Amines, Diamines and Cyclic Organic Nitrogen Compounds - pKa Values

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, pK _a1 and pK _a2 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 compounds , boiling and melting point of organic nitrogen compounds , pKa 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 .

For full table with molweight, melting and boiling temperature and density - rotate the screen!

Amines, Diamines and Cyclic Organic Nitrogen Compounds - pKa Values

Group	Compound name	Common name	#C	#H	#N	pK a1 of the conjugate acid BH +	pK a2 of the conjugate acid BH 2 2+	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, K _a , 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 + H ₂ O   =  A ^- +H ₃ O ⁺

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 H ₃ O ⁺ 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 [H ₃ O ⁺ ]

K _a = [A ^- ]*[H ₃ O ⁺ ] / [HA]*[H ₂ O]

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           K _a = [A ^- ]*[H ⁺ ] / [HA]

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

pK _a = -log ₁₀ K _a

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

A weak acid has a pK _a value in the approximate range -2 to 12 in water.

Strong acids has pK _a 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. pK _a values for strong acids can, however, be estimated by theoretical means.

After rearranging the expression defining K _a , and putting pH = -log ₁₀ [H ⁺ ], one obtains

pH = pK _a + [A ^- ] / [HA]  and further

pH - pK _a = log [A ^- ] / [HA]

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

Polyprotic acids are acids that can lose more than one proton. Then we have more than one dissiciation constant; K _a1 , K _a2 , etc..  and similar pK _a1 , pK _a2 , etc.

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

For amines, the pk _a value is given for the conjugate bases BH ⁺ and BH ₂ ²⁺ .

BH ⁺ =  B  + H ⁺

The pK _b for a base may be calculated from the pK _a value of its conjugate acid:

pK _w =  pK _a + pK _b

At 25°C the pK _w is 14 and

pK _b = 14 - pK _a

Definition of organic compounds

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

Alkane: An acyclic saturated hydrocarbon, with the general formula C _n H _2n+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 C _n H _2n+1.

Phenyl: An phenyl group is a benzene substituent missing one hydrogen, with general formula C ₆ H ₅ .

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.