Fatty acids are carboxylic acids with hydrocarbon chains of 4 to 36 carbons. In some fatty acids, this chain is fully saturated (contains no double bonds) and unbranched; others contain one or more double bonds and are unsaturated ones.

The physical properties of the fatty acids, and of compounds that contain them, are determined by the length and degree of unsaturation of the hydrocarbon chain. The nonpolar hydrocarbon chain accounts for the poor solubility of fatty acids in water. The longer the fatty acyl chain and the fewer the double bonds, the lower the solubility in water. The carboxylic acid group is polar (and ionized at neutral pH), and accounts for the slight solubility of short chain in water.

In the fully saturated compounds, free rotation around each of the carbon-carbon bonds gives the hydrocarbon chain great flexibility; the most stable conformation is this fully extended form, In which the steric hindrance of neighboring atoms is maximized. In a unsaturated fatty acids, a cis double bond forces a kink in the hydrocarbon chain, and fatty acids with one or more kinks cannot pack together as tightly as fully saturated fatty acids and their interactions with each other are therefore weaker.

The melting points of fatty acids and of compounds that contain them are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain. At room temperature (25°C), the saturated fatty acids have a waxy consistence, whereas unsaturated fatty acids are oil liquids because it takes less thermal energy to disorder its poorly ordered arrays of unsaturated fatty acids.

In the figures below, take a look in the carboxyl groups represented in blue.

Fatty Acids