Of double bonds and rings

Other terms that mean the same thing as the index of hydrogen deficiency include elements of unsaturation sites of unsaturation and the sum of double bonds and rings... 

Section 13 23 A compound s molecular formula gives information about the number of double bonds and rings it contains and is a useful complement to spec troscopic methods of structure determination... 

Given a molecular formula, calculate the number of double bonds and rings. 

You will notice that they are all aliphatic compounds with a scattering of double bonds and rings, few functional groups, and an abundance of methyl groups. A better definition (that is, a bio synthetically based definition) arose when it was noticed that all these compounds have 5n carbon atoms. Pinene and camphor are Cto compounds, humulene is Cl5> and phytol is C20 It seemed obvious that terpenes were made from a C5 precursor and the favourite candidate was isoprene (2-methylbuta-l,3-diene) as all these structures can be drawn by joining together 2-, 3-, or 4-isoprene skeletons end to end. Humulene illustrates this idea. 

A better term would be all-antiperiplanar (terminology of W. Klyne and V. Prelog, Experientia 16 (1960) 521) to avoid confusion with trans as being used for one of the geometric isomers of double bonds and ring systems Icis-trans system). 

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