312 Green Chemistry, 2nd ed
photosynthesis. Corn is one of the more productive field crops, and hybrid varieties
produce large quantities of grain and plant biomass. Sugar cane is noted for its ability to
produce biomass, some in the form of sugar, much more in the cane stalk biomass, which
has relatively few uses, other than for fuel. One of the more exciting developments of
productive hybrid plants is the hybrid poplar tree which, nourished by minimal amounts
of fertilizer and watered by economical trickle irrigation systems, grows within a few
years to a harvestable size for the production of wood pulp, and wood for plywood. The
ability of these trees to generate cellulose that can be converted to glucose means that
they may serve as the basis of an entire plant-based chemicals industry. The possibility
exists that they can be genetically engineered to produce other chemicals as well.
12.5. Glucose As Feedstock
The glucose molecule (right) provides a promising
platform for a number of different organic syntheses.
In addition to being produced in abundance by plants,
glucose is a partially oxidized material, advantageous
where a partially oxidized product is made. It also
contains hydroxyl groups (-OH) around the molecule,
which act as sites for the attachment of various
functionalities. Glucose is metabolized by essentially
all organisms, so it serves as an excellent starting
point for biosynthesis reactions using enzymes, and it and many of its products are
biodegradable, adding to their environmental acceptability.
Glucose can be obtained by enzyme-catalyzed processes from other sugars, including
sucrose and fructose. Virtually all of the glucose that is now used is obtained from the
enzymatic hydrolysis of cornstarch. It is also possible to obtain glucose by the enzymatic
hydrolysis of cellulose. The difficulty in doing this is compensated by the enormous
quantities of cellulose available in wood and other biomass sources. The greatest use of
glucose for synthesis is by fermentation with yeasts to produce ethanol,
H
H
H
C
H
H
C OH
Ethanolan alcohol widely used as a gasoline additive, solvent, and chemical feedstock. A
byproduct of this fermentation process is carbon dioxide, the potential of which in green
chemical applications as a supercritical fluid solvent are discussed in Section 11.9.
Glucose is widely used as a starting material for the biological synthesis of a
number of different biochemical compounds. These include ascorbic acid, citric acid,
and lactic acid. Several amino acids used as nutritional supplements, including lysine,
phenylalanine, threonine, and tryptophan, are biochemically synthesized starting
with glucose. The vitamins folic acid, ubiquinone, and enterochelin are also made
biochemically from glucose.
HO OH
H
OH
H
H
OH
H
CH 2 OH
H
C O
C C
C C
Glucose