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).