Partition coefficients (25 °C)

Partition coefficients lg K_D of neutral molecules between an organic solvent and water (lg P for 1-octanol–water), with the pKa that makes the distribution ratio D depend on pH. The separating funnel and the pH-extraction bench of the extraction lesson use these values.


Benzoic acid	$\ce{C6H5COOH}$	1-octanol/water	1.87	4.20	weak acid
Phenol	$\ce{C6H5OH}$	1-octanol/water	1.46	10.00	weak acid
Aniline	$\ce{C6H5NH2}$	1-octanol/water	0.90	4.60	weak base
Pyridine	$\ce{C5H5N}$	1-octanol/water	0.65	5.20	weak base
Naphthalene	$\ce{C10H8}$	1-octanol/water	3.30	—	neutral
Iodine (teaching value)	$\ce{I2}$	hexane/water	1.70	—	neutral

For weak bases the pKa is that of the conjugate acid BH⁺. The distribution ratio is D = K_D·α(neutral): only the neutral form is extracted, and the small partition of ion pairs is ignored. Iodine’s entry is the site’s shared teaching value; solubility ratios suggest a true value of about 30–45.

Sources

C. Hansch, A. Leo, D. Hoekman, Exploring QSAR: Hydrophobic, Electronic and Steric Constants (ACS, 1995); J. Sangster, Octanol–Water Partition Coefficients (Wiley, 1997).
pKa: D. C. Harris, Quantitative Chemical Analysis, 9th ed. (2016), Appendix G (anilinium 4.601) and the site’s acid-constant table.
Iodine: the site’s shared teaching value K_D(hexane/water) = 50 (±50 %); cf. 85 for CCl₄/water.

Used in these lessons

Extraction: partition equilibria and the separating funnel