Natural Product Biomolecules - Organic Chemistry Concepts: An EFL Approach - page 152

Carbohydrates are classed as either simple or complex. They are simple or com-
plex depending on whether or not there are one or more sugar units in the struc-
ture. Simple sugars, as in Figure 8.1 , are the monosaccharide building blocks for
complex carbohydrates—disaccharides, trisaccharides … polysaccharides.
Monosaccharides are subdivided into aldoses or ketoses, based on whether
there is an aldehyde or ketone as the carbonyl group. The number of carbons in
the sugar backbone is indicated with a prefix such as tri-, tetr-, pent-, and hex-.
See the examples in Figure 8.1 .
As shown in Figure 8.2 , carbohydrates are the products of photosynthesis and
are Nature's energy source.
FIGURE 8.2
Glucose synthesis and metabolism.
The link to simple organic molecules is through the functional groups such as
hydroxy from alcohols, and carbonyl groups from aldehydes and ketones. In addi-
tion, there can be chirality centers. These functional groups and stereochemical
features have the same properties described earlier for their simpler equivalents.
For example, aldoses, like any aldehyde, can be oxidized to the carboxylic acid
(aldonic acid). The Tollens' silver mirror test or the copper(II)-based Fehling's
and Benedict's reagents can be used to show whether a simple sugar is pres-
ent. The metal reduction product of elemental silver, as in Figure 8.3 , or red
copper(I) oxide give clear visual results.
FIGURE 8.3
Tollens' aldose oxidation.
These reagents also give positive test reactions with 2-ketoses. This surprising
result can be explained by the base-catalyzed tautomerism shown in Figure 8.4 ,
which gives the corresponding aldose. This aldose gives a positive aldehyde test.
The equilibrating species differ only in the position of the tautomeric hydrogen.