60
1
General Principles
1 General Properties and Occurrence of Carbohydrates
Carbohydrates have the following major properties: (1) they are polyhydroxy aldehydes or
ketones; (2) they have chiral or asymmetric carbons that are generally manifested by the rotation of plane polarized light; (3) they have the ability to form multiple hydrogen bonds, generally giving them the property of being water-soluble, but they also can be water-insoluble
when they form intermolecular hydrogen bonds with each other to give crystals or large, high
molecular weight, insoluble crystalline aggregates, granules, or fibers; (4) many have reactivities of aldehydes that can be oxidized to acids by reagents that are thereby reduced (e. g.,
reducing an oxidizing agent such as an alkaline solution of copper(II) or ferricyanide/cyanide),
and they, hence, are considered to be reducing sugars, or they can themselves be reduced by
reducing reagents, such as NaBH 4 , to give sugar alcohols; (5) the aldehyde or ketone groups
in carbohydrates with five or more carbons will react with intramolecular alcohol groups to
form cyclic structures with hemiacetal and hemiketal hydroxyl groups; (6) the hemiacetal or
hemiketal hydroxyls are more reactive than the alcohols and can react intermolecularly with
alcohols and amines to give acetals or ketals (glycosidic bonds) that are fairly stable; (7) they
have two kinds of alcohol groups, secondary and primary, that can undergo the usual reactions
of alcohols to give esters and ethers and can be replaced, for example, by hydrogen, halogens
(F, Cl, Br, and I), amino groups, N-acetyl amino groups, and sulfhydryl groups; (8) they are
generally, although not all of them, sweet-tasting (for example, D-glucose, D-glucitol, D-fructose, D-xylose, D-xylitol and sucrose are sweet-tasting) by forming specific hydrogen and
hydrophobic bonds with the sweet-taste receptors on the tongue; and (9) when attached to
proteins or cell surfaces, the structural diversity of oligosaccharides mediate a large number of
biochemical and biological processes.
In the 19th century, several naturally occurring carbohydrates were known, such as glucose
(then called dextrose), fructose (then called levulose), mannose, galactose, sucrose, lactose,
starch, and cellulose. Some of these had been known for thousands of years, for example,
sucrose, starch, and cellulose. Also in the 19th century, the empirical formula for all of these
materials was found to be C n (H 2 O) n and they were originally thought to be hydrates of carbon,
hence the name carbohydrates.
Carbohydrates are now more completely defined as polyhydroxy aldehydes or ketones and
compounds that can be derived from them by reduction to give sugar alcohols, oxidation to
give sugar acids, substitution of hydroxyl group(s) by hydrogen to give deoxy sugars or by
amino or N-acetyl amino groups to give deoxy-amino sugars, derivatization of a hydroxyl
group by phosphate or sulfate to give sugar phosphates or sugar sulfates, and by condensation
reactions of a hydroxyl group of one sugar with the hemiacetal group of another sugar to give
disaccharides, trisaccharides, oligosaccharides, and polysaccharides.
2 Carbohydrate Property of Optical Rotation of Plane Polarized Light
An important property of carbohydrates that was recognized in the 19th century was that they
generally, but not always, rotated plane polarized light and that this was specific for each carbohydrate. This property is due to the presence of asymmetric or chiral carbons that have four
different groups attached to the carbons. Those carbohydrates that rotate plane polarized light
1
General Principles
1 General Properties and Occurrence of Carbohydrates
Carbohydrates have the following major properties: (1) they are polyhydroxy aldehydes or
ketones; (2) they have chiral or asymmetric carbons that are generally manifested by the rotation of plane polarized light; (3) they have the ability to form multiple hydrogen bonds, generally giving them the property of being water-soluble, but they also can be water-insoluble
when they form intermolecular hydrogen bonds with each other to give crystals or large, high
molecular weight, insoluble crystalline aggregates, granules, or fibers; (4) many have reactivities of aldehydes that can be oxidized to acids by reagents that are thereby reduced (e. g.,
reducing an oxidizing agent such as an alkaline solution of copper(II) or ferricyanide/cyanide),
and they, hence, are considered to be reducing sugars, or they can themselves be reduced by
reducing reagents, such as NaBH 4 , to give sugar alcohols; (5) the aldehyde or ketone groups
in carbohydrates with five or more carbons will react with intramolecular alcohol groups to
form cyclic structures with hemiacetal and hemiketal hydroxyl groups; (6) the hemiacetal or
hemiketal hydroxyls are more reactive than the alcohols and can react intermolecularly with
alcohols and amines to give acetals or ketals (glycosidic bonds) that are fairly stable; (7) they
have two kinds of alcohol groups, secondary and primary, that can undergo the usual reactions
of alcohols to give esters and ethers and can be replaced, for example, by hydrogen, halogens
(F, Cl, Br, and I), amino groups, N-acetyl amino groups, and sulfhydryl groups; (8) they are
generally, although not all of them, sweet-tasting (for example, D-glucose, D-glucitol, D-fructose, D-xylose, D-xylitol and sucrose are sweet-tasting) by forming specific hydrogen and
hydrophobic bonds with the sweet-taste receptors on the tongue; and (9) when attached to
proteins or cell surfaces, the structural diversity of oligosaccharides mediate a large number of
biochemical and biological processes.
In the 19th century, several naturally occurring carbohydrates were known, such as glucose
(then called dextrose), fructose (then called levulose), mannose, galactose, sucrose, lactose,
starch, and cellulose. Some of these had been known for thousands of years, for example,
sucrose, starch, and cellulose. Also in the 19th century, the empirical formula for all of these
materials was found to be C n (H 2 O) n and they were originally thought to be hydrates of carbon,
hence the name carbohydrates.
Carbohydrates are now more completely defined as polyhydroxy aldehydes or ketones and
compounds that can be derived from them by reduction to give sugar alcohols, oxidation to
give sugar acids, substitution of hydroxyl group(s) by hydrogen to give deoxy sugars or by
amino or N-acetyl amino groups to give deoxy-amino sugars, derivatization of a hydroxyl
group by phosphate or sulfate to give sugar phosphates or sugar sulfates, and by condensation
reactions of a hydroxyl group of one sugar with the hemiacetal group of another sugar to give
disaccharides, trisaccharides, oligosaccharides, and polysaccharides.
2 Carbohydrate Property of Optical Rotation of Plane Polarized Light
An important property of carbohydrates that was recognized in the 19th century was that they
generally, but not always, rotated plane polarized light and that this was specific for each carbohydrate. This property is due to the presence of asymmetric or chiral carbons that have four
different groups attached to the carbons. Those carbohydrates that rotate plane polarized light