Conjugated double bonds polyunsaturated hydrocarbons

The structures of these compounds have numerous conjugated double-bonds and cyclic end groups thus, carotenoids present a variety of stereoisomers with different chemical and physical proprieties [5]. Carotenoids can be divided in two groups carotenes, constituted by polyunsaturated hydrocarbons C40 (Fig. 135.1), and xanthophylls their oxygenated derivatives and main contain epoxy, carbonyl, hydroxy, methoxy, or carboxylic acid functional groups (Fig. 135.2). 

Fatty acids are carboxylic acids with long hydrocarbon chains. The fatty acids most frequently found in nature are shown in Table 26.1. Because they are synthesized from acetate, a compound with two carbon atoms, most naturally occurring fatty acids contain an even number of carbon atoms and are unbranched. The mechanism for the biosynthesis of fatty acids is discussed in Section 19.21. Fatty acids can be saturated with hydrogen (and therefore have no carbon-carbon double bonds) or unsaturated (have carbon-carbon double bonds). Fatty acids with more than one double bond are called polyunsaturated fatty acids. Double bonds in naturally occurring unsaturated fatty acids are never conjugated—they are always separated by one methylene group. 

---