19. The Simple Compounds
                                   of Carbon
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       Conjugation, Aromaticity, and Colour

The polybutadiene chains of natural and synthetic rubbers on page 20 are not conjugated, because more than one single bond intervenes between double bonds.

The carbon atoms that are surrounded on both sides by single bonds have all of their electrons and orbitals tied up in sigma bonds, and act as "insulators," thereby preventing the double-bond electrons from combining into one delocalized pool.

Many biologically important molecules have these conjugated carbon chains, among them the carotenes shown on the next page. Carotenes are light-absorbing pigments used in photosynthesis by bacteria and green plants. b-Carotene has 11 double bonds in one long conjugated chain, and hence contributes 11 electron pairs, or 22 electrons, to a delocalized electron system.

The delocalized region is shaded in colour in the drawing of bCarotene on the next page. Spirilloxanthin from purple bacteria has 13 double bonds and 26 delocalized electrons, and isorenieratene from green bacteria has the largest delocalized system of the three: 15 double bonds and 30 delocalized electrons.


The electronic structure of butadiene also can be explained in terms of delocalization of electrons in molecular orbitals that extend over the entire carbon skeleton, as in benzene. This is termed conjugation. The butadiene molecule is more stable than would be expected if delocalisation did not occur ( below - all carbon-carbon bonds equivalent s).

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