Approximate pKa chart of the functional groups: values to know
1. Protonated carbonyl pKa = ‐7
2. Protonated alcohol or ether pKa = ‐2 to ‐3
3. Carboxylic acid pKa = 4‐5
4. Ammonium ion pKa = 9‐10
5. Phenol pKa = 10
6. Thiol pKa = 10
7. Alcohol pKa = 16‐18
8. Water pKa = 15.7
9. Amide pKa = 18
10. Alpha proton of ketone/aldehyde pKa = 20
11. Alpha proton of ester pKa = 25
12. Terminal alkyne pKa = 25
13. Amine pKa = 38‐40
14. Aromatic: Aryl 43, benzylic 41
15. Alkene: vinyl 45‐50; allylic 43
16. Alkane pKa = above 50
Examples:
-7
-2 to -3
OH
Other important pKa’s
H2 = 35
-2 to -3
H
O
O
OH2
4-5
OH
H2
N
SH 10
OH
10
9-10
16-18
OH
O
O
20
18
CH3
NH2
38-40
25
O
H
N
H
25
H
O
41
H
(benzylic)
43
45-50
H
(aryl)
(allylic)
H
43
H
(vinyl)
above 50
H