Amines, diamines and cyclic organic nitrogen compounds - pKa values

Values for the negative logarithm of the acid dissociation constant, pKa, of the conjugated acid of amines, diamines and cyclic organic nitrogen compounds, shown together with the molecular structure of the acids.

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 is given together with boiling and melting point, density and molecular weight, as well as number of carbon, hydrogen and oxygen atoms in each molecule.

See also boiling and melting point of different kinds of organic compoundsboiling and melting point of organic nitrogen compoundspKa of phenols, alcohols and carboxylic acids and pKa of inorganic acids and bases, as well as acid and base pH indicators and Acid-base properties of aqueous solutions of salts with ions from both acids and bases.



pKa of amines

pKa of diamines and cyclic nitrogen compounds


Group Compound name Common name #C #H #N pKa1 of the
conjugate acid
BH+
pKa2 of the
conjugate acid
BH22+
Mole
weight
g/mol
Melting
temp
°C
Boiling
temp
°C
Density@20°C
g/ml
1-Amine Methylamine Methanamine 1 5 1 10.59 31.06 -93 -6 0.660
1-Amine Ethylamine Ethanamine 2 7 1 10.67 45.08 -81 17 0.685
1-Amine Propylamine 1-Propanamine 3 9 1 10.69 59.11 -85 47 0.717
1-Amine Butylamine 1-Butanamine 4 11 1 10.61 73.14 -49 77 0.741
1-Amine Pentylamine Amylamine 5 13 1 10.63 87.16 -51 105 0.754
1-Amine Hexylamine 1-Hexanamine 6 15 1 10.56 101.19 -21 132 0.766
1-Amine Heptylamine 1-Heptanamine 7 17 1 10.67 115.22 -23 153 0.775
1-Amine Octylamine 1-Octanamine 8 19 1 10.65 129.24 0 179 0.783
1-Amine Nonylamine 1-Nonanamine 9 21 1 10.64 143.27 -1 198 0.791
1-Amine Decylamine 1-Decanamine 10 23 1 10.64 157.30 15 217 0.794
1-Amine Undecylamine 1-Undecanamine 11 25 1 10.63 171.32 15 229 0.798
1-Amine Dodecylamine 1-Dodecanamine 12 27 1 10.63 185.35 28 255 0.802
1-Amine (Tridecyl)amine 1-Tridecanamine 13 29 1 10.63 199.38 27 273 0.806
1-Amine Tetradecylamine 1-Tetradecanamine 14 31 1 10.62 213.40 39 289 0.808
1-Amine Pentadecylamine Pentadecanamine 15 33 1 10.61 227.43 37 312 0.810
1-Amine Hexadecylamine 1-Hexadecanamine 16 35 1 10.63 241.46 46 321 0.813
1-Amine Octadecylamine 1-Octadecanamine 18 39 1 10.60 269.51 53 350 0.862
Amine Dimethylamine Methyl methylamine 2 7 1 10.73 45.08 -92 7 0.654
Amine Allylamine 2-propen-1-amine 3 7 1 9.49 57.09 -88 54 0.758
Amine Trimethylamine Dimethyl methylamine 3 9 1 9.81 59.11 -117 3 0.631
Amine 2-Propanamine 3 9 1 10.60 59.11 -95 32 0.682
Amine sec-Butylamine 2-Butanamine 4 11 1 10.60 73.14 -105 63 0.725
Amine tert-Butylamine 2-Methyl-2-propanamine 4 11 1 10.68 73.14 -67 46 0.696
Amine Butylamine 1-Butanamine 4 11 1 10.77 73.14 -49 77 0.741
Amine Diethylamine Ethyl ethylamine 4 11 1 10.98 73.14 -50 56 0.706
Amine iso-Butylamine 2-Methyl-1-propanamine 5 12 1 10.43 86.16 -85 68 0.730
Amine Aniline 6 7 1 4.61 93.13 -6 184 1.025
Amine Cyclohexylamine 6 13 1 10.66 99.17 -18 134 0.819
Amine Triethylamine Diethyl ethylamine 6 15 1 10.75 101.19 -115 90 0.727
Amine o-Toluidine 2-Aminotoluene, 2-Methylaniline 7 9 1 4.45 107.15 -28 199 1.010
Amine m-Toluidine 3-Aminotoluene, 3-Methylaniline 7 9 1 4.71 107.15 -30 203 1.001
Amine p-Toluidine 4-Aminotoluene, 4-Methylaniline 7 9 1 5.08 107.15 44 201 0.975
Amine 2-Heptylamine 2-Heptanamine, 1-Methylhexylamine 7 17 1 10.70 115.22 143 0.766
Amine Dibutylamine 8 19 1 11.25 129.24 -62 162 0.767
Amine n-Allylaniline Allylphenylamine 9 11 1 4.17 133.19 219 0.977
Amine 2-Naphthylamine 2-Aminonaphthalene, beta-naphthylamine 10 9 1 4.16 143.19 112 306 1.063
Amine Diphenylamine N-phenyl-aminobenzene 12 11 1 0.79 169.22 51 302 1.160
Amine 2-Aminobiphenyl 2-Biphenylylamine, 2-Phenylaniline 12 11 1 3.82 169.22 48 299
Amine 4-Aminobiphenyl 4-Phenylaniline, Xenylamine 12 11 1 4.35 169.22 53 348
Amine 4-Benzylaniline 13 13 1 2.17 183.25 35 300 1.038
Diamine 1,2-Propandiamine 3 10 2 9.82 6.61 74.12 118 0.875
Diamine 1,3-Propandiamine 3 10 2 10.55 8.88 74.12 -11 139 0.887
Diamine 1,3-diaminobenzene m-phenylenediamine 6 8 2 2.3 5.0 108.14 65 282
Diamine 1,2-diaminobenzene o-phenylenediamine 6 8 2 0.6 4.74 108.14 102 256
Diamine 1,4-diaminobenzene p-Phenylenediamine 6 8 2 2.7 6.2 108.14 140 267
Diamine 1,6-hexanediamine 6 16 2 11.86 10.76 116.20
Diamine p-Benzidine 1,1-biphenyl-4,4-diamine 12 12 2 1.66 3.57 184.24 127 401
Piperidine Piperidine 5 11 1 11.12 85.15 -13 106 0.862
Pyridine Pyridine Azine 5 5 1 5.23 79.10 -42 115 0.982
Pyridine 3-Methylpyridine 6 7 1 5.52 93.13 -18 144 1.504
Pyridine 4-Methylpyridine 6 7 1 6.08 93.13 4 145 1.504
Pyridine 2-Methylpyridine 6 7 1 6.20 93.13 -67 129 1.499
Pyridine 2-Ethylpyridine 7 9 1 5.89 107.15 -63 149 0.952
Pyridine 2,4,6-trimethylpyridine 2,4,6-Collidine 8 11 1 7.43 121.18 -44 170 0.917
Pyridine 2-Benzylpyridine 12 11 1 5.13 169.22 13 277 1.059
Pyrrole Pyrrolidine Azacyclopentane, tetrahydropyrrole 4 9 1 11.27 71.12 -58 87 0.859
Quinoline Quinoline 1-Azanapthalene 9 7 1 4.90 129.16 -15 237 1.096
Quinoline Isoquinoline 9 7 1 5.42 129.16 27 242 1.101
Quinoline Acridine Dibenzo[b,e]pyridine 13 9 1 5.58 179.22 111 347 1.005
Quinoline Benzo[c]quinoline Phenanthridine 13 9 1 5.58 179.22 107 350



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.

For amines, the pka value is given for the conjugate bases BH+ and BH22+.

BH+  =  B  + H+

The  pKb for a base may be calculated from the pKa value of its conjugate acid:

pKw  =  pKa  + pKb

At 25°C the pKw is 14 and

pKb = 14 - pKa

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.

Amine: A compound or functional group that contain a basic nitrogen atom with a lone pair. It can be a primary (R-NH2), a secondary (R',R''-NH) or a tertiary amine (R',R'',R'''-N), where R represent an alkyl or other organic substituent. For 1-amines in this document the R represents an alkyl group, in which the NH2-group is placed at the end of the the alkane chain.

Diamine: An amine with two amino groups.

Pyrrole: A heterocyclic aromatic organic compound, a five-membered ring with two double bounds, with the formula C4H4NH. Substituted derivatives are called pyrroles.

Dihydropyrrole: A compound formally derived from the aromate pyrrole by partial hydrogenation, containing one double bound. Also called pyrroline.

Pyridine: A heterocyclic six-membered ring compound with the chemical formula C5H5N. It is structurally similar to benzene, with one methine group (=CH−) replaced by a nitrogen atom.

Piperidine: A heterocyclic amine consisting of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). The molecular formula is (CH2)5NH.

Quinoline: A heterocyclic aromatic organic compound, consisting of a benzene ring fused to a pyridine ring, with the molecular formula C9H7N and the nitrogen atom in position 1.

Isoquinoline: An analog to quinoline with the nitrogen atom in position 2.



Related Topics

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