A common method for evaluating the strength of bases is to report the acidities of the conjugate acids of the bases (these conjugate acids are often "onium" cations). The resulting pKa's are proportional to the base strength of the base.

A useful rule is: pKa + pKb = 14.

Ionization Constants of B-H Onium Acids

Name of Base (B)

Formula of B

Ka (BH+)

pKa

aliphatic nitro cpd.

R-NO2

ca. 1012

ca. -12

aryl nitro cpd.

C6H5NO2

ca. 1011

ca. -11

nitriles

R-C≡N

ca. 1010

ca. -10

aliphatic aldehydes

R-CH=O

ca. 108

ca. -8

aryl aldehydes

C6H5CH=O

ca. 107

ca. -7

aliphatic ketones

R2C=O

ca. 107

ca. -7

esters

R-CO2R'

ca. 3 * 106

ca. -6.5

aryl alkyl ethers

C6H5OCH3

ca. 3 * 106

ca. -6.5

phenols

C6H5OH

ca. 3 * 106

ca. -6.5

aryl ketones

C6H5COCH3

ca. 106

ca. -6

carboxylic acids

R-CO2H

ca. 106

ca. -6

sulfides

RSR

ca. 105

ca. -5

diethyl ether

C2H5OC2H5

ca. 3 * 103

ca. -3.5

dioxane

O(CH2CH2)2O

ca. 103

ca. -3

pyran

(CH2)5O

ca. 3 * 102

ca. -2.5

tetrahydrofuran

(CH2)4O

ca. 102

ca. -2

alcohols

R-CH2-OH

ca. 102

ca. -2

aryl amides

C6H5C(NH2)=O

ca. 102

ca. -2

indole

ca. 102

ca. -2

water

H2O

55

-1.74

aliphatic amides

RC(NH2)=O

3.2

-0.5

dimethyl sulfoxide

(CH3)2SO

1

0

phosphine oxides

R3PO

1

0

pyrrole

C4H4NH

1

0

urea

(NH2)2C=O

0.8

0.1

diphenylamine

(C6H5)2NH

0.15

0.8

p-nitroaniline

4-O2NC6H4NH2

0.1

1.0

aniline

C6H5NH2

2.5 * 10-5

4.6

trimethylamine N-oxide

(CH3)3NO

2.5 * 10-5

4.6

N,N-dimethylaniline

C6H5N(CH3)2

10-5

5.1

pyridine

C5H5N

6.3 * 10-6

5.2

hydroxyl amine

HONH2

1.3 * 10-6

5.9

2,6-dimethylpyridine

2.0 * 10-7

6.7

imidazole

10-7

7.0

hydrazine

H2NNH2

10-8

8.0

alkyl phosphines

R3P

10-8

8.0

aziridine

10-8

8.0

2,2,2-trifluoroethylamine

CF3CH2NH2

5.0*10-9

8.3

morpholine

O(CH2CH2)2NH

5.0*10-9

8.3

DABCO

K1 = 2.0 * 10-9
K2 = 6.3 * 10-5

 8.7
 4.2

ammonia

NH3

5.62 * 10-10

9.25

4-dimethylaminopyridine

4-(CH3)2N-C5H4N

2.0 * 10-10

9.7

ethyl amine

C2H5NH2

2.0 * 10-11

10.7

triethyl amine

(C2H5)3N

1.8 * 10-11

10.8

diethyl amine

(C2H5)2NH

10-11

11.0

piperidine

(CH2)5NH

10-11

11.0

pyrrolidine

(CH2)4NH

6.3 * 10-12

11.2

Hünig's base

3.4 * 10-12

11.4

DBU

10-12

12

proton sponge

4.4 * 10-13

12.3

guanidine

(NH2)2C=NH

2.0 * 10-14

13.6

pentamethylguanidine

[(CH3)2N]2C=NCH3

1.8 * 10-14

13.8