Importance of chain length on physicochemical and crystalline properties of organic homologs

It has been observed that certain physical properties of normal alkyl p‐aminobenzoates (i.e., their melting behavior and their solubilities in water, silicone oil, and hexane) exhibit an unusual dependency upon chain length. Each property is characterized by a definite break in the chain length profile about the butyl ester. However, the silicone oil‐water and hexane‐water partition coefficients of these esters show a regular increase with chain length extending over many orders of magnitude. The overall behavior of these homologs is attributed to basic difference in the crystal lattices of short (≤four carbons) and long (⩽four carbons) homologs. X‐ray patterns and thermal data were obtained and are supportive of this conclusion. As an additional verification, the hexane solubility of each compound was calculated by Scatchard‐Hildebrand theory using the thermal data and empirically estimated solubility parameters. Agreement between calculated and experimental values is excellent. Because of relationships drawn for the influence of chain length on solubility and relative solubility (partition coefficients), these experiments also indicate a convenient method of estimating congeneric solubilities in other solvents.