log P Estimator
Count the functional-group fragments in a molecule and get an additive estimate of its octanol-water partition coefficient (log P / log Kow) and water-solubility class. No structure drawing needed, useful for solvent selection, formulation, and feeding the BCF environmental-fate tool.
How to use this tool
Estimate how oil-loving or water-loving a molecule is, its log P, without drawing a structure. You just tally up the functional groups it contains, and the tool adds their known contributions together.
What to enter
- Fragment counts: for each functional group (–CH₃, –OH, aromatic carbon, –COOH…), type how many the molecule has. Each shows its contribution constant f; positive raises log P, negative lowers it.
- Quick-load links: click Ethanol, benzene or aspirin to fill in a worked molecule and see the method in action.
Reading the result
The headline is the estimated log P: roughly 0 means it splits evenly between oil and water, positive is fat-/solvent-loving (lipophilic), negative is water-loving (hydrophilic). You also get a water-solubility class and a term-by-term breakdown. Treat it as a screening estimate (≈ ±0.5 log unit), not a measured value.
Worked example
Ethanol, one –CH₃, one –CH₂–, one –OH, comes out near zero / slightly negative, i.e. freely water-soluble; load benzene's six aromatic carbons instead and log P climbs above +2, clearly oil-loving.
Methodology
log P is estimated additively in the Hansch-Leo fragment tradition: log P = base constant + Σ (count × fragment constant), summing a contribution for each functional group. Negative constants (–OH, –NH₂, –COOH) lower log P toward water solubility; carbon and halogen fragments raise it. The water-solubility class is read from the resulting log P.
Known limits
- This is a screening estimate, typically within about ±0.5 log unit. It omits the proximity, branching, and intramolecular-bonding correction factors of the full Hansch-Leo system.
- Strongly ionised species, internal H-bonding, and zwitterions are not modelled, for those, measure log D at the relevant pH.
- Use it for ranking and triage; cite a measured or QSAR-validated value where a number must be defended.