Table 3.1 Relative Strength of Selected Acids and their Conjugate Bases
Acid Approximate pKa Conjugate Base
Strongest Acid HSbF 6 (a super acid) < − 12 SbF 6 − Weakest Base
Incr
easing ac
id stre
ngth
HI
H 2 SO 4
HBr
HCl
C 6 H 5 SO 3 H
(CH 3 ) 2 O+H
(CH 3 ) 2 C=O+H
CH 3 O+H 2
H 3 O+
HNO 3
CF 3 CO 2 H
HF
H 2 CO 3
CH 3 CO 2 H
CH 3 COCH 2 COCH 3
NH 4 +
C 6 H 5 OH
HCO 3 −
CH 3 NH 3 +
H 2 O
CH 3 CH 2 OH
(CH 3 ) 3 COH
CH 3 COCH 3
HC≡CH
H 2
NH 3
CH 2 =CH 2
− 10
− 9
− 9
− 7
−6.5
−3.8
−2.9
−2.5
−1.74
−1.4
0.18
3.2
3.7
4.75
9.0
9.2
9.9
10.2
10.6
15.74
16
18
19.2
25
35
38
44
I−
HSO 4 −
Br−
Cl−
C 6 H 5 SO 3 −
(CH 3 ) 2 O
(CH 3 ) 2 C=O
CH 3 OH
H 3 O
HNO 3 −
CF 3 CO 2 −
F−
HCO 3 −
CH 3 CO 2 −
CH 3 COCH−COCH 3
NH 4 +
C 6 H 5 O−
HCO 32 −
CH 3 NH 3
HO−
CH 3 CH 2 O−
(CH− 3 ) 3 CO−
CH 2 COCH 3
HC≡C−
H−
NH 2 −
CH 2 =CH−
Incre
asing base strength
Weakest Acid CH 3 CH 3 50 CH 3 CH 2 − Strongest Base
3.5C PREDICTING THE STRENGTH OF BASES
- The stronger the acid, the weaker will be its conjugate base.
- The larger the pKa of the conjugate acid, the stronger is the base.