Carboxylic acids are widespread in nature, often combined with other functional groups. Simple alkyl carboxylic acids, composed of four to ten carbon atoms, are liquids or low melting solids having very unpleasant odors. The fatty acids are important components of the biomolecules known as lipids, especially fats and oils. As shown in the following table, these long-chain carboxylic acids are usually referred to by their common names, which in most cases reflect their sources. A mnemonic phrase for the C 10 to C 20 natural fatty acids capric, lauric, myristic, palmitic, stearic and arachidic is: " C urly, L arry & M oe P erform S illy A ntics" (note that the names of the three stooges are in alphabetical order). Formula Common Name Melting Point CH3(CH2)10CO2H lauric acid 45 ºC CH3(CH2)12CO2H myristic acid 55 ºC CH3(CH2)14CO2H palmitic acid 63 ºC CH3(CH2)16CO2H stearic acid 69 ºC CH3(CH2)18CO2H arachidic acid 76 ºC Interestingly, the molecules of most natural fatty acids have an even number of carbon atoms . Analogous compounds composed of odd numbers of carbon atoms are perfectly stable and have been made synthetically. Since nature makes these long-chain acids by linking together acetate units, it is not surprising that the carbon atoms composing the natural products are multiples of two. The double bonds in the unsaturated compounds listed on the right are all cis (or Z). Formula Common Name Melting Point CH3(CH2)5CH=CH(CH2)7CO2H palmitoleic acid 0 ºC CH3(CH2)7CH=CH(CH2)7CO2H oleic acid 13 ºC CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H linoleic acid -5 ºC CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H linolenic acid -11 ºC CH3(CH2)4(CH=CHCH2)4(CH2)2CO2H arachidonic acid -49 ºC The following formulas are examples of other naturally occurring carboxylic acids. The molecular structures range from simple to complex, often incorporate a variety of other functional groups, and many are chiral. Contributors William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry