Chemistry Reference
In-Depth Information
16
Carbohydrates
Chapter Summary
Carbohydrates
are polyhydroxy aldehydes or polyhydroxy ketones, or substances that give
such compounds on hydrolysis. They are classified as monosaccharides, oligosaccharides,
and polysaccharides.
Monosaccharides,
also called simple sugars, are classified by the number of
carbon atoms (triose, tetrose, pentose, and so on) and by the nature of the carbonyl group
(
aldose
or
ketose
).
R-(+)-Glyceraldehyde
is an aldotriose designated by the following
Fischer
projection formula:
CH
O
CH
O
CH
O
H
OH
H
OH
OH
H
HO
CH
2
CH
2
OH
CH
2
OH
This configuration is designated
D
, whereas the enantiomer with the H and OH
positions reversed is
L
. In larger monosaccharides, the letters
D
and
L
are used to designate
the configuration of the stereogenic center with the
highest
number
,
the stereogenic carbon
most remote from the carbonyl group. The
D
-aldoses through the hexoses are listed in
Figure 16.1.
Epimers
are stereoisomers that differ in configuration at only one stereogenic
center.
Monosaccharides with four or more carbons usually exist in a cyclic hemiacetal form,
in which a hydroxyl group on C-4 or C-5 reacts with the carbonyl group (at C-1 in aldoses) to
form a hemiacetal. In this way, C-1 also becomes stereogenic and is called the
anomeric
carbon. Anomers
differ in configuration only at this stereogenic center and are designated
α
or
β
. The common ring sizes for the cyclic hemiacetals are six-membered (called
pyranoses)
or five-membered (called
furanoses).
The rings contain one oxygen atom and
five or four carbon atoms, respectively.
Anomers usually interconvert in solution, resulting in a gradual change in optical
rotation from that of the pure anomer to an equilibrium value for the mixture. This rotational
change is called
mutarotation.
Haworth formulas
are a useful way of representing the cyclic forms of
monosaccharides. The rings are depicted as flat, with hydroxyl groups or other substituents
above or below the ring plane.
Monosaccharides can be oxidized at the aldehyde carbon to give carboxylic acids
called
aldonic acids.
Oxidation at both ends of the carbon chain gives
aldaric acids.
Reduction of the carbonyl group to an alcohol gives polyols called
alditols.
The -OH groups
in sugars, like those in simpler alcohols, can be esterified or etherified.
