Ionization Constants of Organic Acids

Because of the very large range of acid strengths (greater than 10⁴⁰), a logarithmic scale of acidity (pK_a) is normally employed. Stronger acids have smaller or more negative pK_a values than do weaker acids. A discussion of acid-base terminology is available here. The pK_a values given here are extrapolated for water at 25 °C. Many of the pK_a values given for weak carbon acids are estimated from the rates of hydrogen exchange with very strong bases.

The ACS Organic Division's Organic Chemistry Data Website has a list of other collections of pK_a values.

Common Name	Formula	Acidity Constant	pKa
trifluoromethanesulfonic acid	CF3SO3H	ca. 1013	ca. -13
benzenesulfonic acid	C6H5SO3H	ca. 103	ca. -2.5
methanesulfonic acid	CH3SO3H	ca. 3 * 102	ca. -2.0
trifluoroacetic acid	CF3CO2H	1.0	0.0
picric acid	(O2N)3C6H2OH	0.5	0.3
squaric acid		K1 = 0.33 K2 = 3.3 * 10-4	1.5 3.5
trichloroacetic acid	CCl3CO2H	0.23	0.77
oxalic acid	(CO2H)2	K1 = 6.5 * 10-2 K2 = 6.1 * 10-5	1.2 4.2
dichloroacetic acid	CHCl2CO2H	5.5 * 10-2	1.25
fluoroacetic acid	FCH2CO2H	2.5 * 10-3	2.6
chloroacetic acid	ClCH2CO2H	1.36 * 10-3	2.87
citric acid	C(OH)(CH2CO2H)2CO2H	K1 = 7.4 * 10-4 K2 = 1.7 * 10-5 K3 = 4.0 * 10-7	3.13 4.76 6.40
formic acid	HCO2H	1.77 * 10-4	3.75
ascorbic acid		K1 = 6.7 * 10-5 K2 = 2.5 * 10-12	4.17 11.6
benzoic acid	C6H5CO2H	6.3 * 10-5	4.20
acetic acid	CH3CO2H	1.77 * 10-5	4.75
thiophenol	C6H5SH	2.5 * 10-7	6.6
tropolone		2.0 * 10-7	6.7
p-nitrophenol	O2NC6H4OH	5.7 * 10-8	7.2
peracetic acid	CH3COO2H	5.7 * 10-9	8.2
succinimide	(CH2CO)2NH	2.5 * 10-10	9.6
phenol	C6H5OH	10-10	10.0
chloral hydrate	CCl3CH(OH)2	10-10	10.0
benzenesulfonamide	C6H5SO2NH2	8 * 10-11	10.1
2-hydroxy-2-cyclohexenone		5.0 * 10-11	10.3
ethanethiol	C2H5SH	2.5 * 10-11	10.6
acetoxime	(CH3)2C=NOH	6 * 10-13	12.2
2,2,2-trifluoroethanol	CF3CH2OH	4 * 10-13	12.4
imidazole	C3H3N2H	3.3 * 10-15	14.5
pyrrole	C4H4NH	10-15	15
ethanol	C2H5OH	10-16	16
1°-amides	RCONH2	10-17	17
p-nitroaniline	O2NC6H4NH2	3.3 * 10-19	18.5
t-butanol	(CH3)3COH	10-19	19
aniline	C6H5NH2	10-27	27
1,1,1,3,3,3,-hexamethyldisilazane	[(CH3)3Si]2NH	10-30	30
pyrrolidine	C4H8NH	10-32	32
diisopropylamine	[(CH3)2CH]2NH	1.9 * 10-36	35.7
tetramethylpiperidine		10-37	37

Ionization Constants of Carbon Acids

Common Name	Formula	Acidity Constant	pKa
tricyanomethane	CH(CN)3	105	-5
dinitromethane	CH2(NO2)2	2.5 * 10-4	3.6
5,5-dimethyl-1,3-cyclohexadione		1.6 * 10-5	4.8
malonaldehyde	CH2(CHO)2	10-5	5.0
Meldrum's acid		8 * 10-6	5.1
2,4-pentanedione	CH2(COCH3)2	1.26 * 10-9	8.9
hydrocyanic acid	HCN	6.3 * 10-10	9.2
nitromethane	CH3NO2	6.3 * 10-11	10.2
malononitrile	CH2(CN)2	10-11	11.0
ethyl acetoacetate	CH3COCH2CO2C2H5	10-11	11.0
tris(trifluoromethyl)methane	(CF3)3CH	10-11	11.0
bis(methylsulfonyl)methane	CH2(SO2CH3)2	2.0 * 10-13	12.7
diethyl malonate	CH2(CO2C2H5)2	3.2 * 10-14	13.5
bromoform	Br3CH	3 * 10-14	13.7
chloroform	Cl3CH	3.2 * 10-16	15,5
cyclopentadiene		10-16	16.0
aldehydes	RCH2CHO	10-17	17
cyclohexanone		10-17	17
phenylacetylene	C6H5C≡CH	10-19	19.0
indene		10-20	20
methyl ketones	RCOCH3	10-20	20
tris(phenylthio)methane	(C6H5S)3CH	10-23	23
methylsulfones	RSO2CH3	10-25	25
alkyl esters	RCH2CO2C2H5	10-25	25
nitriles	RCH2CN	10-25	25
acetylene	HC≡CH	10-25	25
fluroform	F3CH	10-28	28
methylsulfoxides	RSOCH3	10-30	30
1,3-dithiane	(CH2)3S2CH2	10-31	31
triphenylmethane	(C6H5)3CH	3.2 * 10-32	31.5
diphenylmethane	(C6H5)2CH2	3.2 * 10-34	33.5
1-methylnaphthylene	1-C10H7CH3	10-37	37
2-methylnaphthylene	2-C10H7CH3	3.2 * 10-38	37.5
toluene	C6H5CH3	10-40	40
benzene	C6H6	10-43	43
ethene	C2H4	10-44	44
cyclopropane	C3H6	10-46	46
ethane	C2H6	10-48	48

The ACS Organic Division's Organic Chemistry Data Website has a list of other collections of pK_a values.