Phenols, alcohols and carboxylic acids - pKa values

For oxygen containing organic compounds this is given: pKa (the negative logarithm of the acid dissociation constant), molecular structures, molar weights, density and melting and boiling points.

Definitions of the acid dissociation constant and pKa are given below the figures, together with the definition of some classes of organic acids.

In the table below, pKa1 and pKa2 for water solutions at 25°C are given together with boiling and melting point, density and molecular weight, as well as number of carbon, hydrogen and oxygen atoms in each molecule.

Below the table, figures showing the fractions of the different acid forms in aqueous solutions at varying pH are given for some common diprotic organic acids (values calculated from the tabulated pKas).

See also boiling and melting point of hydrocarbons, alcohols and acidspKa of amines, diamines and cyclic organic nitrogen compounds and pKa of inorganic acids and bases, as well as acid base indicatorsAcid-base properties of aqueous solutions of salts with ions from both acids and bases and Buffer solutions



pKa for alcohols

pKa for paraffinic monoacids



pKa for benzoic and hydroxy acids




pKa for dioic acids



Group IUPAC name Common name #C #H #O pKa1 pKa2 Molweight
g/mol
Melting temp
°C
Boiling temp
°C
Density
@20°C5*

g/ml
1-alkanol Methanol Methyl alcohol 1 4 1 15.5 32.04 -98 65 0.791
1-alkanol Ethanol Ethyl alcohol 2 6 1 15.5 46.07 -114 78 0.789
1-alkanol 1-propanol Propyl alcohol 3 8 1 16.1 60.09 -124 97 0.804
1-alkanol 1-butanol Butyl alcohol 4 10 1 16.1 74.12 -89 118 0.810
2-alkanol 2-propanol Isopropyl alcohol, isopropanol 3 8 1 17.2 60.09 -88 82 0.785
2-alkanol 2-butanol sec-butyl alcohol 4 10 1 17.6 74.12 -88 99 0.806
2-alkanol 2-pentanol sec-amyl alcohol 5 12 1 17.8 88.15 -73 119 0.809
3-alkanol 3-pentanol Diethyl carbinol 5 12 1 18.2 88.15 -70 123 0.820
Phenols Phenol Hydroxybenzene 6 6 1 9.98 94.11 41 181 1.132*
Phenols 1,2-Benzenediol Catechol, pyrocatechol 6 6 2 9.45 12.8 110.11 102 245 1.344*
Phenols 1,3-Benzenediol Resorcinol 6 6 2 9.2 11.3 110.11 110 277 1.278
Phenols 1,4-Benzenediol Hydroquinone, 1,4-Dihydroxybenzene 6 6 2 10.9 11.4 110.11 171 285
Phenols 1,3,5-Benzenetriol Phloroglucinol, 1,3,5-trihydroxybenzene 6 6 3 8.45 126.11 216 s 1.460*
Phenols 2-Methylphenol o-cresol, 2-methylhydroxybenzene 7 8 1 10.29 108.13 31 191 1.135*
Phenols 3-Methylphenol m-cresol, 3-methylhydroxybenzene 7 8 1 10.09 108.13 12 202 1.03*
Phenols 4-Methylphenol p-cresol, 4-methylhydroxybenzene 7 8 1 10.26 108.13 35 202 1.154*
Phenols 2-Methoxyphenol Guaiacol 7 8 2 9.98 124.13 28 204 1.1287*
Phenols 3-Methoxyphenol Resorcinol, monomethyl ether 7 8 2 9.65 124.13 -18 244 1.145*
Phenols 4-Methoxyphenol 7 8 2 10.21 124.13 55 253
Phenols 4-Methyl-1,2-benzenediol 4-Methylcatechol 7 8 2 9.55 124.13 68 251 1.129*
Phenols 2-Ethylphenol 2-Ethylhydroxybenzene 8 10 1 10.2 122.16 -3 205 1.015*
Phenols 3-Ethylphenol 3- Ethylhydroxybenzene 8 10 1 9.9 122.16 -4 218 1.008*
Phenols 4-Ethylphenol 4-Ethylhydroxybenzene 8 10 1 10.00 122.16 45 218 1.05*
Phenols 2-Propylphenol o-propylphenol 9 12 1 10.47 136.19 7 223 1.015
Phenols 4-Propylphenol p-propylphenol 9 12 1 10.34 136.19 22 232 1.009*
Alkanoic acid Formic acid Methanoic acid 1 2 2 3.74 46.0 8 101 1.220
Alkanoic acid Acetic acid Ethanoic acid 2 4 2 4.76 60.1 17 118 1.0446*
Alkanoic acid Butanoic acid Butyric acid 4 8 2 4.82 88.1 -5 164 0.9528*
Alkanoic acid Pentanoic acid Valeric acid 5 10 2 4.86 102.1 -34 186 0.9339*
Alkanoic acid Propanoic acid Propionic acid 3 6 2 4.87 74.1 -21 142 0.9882*
Alkanoic acid Hexanoic acid Caproic acid 6 12 2 4.87 116.2 -4 202 0.9212*
Alkanoic acid Heptanoic acid Enanthic acid 7 14 2 4.89 130.2 -7 222 0.9124*
Alkanoic acid Octanoic acid Caprylic acid 8 16 2 4.89 144.2 17 240 0.907*
Alkanoic acid Nonanoic acid Pelargonic acid 9 18 2 4.96 158.2 12 256 0.905
Alkanoic acid, branched 2-Methylpropanoic acid Isobutyric acid 4 8 2 4.84 88.1 -46 155 0.943*
Alkanoic acid, branched 2,2-Dimethylpropanoic acid Trimethylacetic acid 5 10 2 4.78 102.1 35 164 0.9052*
Alkanoic acid, branched 2-Methylbutanoic acid 5 10 2 4.80 102.1 -70 176 0.934
Alkanoic acid, branched 3-Methylbutanoic acid Isovaleric acid 5 10 2 4.77 102.1 -30 176 0.925
Alkanoic acid, branched 4-Methylpentanoic acid 4-Methylvaleric acid, Isocaproic acid 6 12 2 4.84 116.2 -33 200 0.923
Alkanoic acid, branched 2-Propylpentanoic acid Valproic acid 8 16 2 4.60 144.2 223 0.904*
Phenylalkanoic acid Phenylethanoic acid α-Tolylic acid, Benzeneacetic 8 8 2 4.31 136.1 77 266 1.081*
Phenylalkanoic acid 2-Phenylbutyric acid a-Ethyl-a-toluic acid 10 12 2 4.66 47.5
Benzoic acid Benzoic acid Benzenecarboxylic acid 7 6 2 4.20 122.1 122 249 1.2663*
Benzoic acid 2-Methyl-benzoic acid o-Toluic acid 8 8 2 3.91 136.1 107 258 1.0624*
Benzoic acid 3-Methyl-benzoic acid m-Toluic acid 8 8 2 4.25 136.1 111 1.0544*
Benzoic acid 4-Methyl-benzoic acid p-Toluic acid 8 8 2 4.37 136.1 182 275
Benzoic acid 2-Phenylbenzoic acid 13 10 2 3.46 198.2 112 344
Cinnamic acid trans-m-Methylcinnamic acid 10 10 2 4.44 115
Cinnamic acid trans-o-Methylcinnamic acid 10 10 2 4.50 162.2 175
Cinnamic acid trans-p-Methylcinnamic acid 10 10 2 4.56 199
Hydroxy acid Hydroxyethanoic acid Glycolic acid 2 4 3 3.88 76.1 80 d
Hydroxy acid 2-Hydroxy-benzoic acid Salicylic acid 7 6 3 2.97 13.6 138.1 159 1.443
Hydroxy acid 3-Hydroxy-benzoic acid 7 6 3 4.80 9.9 138.1 202
Hydroxy acid 4-hydroxy-benzoic acid 7 6 3 4.58 9.4 138.1 215
Dioic acid Ethanedioic acid Oxalic acid 2 2 4 1.25 4.29 90.0 d 190 s 157 1.93*
Dioic acid Propanedioic acid Malonic acid 3 4 4 2.85 5.69 104.1 136 d 140 1.6193*
Dioic acid cis-Butenedioic acid Maleic 4 4 4 1.92 6.22 116.1 139 d 1.59*
Dioic acid trans-Butenedioic acid Fumaric 4 4 4 3.02 4.39 116.1 s 300 1.635
Dioic acid 2-Methylpropanedioic acid Methylmalonic acid 4 6 4 3.07 5.76 118.1 129 1.455
Dioic acid Butanedioic acid Succinic acid 4 6 4 4.21 5.64 118.1 185 d 235 1.572*
Dioic acid Pentanedioic acid Glutaric acid 5 8 4 3.22 4.82 132.1 98 273 1.4293*
Dioic acid 3-Methylpentanedioic acid 3-Methylglutaric acid 6 10 4 4.24 146.1 83
Dioic acid Hexanedioic acid Adipic acid 6 10 4 4.34 5.41 146.1 152 338
Dioic acid Heptanedioic acid Pimelic acid 7 12 4 4.71 5.58 160.2 104 357 d?
Dioic acid 1,2-Benzenedicarboxylic acid o-Phthalic 8 6 4 4.42 5.41 166.1 d 210
Dioic acid Octanedioic acid Suberic acid 8 14 4 4.52 5.50 174.2 142 230 d?
Dioic acid Nonanedioic acid Azelaic acid 9 16 4 4.53 5.33 188.2 110 336 d?
Dioic acid Decanedioic acid Sebacic acid 10 18 4 4.59 5.59 202.2 131 374 d? 1.2705
Dioic acid Undecanedioic acid 1,9-Nonanedicarboxylic acid 11 20 4 4.65 216.3 109
Dioic acid Dodecanedioic acid Decane-1,10-dicarboxylic acid 12 22 4 4.65 230.3 128 417 d?
Dioic acid Tridecanedioic acid Brassylic acid 13 24 4 4.65 244.3 113
Chlorocarboxylic acid Chloroacetic acid CH2ClCO2H 2 3 2 2.87 94.5 62 189 1.40435*
Chlorocarboxylic acid Dichloroacetic acid CHCl2CO2H 2 2 2 1.35 128.9 12 193 1.564
Chlorocarboxylic acid Trichloroacetic acid CCl3CO2H 2 1 2 0.66 163.4 59 198 1.612*
Chlorocarboxylic acid 2-Chloropropanoic acid C2H4ClCO2H 3 5 2 2.83 108.5 185 1.26
Chlorocarboxylic acid 3-Chloropropanoic acid C2H4ClCO2H 3 5 2 3.98 108.5 204 d
Chlorocarboxylic acid 2-Chlorobutanoic acid C3H6ClCO2H 4 7 2 2.86 122.6 1.18
Chlorocarboxylic acid 3-Chlorobutanoic acid C3H6ClCO2H 4 7 2 4.05 122.6 16 1.19
Chlorocarboxylic acid 4-Chlorobutanoic acid C3H6ClCO2H 4 7 2 4.52 122.6 16 1.22
Chlorocarboxylic acid 2-Chlorobenzoic acid C6H4ClCO2H 7 5 2 2.90 156.6 140 274 1.5445*
Chlorocarboxylic acid 3-Chlorobenzoic acid C6H4ClCO2H 7 5 2 3.84 156.6 154 283 1.4965*
Chlorocarboxylic acid 4-Chlorobenzoic acid C6H4ClCO2H 7 5 2 4.00 156.6 240
Bromocarboxylic acid Bromoacetic acid CH2BrCO2H 2 3 2 2.90 139.0 50 208 1.93352*
Bromocarboxylic acid 3-Bromopropanoic acid C2H4BrCO2H 3 5 2 4.00 153.0 63 1.485*
Bromocarboxylic acid 2-Bromobenzoic acid C6H4BrCO2H 7 5 2 2.85 201.0 149 295 1.9295*
Bromocarboxylic acid 3-Bromobenzoic acid C6H4BrCO2H 7 5 2 3.81 201.0 157 285 1.8455*
Bromocarboxylic acid 4-Bromobenzoic acid C6H4BrCO2H 7 5 2 3.96 201.0 254 1.8945*
Fluorocarboxylic acid Fluoroacetic acid CH2FCO2H 2 3 2 2.59 78.1 35 168 1.3682*
Fluorocarboxylic acid Trifluoroacetic acid CF3CO2H 2 1 2 0.52 114.0 -15 72 1.5351*
Fluorocarboxylic acid 2-Fluorobenzoic acid C6H4FCO2H 7 5 2 3.27 140.1 124 1.465*
Fluorocarboxylic acid 3-Fluorobenzoic acid C6H4FCO2H 7 5 2 3.86 140.1 124 0.475*
Fluorocarboxylic acid 4-Fluorobenzoic acid C6H4FCO2H 7 5 2 4.15 140.1 184 1.485*
Fluorocarboxylic acid Pentafluorobenzoic acid C6F5CO2H 7 1 2 1.75 212.1 103 220
s = sublimes,   d = decomposes,     * measured at 25°C,     2* measured at 50°C,      3* measured at 15°C,      4* measured at 115°C,      5* Measured at solid state
d? = Boiling temperature at 1 atm is estimated from low pressure measurements. The decomposition temperature is not known.



An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation of acid–base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

HA + H2O   =  A- +H3O+

where HA is an acid that dissociates into A, (known as the conjugate base of the acid) and a hydrogen ion which combines with a water molecule to make a hydronium ion.

The chemical species HA, A and H3O+ are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A] and [H3O+]

Ka = [A-]*[H3O+] / [HA]*[H2O]

In all, but the most concentrated, aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply

HA = A- + H+        and           Ka = [A-]*[H+] / [HA]

This is the definition in common usage. For many practical purposes it is more convenient to discuss the logarithmic constant, pKa

pKa = -log10 Ka

The larger the value of pKa, the smaller the extent of dissociation at any given pH - that is, the weaker the acid.

A weak acid has a pKa value in the approximate range −2 to 12 in water.

Strong acids has pKa values of less than about −2; the dissociation of a strong acid is effectively complete such that concentration of the undissociated acid is too small to be measured. pKa values for strong acids can, however, be estimated by theoretical means.

After rearranging the expression defining Ka, and putting pH = −log10[H+], one obtains

pH = pKa + [A-] / [HA]  and further

pH - pKa = log [A-] / [HA]

Then, a solution with 50% dissociation has pH equal to the pKa of the acid.

Polyprotic acids are acids that can lose more than one proton. Then we have more than one dissiciation constant; Ka1, Ka2, etc..  and similar pKa1, pKa2, etc.

All data given in the figures apply to dilute aqueous solutions at ambient temperature.

The figure below shows the fractions of the different acid forms (H2A, HA- and A2- for twoprotic acids) in aqueous solutions at varying pH:

Fractions of the different acid forms ions as function of pH 

Definition of organic compounds

Hydrocarbon: An organic compound consisting entirely of hydrogen and carbon.

Alkane: An acyclic saturated hydrocarbon, with the general formula CnH2n+2. Also called paraffin.

Aromatic hydrocarbon: A cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. The simplest of the aromatics have 6 carbon atoms and contains 3 double bounds.  A one ring aromatic without any substituents is called benzene, with the formula C6H6.

Alkyl: An alkyl group is an alkane substituent missing one hydrogen, with general formula CnH2n+1.

Phenyl: An phenyl group is a benzene substituent missing one hydrogen, with general formula C6H5.

Alcohol: An organic compound in which the hydroxyl functional group (–OH) is bound to a saturated carbon atom.

Alkanol: An alcohol where the hydroxyl group is bound to an alkyl group, with the general formula CnH2n+1OH.

Phenol: An alcohol where the hydroxyl group is bound to a phenyl group, with the formula C6H5OH. 

Phenols: A group of compounds consisting of a phenol molecule with one or more substiuents.

Diol: An organic compound containing two hydroxyl groups, -OH.

Triol: An organic compound containing three hydroxyl groups, -OH.

Carboxylic acid: an organic compound that contains a carboxyl group (C(=O)OH). The general formula of a carboxylic acid is R–COOH, with R referring to the rest of the molecule.

Paraffinic acid = Alkanoic acid: A carboxylic acid where the R is an alkyl group.

Dioic acid: A carboxylic acid with two acid groups (-COOH).

Benzoic acid: A carboxylic acid where the acid group is substituted to one carbon of a benzene ring.

Hydroxy acid: An carboxylic acid containing an additional hydroxy group (-OH), substistuted to another carbon atom than the acid group.




Related Topics

  • Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more

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