Consequently, their carbon skeletons usually contain five or multiples of five
carbon atoms. The structures of terpenes may also contain functional groups,
such as alcohols, ethers, esters and ketones.
Two isoprene unitsα-PineneMyrcene (oil of bay)Two isoprene unitsIsopreneFigure 1.25 The isoprene rule for classifying compounds as terpenes. To apply the rule one
ignores the double bonds (see myrcene) and in some cases it is also necessary to distort the
isoprene structure (seea-pinene)
Most terpenes are known by their historic trivial names, although systematic
names may be used for simple compounds. Terpenes are also classified
according to the number of carbon atoms in their structures (Table 1.5).
Table 1.5 The classification system for terpenesNumber of C atoms Classification10 Monoterpene
15 Sesquiterpene
20 Diterpene
25 Sesterterpene
30 Triterpene
40 TetraterpeneMonoterpenes are the main constituents of a group of compounds that are
known as theessential oils. They are extracted from plants and animals and have
been used as perfumes, medicines and spices for thousands of years (Figure 1.26).
1.5.6 Phospholipids
Most phospholipids are essentially disubstituted phosphates (Figure 1.27). They
may be initially classified as phosphatidyl compounds, plasmalogens, ether
phospholipids and sphingomyelins depending on the nature of the R 1 group
attached to the phosphate residue (Table 1.6).
LIPIDS 23