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370 Carbohydrates

equilibration to yield a complex mixture of six isomers being replaced in chemistry texts by less ambiguous
(two furanoses, two pyranoses, an acyclic carbonyl names such as monosaccharides and disaccharides.
form,andahydratedcarbonylform).Therotationofthe Uloses (also known as ketoses) Chiral polyhydroxyalka-
equilibrium mixture is naturally different from that of nones that possess at least four carbon atoms in their
the original pure α-D-glucopyranose, which accounts skeleton (lower members are achiral). Uloses that pos-
for the observed change in rotation. sess a sufficient number of carbon atoms to cyclize in
Oligosaccharides Low molecular weight polymers made strainless rings do so and yield furanuloses and pyran-
up of 2 to 10 monosaccharide units linked together uloses.
by acetal linkages. Oligosaccharides are classified ac-
cording to degree of polymerization into disaccha-
rides, trisaccharides, and so on. They are also grouped CARBOHYDRATES are monomeric, oligomeric, or
into reducing and nonreducing oligosaccharides. If one polymeric forms of polyhydroxyalkanals or polymeric
of the terminal monosaccharides possesses a hemiac- forms of polyhydroxyalkanals or polyhydroxyalkanones,
etal group, the oligomer is referred to as a reducing which are collectively called monosaccharides. In the
oligosaccharide (since it is susceptible to mild oxi- nineteenth century, when the empirical formulas of or-
dants). If, on the other hand, it does not possess a ganic compounds were determined, it was found that
hemiacetal group (such as when two anomeric hy- the sugars and polysaccharides known at the time had
droxyl groups on adjacent monosaccharides are in- the formula C n (H 2 O) y . They were accordingly given the
volved in acetal bond formation), the oligosaccharide name carbohydrates (i.e., hydrates of carbon). Today’s
is referred to as nonreducing. Reducing oligosaccha- usage of the word carbohydrate applies to a large num-
rides mutarotate, because in solution one of their ter- ber of monomeric, oligomeric, and polymeric compounds,
minal monosaccharides is in equilibrium with other which do not necessarily have their hydrogen and oxygen
forms, while nonreducing oligosaccharides do not atoms in the molecular ratio of 2:1, but which belong to
mutarotate. the group of compounds called monosaccharides or can be
Polysaccharides Polymers composed of more than readily converted to members of that group by hydrolysis.
10 monosaccharide units linked by acetal bridges. Most
naturally occurring polysaccharides have degrees of
5
2
polymerization ranging between 10 and 10 . Linear I. INTRODUCTION
polysaccharides such as cellulose are usually micro-
crystalline, are insoluble in water, and form strong films
A. Historical Background
and fibers, whereas highly branched polysaccharides
such as gums form gets and produce brittle films. The origins of carbohydrate chemistry can be traced back
Pyranoses Aldoses having five carbon atoms or more and to the civilizations of antiquity. For example, the manufac-
uloseshaving sixcarbon atoms ormore canform strain- ture of beer and wine by alcoholic fermentation of grain
less six-membered rings. Such cyclic forms of saccha- starch and grape sugar is well documented on the walls of
rides are called pyranoses, because they are related to ancient Egyptian tombs. The isolation of cellulose fibers
tetrahydropyran. Aldopentoses having six-membered from cotton and flax was started by the civilizations of the
rings are called aldopentopyranoses, and hexuloses Far and Near East and was introduced by the Greeks to
possessing such rings are designated hexopyranuloses. Europe. Gums and resins were valued commodities at the
The most stable conformations of pyranoses are the two beginning of the Christian era.
1
4
chair conformations C 4 and C 1 , of which the latter is The isolation of sucrose from the juice of sugarcane
usually the more stable. marks an important milestone in sugar chemistry. In the
Saccharide (Sugarlike; synonymous with carbohydrate). Far East, where this plant grew, sugar was isolated as a
It is often prefixed by “mono,”“oligo,” or “poly” to yellowish syrupy concentrate that crystallized, on stand-
designate degree of polymerization. ing, into a brown mass, and a number of Chinese recipes
Sugars Sweet-tasting saccharides (monosaccharides and dating from the fourth century describe in detail how the
disaccharides are sweet tasting, whereas most trisac- sugarcane juice was concentrated (Fig. 1). It is interesting
charides and all higher saccharides are devoid of taste). that the Sanskrit word sugar means “sweet sand,” which
Because the term sugar encompasses compounds pos- aptly describes the properties of crushed, raw sugar. Sig-
sessing a physiological rather than a chemical property, nificant progress in the manufacture of sugar occurred af-
and because the term has been used indiscriminately to ter the French Revolution, when Europe under blockade
designate different saccharides (sucrose by the public had to rely on the sugar beet for the manufacture of this
and D-glucose by the medical profession), it is slowly important commodity. Improved methods of isolation and