The pKa value is a fundamental chemical property that reflects the acidity or alkalinity of a molecule, especially its tendency to donate or accept protons in solution. Amines, diamines, and cyclic nitrogen compounds are key components of organic chemistry. Their ability to act as bases (accepting protons from acids) is greatly influenced by their pKa values. For example, the pKa of an amine tells us about its ability to bind protons under physiological or industrial conditions. A higher pKa indicates that the compound is less acidic but more basic, while a lower pKa indicates more acidic but less basic. Knowing their pKa values allows us to manipulate their reactivity and optimize chemical processes.
Alfa Chemistry has listed typical pKa ranges for different classes of amines, diamines, and cyclic nitrogen compounds in the table below for reference only.
| Group | Compound Name | Common Name | #C | #H | #N | pKa1 of conjugate acid BH+ | pKa2 of conjugate acid BH22+ | Molar Mass (g/mol) | Melting point ℃ | Boiling point ℃ | Density@20℃ (g/ml) |
| 1-Amine | Methylamine | Methanamine | 1 | 5 | 1 | 10.59 | 31.06 | -93 | -6 | 0.66 | |
| Ethylamine | Ethanamine | 2 | 7 | 1 | 10.67 | 45.08 | -81 | 17 | 0.685 | ||
| Propylamine | 1-Propanamine | 3 | 9 | 1 | 10.69 | 59.11 | -85 | 47 | 0.717 | ||
| Butylamine | 1-Butanamine | 4 | 11 | 1 | 10.61 | 73.14 | -49 | 77 | 0.741 | ||
| Pentylamine | Amylamine | 5 | 13 | 1 | 10.63 | 87.16 | -51 | 105 | 0.754 | ||
| Hexylamine | 1-Hexanamine | 6 | 15 | 1 | 10.56 | 101.19 | -21 | 132 | 0.766 | ||
| Heptylamine | 1-Heptanamine | 7 | 17 | 1 | 10.67 | 115.22 | -23 | 153 | 0.775 | ||
| Octylamine | 1-Octanamine | 8 | 19 | 1 | 10.65 | 129.24 | 0 | 179 | 0.783 | ||
| Nonylamine | 1-Nonanamine | 9 | 21 | 1 | 10.64 | 143.27 | -1 | 198 | 0.791 | ||
| Decylamine | 1-Decanamine | 10 | 23 | 1 | 10.64 | 157.3 | 15 | 217 | 0.794 | ||
| Undecylamine | 1-Undecanamine | 11 | 25 | 1 | 10.63 | 171.32 | 15 | 229 | 0.798 | ||
| Dodecylamine | 1-Dodecanamine | 12 | 27 | 1 | 10.63 | 185.35 | 28 | 255 | 0.802 | ||
| (Tridecyl)amine | 1-Tridecanamine | 13 | 29 | 1 | 10.63 | 199.38 | 27 | 273 | 0.806 | ||
| Tetradecylamine | 1-Tetradecanamine | 14 | 31 | 1 | 10.62 | 213.4 | 39 | 289 | 0.808 | ||
| Pentadecylamine | Pentadecanamine | 15 | 33 | 1 | 10.61 | 227.43 | 37 | 312 | 0.81 | ||
| Hexadecylamine | 1-Hexadecanamine | 16 | 35 | 1 | 10.63 | 241.46 | 46 | 321 | 0.813 | ||
| Octadecylamine | 1-Octadecanamine | 18 | 39 | 1 | 10.6 | 269.51 | 53 | 350 | 0.862 | ||
| Amine | Dimethylamine | Methyl methylamine | 2 | 7 | 1 | 10.73 | 45.08 | -92 | 7 | 0.654 | |
| Allylamine | 2-propen-1-amine | 3 | 7 | 1 | 9.49 | 57.09 | -88 | 54 | 0.758 | ||
| Trimethylamine | Dimethyl methylamine | 3 | 9 | 1 | 9.81 | 59.11 | -117 | 3 | 0.631 | ||
| 2-Propanamine | 3 | 9 | 1 | 10.6 | 59.11 | -95 | 32 | 0.682 | |||
| sec-Butylamine | 2-Butanamine | 4 | 11 | 1 | 10.6 | 73.14 | -105 | 63 | 0.725 | ||
| tert-Butylamine | 2-Methyl-2-propanamine | 4 | 11 | 1 | 10.68 | 73.14 | -67 | 46 | 0.696 | ||
| Butylamine | 1-Butanamine | 4 | 11 | 1 | 10.77 | 73.14 | -49 | 77 | 0.741 | ||
| Diethylamine | Ethyl ethylamine | 4 | 11 | 1 | 10.98 | 73.14 | -50 | 56 | 0.706 | ||
| iso-Butylamine | 2-Methyl-1-propanamine | 5 | 12 | 1 | 10.43 | 86.16 | -85 | 68 | 0.73 | ||
| Aniline | 6 | 7 | 1 | 4.61 | 93.13 | -6 | 184 | 1.025 | |||
| Cyclohexylamine | 6 | 13 | 1 | 10.66 | 99.17 | -18 | 134 | 0.819 | |||
| Triethylamine | Diethyl ethylamine | 6 | 15 | 1 | 10.75 | 101.19 | -115 | 90 | 0.727 | ||
| o-Toluidine | 2-Aminotoluene, 2-Methylaniline | 7 | 9 | 1 | 4.45 | 107.15 | -28 | 199 | 1.01 | ||
| m-Toluidine | 3-Aminotoluene, 3-Methylaniline | 7 | 9 | 1 | 4.71 | 107.15 | -30 | 203 | 1.001 | ||
| p-Toluidine | 4-Aminotoluene, 4-Methylaniline | 7 | 9 | 1 | 5.08 | 107.15 | 44 | 201 | 0.975 | ||
| 2-Heptylamine | 2-Heptanamine, 1-Methylhexylamine | 7 | 17 | 1 | 10.7 | 115.22 | 143 | 0.766 | |||
| Dibutylamine | 8 | 19 | 1 | 11.25 | 129.24 | -62 | 162 | 0.767 | |||
| n-Allylaniline | Allylphenylamine | 9 | 11 | 1 | 4.17 | 133.19 | 219 | 0.977 | |||
| 2-Naphthylamine | 2-Aminonaphthalene, beta-naphthylamine | 10 | 9 | 1 | 4.16 | 143.19 | 112 | 306 | 1.063 | ||
| Diphenylamine | N-phenyl-aminobenzene | 12 | 11 | 1 | 0.79 | 169.22 | 51 | 302 | 1.16 | ||
| 2-Aminobiphenyl | 2-Biphenylylamine, 2-Phenylaniline | 12 | 11 | 1 | 3.82 | 169.22 | 48 | 299 | |||
| 4-Aminobiphenyl | 4-Phenylaniline, Xenylamine | 12 | 11 | 1 | 4.35 | 169.22 | 53 | 348 | |||
| 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 | ||
| 1,3-Propandiamine | 3 | 10 | 2 | 10.55 | 8.88 | 74.12 | -11 | 139 | 0.887 | ||
| 1,3-diaminobenzene | m-phenylenediamine | 6 | 8 | 2 | 2.3 | 5 | 108.14 | 65 | 282 | ||
| 1,2-diaminobenzene | o-phenylenediamine | 6 | 8 | 2 | 0.6 | 4.74 | 108.14 | 102 | 256 | ||
| 1,4-diaminobenzene | p-Phenylenediamine | 6 | 8 | 2 | 2.7 | 6.2 | 108.14 | 140 | 267 | ||
| 1,6-hexanediamine | 6 | 16 | 2 | 11.86 | 10.76 | 116.2 | |||||
| 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 | 79.1 | -42 | 115 | 0.982 | |
| 3-Methylpyridine | 6 | 7 | 1 | 5.52 | 93.13 | -18 | 144 | 1.504 | |||
| 4-Methylpyridine | 6 | 7 | 1 | 6.08 | 93.13 | 4 | 145 | 1.504 | |||
| 2-Methylpyridine | 6 | 7 | 1 | 6.2 | 93.13 | -67 | 129 | 1.499 | |||
| 2-Ethylpyridine | 7 | 9 | 1 | 5.89 | 107.15 | -63 | 149 | 0.952 | |||
| 2,4,6-trimethylpyridine | 2,4,6-Collidine | 8 | 11 | 1 | 7.43 | 121.18 | -44 | 170 | 0.917 | ||
| 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 | 4.9 | 129.16 | -15 | 237 | 1.096 | |
| Isoquinoline | 9 | 7 | 1 | 5.42 | 129.16 | 27 | 242 | 1.101 | |||
| Acridine | Dibenzo[b,e]pyridine | 13 | 9 | 1 | 5.58 | 179.22 | 111 | 347 | 1.005 | ||
| Benzo[c]quinoline | Phenanthridine | 13 | 9 | 1 | 5.58 | 179.22 | 107 | 350 |
Definition of Organic Compounds
a. Amines: compounds or functional groups containing basic nitrogen atoms with lone pairs of electrons. It can be a primary amine (R-NH2), secondary amine (R',R''-NH), or tertiary amine (R',R'',R'''-N), where R represents an alkyl group or other organic substituent. The pKa of primary amines is usually lower than that of secondary and tertiary amines due to space barrier and electronic effects. Spatial hindrance reduces the probability that the lone pair of electrons on the nitrogen will be available for protonation, and electronic factors (such as induced effects) can stabilize or destabilize the conjugate acid, thus affecting the pKa. for the 1-amines on this page, R represents the alkyl group, where the NH2 group is located at the end of the alkane chain.
b. Diamines: Diamines contain two amine groups (-NH2) in the same molecule and their pKa behavior is more complex due to potential intramolecular interactions. The pKa value of a diamine depends on the spacing between the two amine groups and their proximity to other functional groups that may affect alkalinity. Diamines usually have two different pKa values, with the first pKa value being higher (more basic) because the first proton is accepted. The second protonation is usually more difficult due to electrostatic repulsion with the first proton, resulting in a lower pKa value for the second.
c. Cyclic nitrogen compounds: Their pKa values vary depending on the ring structure and the role of nitrogen in the ring. Compounds such as pyridine, pyrrole, and imidazole are common examples of compounds with different basicity profiles.
d. Pyridine: A six-membered heterocyclic compound with the chemical formula C5H5N. its structure is similar to that of benzene, but with one of the hypomethyl groups (=CH-) replaced by a nitrogen atom. Pyridine has a lone pair of electrons on the nitrogen atom, it is not part of the aromatic system and is relatively basic.
e. Pyrrole: A heterocyclic aromatic organic compound with a five-membered ring with two double bonds and the molecular formula C4H4NH. the lone pair of electrons in pyrrole is off-domain, which increases the aromaticity, and it has a much lower basicity.
f. Dihydropyrrole: A compound obtained by partial hydrogenation of aromatic pyrrole, containing a double bond, also known as pyrroline.
g. 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.
h. Quinoline: A heterocyclic aromatic organic compound consisting of a benzene ring thickened with a pyridine ring, with the molecular formula C9H7N and the nitrogen atom in the 1 position.
i. Isoquinoline: A quinoline analog with the nitrogen atom in the 2-position.
Practical Applications and Considerations
At Alfa Chemistry, understanding these pKa values is critical to adjusting chemical reactions. Example:
- Synthesis - The basicity of amines is utilized in nucleophilic substitution reactions, where their nucleophilicity is a key factor in the rate of the reaction.
- Pharmaceuticals - The pKa value of cyclic nitrogen compounds such as imidazole is important in drug design because the ionization state of the molecule at physiological pH affects its bioavailability and receptor binding.
- Polymer chemistry - Diamines such as ethylenediamine are used in the production of polyamides, where the control of pH during polymerization affects the molecular weight and properties of the resulting polymer.
