Acidity of Representative Organic and Inorganic Acids

For some particularly thorough comparisons of pKa values, consult:

  1. Miller, Table 1-2
  2. March, Table 8.1

Note that the pKa value will be different in different solvents; consult one of the above texts for a complete description of how the pKa value is determined. Tables below are intended to give you a comparative list of Bronsted acids and their relative acidity. The quantitative application of this data requires knowing the pKa for all acids in the solvent of interest.

Some representative values.

Inorganic and sulfonic acids.

Acid Conjugate Base pKa
HCl Cl- -8
CF3SO3H CF3SO3- -5.9
PhSO3H PhSO3- -2.8
H2SO4 HSO4- -2.8
H3O+ H2O -1.7
H3PO4 H2PO4- 2.1
HF F- 3.2
H2CO3 HCO3- 3.7
H2S HS- 7.0
NH4+ NH3 9.2
HCO3- CO3-2 10.2
HOOH HOO- 11.6
H2O HO- 15.74

Protonated organic molecules

Acid Conjugate Base pKa
Me2SH+ Me2S -6.99
PhOCH3 -6.5
Ethyl benzoate -6.2
Benzoic acid -4.7
Acetophenone -4.3
Benzaldehyde -3.9
Me2OH+ Me2O -3.8
(CH3)2C=OH+ Acetone -2.85
CH3OH2+ Methanol -2.5
Benzamide -1.74
(CH3)2S=OH+ DMSO -1.5
N, N-dimethylacetamide 0.1
Aniline 4.6
Pyridine 5.2
Cyclohexylamine 10.7
Et3NH+ Triethylamine 10.8
Et2NH+ Diethylamine 11.0

Carboxylic Acids

in water unless otherwise noted

Acid Conjugate Base pKa
CF3CO2H CF3CO2- -0.6
CCl3CO2H CCl3CO2- -0.5
HO2CCO2H HO2CCO2- 1.25
Cl2CHCO2H Cl2CHCO2- 1.35
FCH2CO2H FCH2CO2- 2.6
ClCH2CO2H ClCH2CO2- 2.86
HCO2H HCO2- 3.75
PhCO2H PhCO2- 4.2
CH3CO2H CH3CO2- 4.76
PhCO2H (DMSO) PhCO2- 11.0
CH3CO2H (DMSO) CH3CO2- 12.3

Alcohols and amines

Acid Conjugate Base pKa
p-Nitrophenol p-Nitrophenoxide 7.2
Phenol Phenoxide 10.0
EtSH EtS- 10.6
CF3CH2OH CF3CH2O- 12.4
Imidazole 14.5
CH3OH CH3O- 15.5
CH3CH2OH CH3CH2O- 15.9
(CH3)2CHOH (CH3)2CHO- 17.1
(CH3)3COH (CH3)3CO- 17.2
Ph2NH (DMSO/H2O) Ph2N- 22.4
iPr2NH (THF) iPr2N- 35.7
NH3 NH2- 41

"Activated" C-H bonds

Acid Conjugate Base Solvent pKa
H2O 9.0
    DMSO 13.4
CH3NO2 -CH2NO2 H2O 10.0
    DMSO 17.2
H2O 10.9
NC-CH2-CO2Et NC-CH--CO2Et DMSO 12.8
CH3CO-CH2-CO2Et CH3CO-CH--CO2Et DMSO 14.2
    H2O 11
CH3SO2-CH2-SO2CH3 CH3SO2-CH--SO2CH3 H2O 12.5
EtO2C-CH2-CO2Et EtO2C-CH--CO2Et H2O 13
CH3CHO -CH2CHO H2O 16.5
Acetone -CH2COCH3 H2O 19.2
    DMSO 26.5
CH3SO2Ph -CH2SO2Ph DMSO 29
CH3CO2Et -CH2CO2Et DMSO 30.5
CH3CN -CH2CN DMSO 31.2

Hydrocarbons

All are in DMSO unless otherwise noted.

Acid Conjugate Base pKa
18.1
22.6
HCCH HCC- 25
Ph3CH Ph3C- 30.6
Ph2CH2 Ph2CH- 32.3
PhCH3 PhCH2- 43
Benzene Ph- 43
CH2=CH2 CH2=CH- 44
CH3CH=CH2 -CH2-CH=CH2 48
CH3-CH3 -CH2-CH3 50
CH4 -CH3 58-60

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Last updated: 09/21/00