H C C C C C C C C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H C C C C C
H
H
H
H
H
H
H
H
C
H C H
H H
H
H
H C H
H
C
C C
C
C
C C
C
H H
H H
H H
H H
H H
H
H
H H
H
H
An 8-carbon straight-chain
hydrocarbon, formula C 8 H 18An 8-carbon branched-chain
hydrocarbon, formula C 8 H 18An 8-carbon cyclic hydro-
carbon, formula C 8 H 16Figure 2.5. Illustration of the bonding versatility of carbon with three carbon-containing hydrocarbon
compounds arranged as straight (continuous) chains, branched chains, and in a cyclic compound.
Green Carbon from the Air
Carbon is present in the air as gaseous carbon dioxide, CO 2. Although air is only
about 0.038% CO 2 by volume, it serves as the source of carbon for the growth of green
plants. In so doing, the chlorophyll in plants captures solar energy in the form of visible
light, represented hν, and use it to convert atmospheric carbon dioxide to high-energy
glucose sugar, C 6 H 12 O 6 , as shown by the following reaction:
hν
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 (2.5.2)The carbon fixed in the form of C 6 H 12 O 6 and related compounds provides the basis
of the food chains that sustain all organisms. Organic carbon produced by photosyn-
thesis in eons past also provided the raw material for the formation of petroleum, coal,
and other fossil fuels. Now, as supplies of these scarce resources dwindle and as the
environmental costs of their extraction, transport, and utilization mount, there is much
renewed interest in photosynthetically produced carbon compounds as raw materials and
even fuels. Despite the low levels of carbon dioxide in the atmosphere and the relatively
low efficiency of photosynthesis, rapidly growing plants, such as some varieties of
hybrid poplar trees, can produce enormous quantities of carbon compounds very rapidly
and in a sustainable manner.
Chap. 2, The Elements: Basic Building Blocks of Green Chemicals 39