
Acid
Conjugate base
pK a
CH 4
CH 3
48
NH 3
NH 2
38
H 2
H
35
H 2 O
H O
15.7
H–C≡N
C≡N
9 . 1
H 2 S
H S
7
HF
F
3.2
H 3 PO 4
H 2 PO 4
2.1
HNO 3
NO 3
−1.4
H 2 SO 4
HSO 4
−3.0
HCl
Cl
−7
HBr
Br
−9
HI
I
−10
Table 4.2 pK a values of C–H acids
Acid
Conjugate base
pK a
H
H
H
52
H 3 C–CH 3
H 3 C–CH 2
50
CH 4
CH 3
48
H 2 =CH 2
H 2 =CH
44
H
44
H 2 C=CH–CH 3
H 2 C=CH–CH 2
43
CH 3
CH 2
41
Ph 3 C–H
Ph 3 C
32
H 3 C–C≡N
H 2 C–C≡N
2 5
HC≡C–H
HC≡C
25
or even by indirect methods; results are then extrapolated to give the value in water. The figures presented
in Tables 4.1–4.6 have been intentionally rounded to
stress that a high level of accuracy is usually inappropriate.
The range of pK a values that can be measured in
water is determined by the ionization of water itself,
i.e. −1.74 (the pK a of H 3 O
+ ) to 15.74 (the pK a
of H 2 O); see Box 4.1. Acids that are stronger than
H 3 O
+ simply protonate water, whereas bases that
are stronger than HO
− remove protons from water.
Table 4.3 pK a values of N–H, O–H, and S–H acids
Acid
Conjugate base
pK a
NH 3
NH 2
38
N H
(CH 3 ) 2 CH
(CH 3 ) 2 CH
N
(CH 3 ) 2 CH
(CH 3 ) 2 CH
36
CH 3 CH 2 NH 2
CH 3 CH 2 NH
35
Ph–NH 2
Ph–NH
28
CH 3 CONH 2
CH 3 CONH
15
(CH 3 ) 3 C–OH
(CH 3 ) 3 C–O
19
CH 3 OH
CH 3 O
15.5
CH 3 CH 2 OH
CH 3 CH 2 O
16
H 2 O
H O
15.7
Ph–OH
Ph–O
10
CH 3 SH
CH 3 S
10.5
H 2 S
H S
7
Ph–SH
Ph–S
6.5
Table 4.4 pK a values of CO 2 H and SO 3 H acids
Acid
Conjugate base
pK a
CH 3 CO 2 H
C H 3 CO 2
4.8
Ph–CO 2 H
Ph–CO 2
4.2
HCO 2 H
H C O 2
3.7
ClCH 2 CO 2 H
C l C H 2 CO 2
2.9
Cl 2 CHCO 2 H
C l 2 CHCO 2
1.3
Cl 3 CCO 2 H
C l 3 CCO 2
0.7
F 3 CCO 2 H
F 3 CCO 2
−0.3
Me–SO 3 H
M e – S O 3
−1.2
SO 3 H
H 3 C
SO 3
H 3 C
−1.3
However, the fact that they do have to be measured
means that as you look in the literature for the pK a of
a particular compound you may find slightly different
values can be presented. Do not let this confuse you.
As mentioned above, relative, rather than specific,
values are our main concern.
We have chosen to present the pK a values as a
series of tables, rather than in a single one. This
should help you to locate a particular compound
123
Table 4.1 pK a values of H–X acids
Acid
Conjugate base
pK a
CH 4
CH 3
48
NH 3
NH 2
38
H 2
H
35
H 2 O
H O
15.7
H–C≡N
C≡N
9 . 1
H 2 S
H S
7
HF
F
3.2
H 3 PO 4
H 2 PO 4
2.1
HNO 3
NO 3
−1.4
H 2 SO 4
HSO 4
−3.0
HCl
Cl
−7
HBr
Br
−9
HI
I
−10
Table 4.2 pK a values of C–H acids
Acid
Conjugate base
pK a
H
H
H
52
H 3 C–CH 3
H 3 C–CH 2
50
CH 4
CH 3
48
H 2 =CH 2
H 2 =CH
44
H
44
H 2 C=CH–CH 3
H 2 C=CH–CH 2
43
CH 3
CH 2
41
Ph 3 C–H
Ph 3 C
32
H 3 C–C≡N
H 2 C–C≡N
2 5
HC≡C–H
HC≡C
25
or even by indirect methods; results are then extrapolated to give the value in water. The figures presented
in Tables 4.1–4.6 have been intentionally rounded to
stress that a high level of accuracy is usually inappropriate.
The range of pK a values that can be measured in
water is determined by the ionization of water itself,
i.e. −1.74 (the pK a of H 3 O
+ ) to 15.74 (the pK a
of H 2 O); see Box 4.1. Acids that are stronger than
H 3 O
+ simply protonate water, whereas bases that
are stronger than HO
− remove protons from water.
Table 4.3 pK a values of N–H, O–H, and S–H acids
Acid
Conjugate base
pK a
NH 3
NH 2
38
N H
(CH 3 ) 2 CH
(CH 3 ) 2 CH
N
(CH 3 ) 2 CH
(CH 3 ) 2 CH
36
CH 3 CH 2 NH 2
CH 3 CH 2 NH
35
Ph–NH 2
Ph–NH
28
CH 3 CONH 2
CH 3 CONH
15
(CH 3 ) 3 C–OH
(CH 3 ) 3 C–O
19
CH 3 OH
CH 3 O
15.5
CH 3 CH 2 OH
CH 3 CH 2 O
16
H 2 O
H O
15.7
Ph–OH
Ph–O
10
CH 3 SH
CH 3 S
10.5
H 2 S
H S
7
Ph–SH
Ph–S
6.5
Table 4.4 pK a values of CO 2 H and SO 3 H acids
Acid
Conjugate base
pK a
CH 3 CO 2 H
C H 3 CO 2
4.8
Ph–CO 2 H
Ph–CO 2
4.2
HCO 2 H
H C O 2
3.7
ClCH 2 CO 2 H
C l C H 2 CO 2
2.9
Cl 2 CHCO 2 H
C l 2 CHCO 2
1.3
Cl 3 CCO 2 H
C l 3 CCO 2
0.7
F 3 CCO 2 H
F 3 CCO 2
−0.3
Me–SO 3 H
M e – S O 3
−1.2
SO 3 H
H 3 C
SO 3
H 3 C
−1.3
However, the fact that they do have to be measured
means that as you look in the literature for the pK a of
a particular compound you may find slightly different
values can be presented. Do not let this confuse you.
As mentioned above, relative, rather than specific,
values are our main concern.
We have chosen to present the pK a values as a
series of tables, rather than in a single one. This
should help you to locate a particular compound