Estimator
Results
| # | Functional group | Substituents (pos:group) | Σσ | ρ | Base pKa | Predicted pKa | pH | Acid/Base | Major species | Fraction major |
|---|
Example data table
Reference values are approximate and for demonstration only.
| Molecule | Functional group | Substituent | Position | Reported pKa (approx) |
|---|---|---|---|---|
| Benzoic acid | Benzoic acid | — | — | 4.20 |
| p-Nitrobenzoic acid | Benzoic acid | NO2 | para | 3.44 |
| p-Methylbenzoic acid | Benzoic acid | CH3 | para | 4.37 |
| Phenol | Phenol | — | — | 10.00 |
| p-Nitrophenol | Phenol | NO2 | para | 7.15 |
| Anilinium | Anilinium | — | — | 4.60 |
| Aliphatic carboxylic acid | Aliphatic carboxylic acid | — | — | 4.75 |
| Aliphatic ammonium | Aliphatic ammonium | — | — | 10.60 |
Formula used
The estimator applies a simple linear free-energy model inspired by Hammett and Taft correlations. For aromatic systems (benzoic acids, phenols, anilinium), we sum substituent constants σ at the declared positions (meta or para) and use a reaction constant ρ characteristic of the scaffold.
For bases (pKa of the conjugate acid): pKa = pKa,0 + ρ · Σσ
Typical base values pKa,0 and ρ used here: Benzoic acid (4.20, 1.0), Phenol (10.0, 2.0), Anilinium (4.60, −1.0). For aliphatic analogs we approximate α‑substituent effects with Taft σ* values and use ρ ≈ 1.3 (acids) or −1.5 (ammonium). These are educational approximations and not a replacement for quantum or data‑driven predictions.
Speciation (Henderson–Hasselbalch)
Base: fraction BH+ = 1 / (1 + 10^(pH − pKa)) ; fraction B = 1 − fraction BH+
Hammett σ (m, p) and Taft σ* constants used here are representative values for common substituents (NO2, CN, CF3, Cl, F, CH3, OCH3, t‑Bu). Summation assumes additivity and independent effects. Always validate with experimental data for critical work.