Principles of Biochemistry

Solubility: Solubility of fatty acid is a direct function of carbon chain length. Acetic acid and butric acid are highly soluble in water. Solubility decreases as carbon chain length increases.

Melting point: The melting point of the acids with even number of carbon atoms increases with increasing chain length from-7.9ºC for C₄ to 90ºC for C₂₈. Saturated fatty acids have higher melting points than unsaturated fatty acids with the same number of carbon atoms. Long chain saturated fatty acids tend to be solids at room temperature, whereas long chain unsaturated fatty acids tend to be liquids at room temperature.

The double bonds in unsaturated fatty acids, which generally have the 'cis' configuration, produce “bends” in the carbon chains of these molecules. These bends prevent unsaturated fatty acids from packing together as tightly as saturated fatty acids. The greater the number of double bonds, the less efficient as packaging and hence have lower melting points than the respective saturated fatty acids.

Boiling point: The boiling points of the saturated fatty acids increase regularly with increasing chain length.

Absorption spectra: The carboxyl group of the free fatty acids absorb light in the short wavelength, but this is not useful. Conjugate bonds show more absorption at longer wavelengths in the ultraviolet, and this character is of considerable use in analytical procedure.