# 20: Carbohydrates

Carbohydrates are a major class of naturally occurring organic compounds, which come by their name because they usually have, or approximate, the general formula $$\ce{C}_n \ce{(H_2O)}_m$$, with $$n$$ equal to or greater than three. Among the well-known carbohydrates are various sugars, starches, and cellulose, all of which are important for the maintenance of life in both plants and animals. Although the structures of many carbohydrates appear to be quite complex, the chemistry of these substances usually involves only two functional groups - ketone or aldehyde carbonyls and alcohol hydroxyl groups. The carbonyl groups normally do not occur as such, but are combined with hydroxyl groups to form hemiacetal or acetal linkages of the kind discussed in Section 15-4E. An understanding of stereochemistry is particularly important to understanding the properties of carbohydrates. Configurational and conformational isomerism play an important role. For this reason, you may wish to review Chapter 5 and Sections 12-3 and 19-5.

• 20.1: Prelude to Carbohydrates
Although the structures of many carbohydrates appear to be quite complex, the chemistry of these substances usually involves only two functional groups - ketone or aldehyde carbonyls and alcohol hydroxyl groups. The carbonyl groups normally do not occur as such, but are combined with hydroxyl groups to form hemiacetal or acetal linkages.
• 20.2: Classification and Occurrence of Carbohydrates
The simple sugars, or monosaccharides, are the building blocks of carbohydrate chemistry. They are polyhydroxy aldehydes or ketones with five, six, seven, or eight carbon atoms that are classified appropriately as pentoses, hexoses, heptoses, or octoses, respectively. They can be designated by more specific names, such as aldohexose or ketohexose, to denote the kind of carbonyl compound they represent.
• 20.3: The Structure and Properties of D-Glucose
Glucose is by far the most abundant monosaccharide; it occurs free in fruits, plants, honey, in the blood of animals, and combined in many glycosides, disaccharides, and polysaccharides. The structure and properties of glucose will be considered in greater detail than those of the other monosaccharides, not only because of its importance, but because much of what can be said about glucose also can be said about the other monosaccharides.
• 20.4: Conventions for Indicating Ring Size and Anomer Configurations of Monosaccharides
The oxide ring is six-membered in some sugars and five-membered in others, and it is helpful to use names that indicate the ring size. The five- and six-membered oxide rings bear a formal relationship to oxa-2,5-cyclohexadiene and oxa-2,4-cyclopentadiene that commonly are known as pyran and furan, respectively. For this reason, the names furanose and pyranose have been coined to denote five- and six-membered rings in cyclic sugars.
• 20.5: Derivatives of Glucose
Although we now have powerful spectroscopic methods available to determine the sizes of the oxide rings formed by the simple monosaccharides, the way in which this was done chemically for glucose highlights the difference in reactivity between ether and alcohol functions. The acid-catalyzed methylation of glucose with methanol to give two distinct glucosides corresponds to displacement of the hemiacetal hydroxyl by methoxyl to form an acetal.
• 20.6: Glycosides
Although abundant quantities of glucose and fructose are found in the free state, they and less common sugars occur widely in plants and animals combined with various hydroxy compounds. The bonding is through oxygen to the carbonyl carbon, as in the αα - and ββ -methylglucosides discussed in Section 20-4A, to give acetal or ketal structures. These substances are sometimes simply called glycosides.
• 20.7: Disaccharides
Combinations of two or more of the simple sugars through glycoside linkages give substances known as polysaccharides. They also are called oligosaccharides if made from two to ten sugar units. The simplest oligosaccharides are disaccharides made of two molecules of simple sugars.
• 20.8: Polysaccharides
The fibrous tissue in the cell walls of plants contains the polysaccharide cellulose, which consists of long chains of glucose units.  A second, very widely distributed polysaccharide is starch, which is stored in the seeds, roots, and fibers of plants as a food reserve - a potential source of glucose. The chemical composition of starch varies, but there are two structurally different polysaccharides. One is a linear structure (amylose) and the other is a branched structure (amylopectin).
• 20.9: Vitamin C
The "antiscorbutic" factor of fresh fruits, which prevents the development of the typical symptoms of scurvy in humans, is a carbohydrate derivative known as vitamin C or ascorbic acid. This substance is not a carboxylic acid, but a lactone, and owes its acidic properties (and ease of oxidation) to the presence of an enediol grouping. It belongs to the L series by the glyceraldehyde convention.
• 20.10: Formation of Carbohydrates by Photosynthesis
Carbohydrates are formed in green plants by photosynthesis, which is the chemical combination, or "fixation", of carbon dioxide and water by utilization of energy from the absorption of visible light. The overall result is the reduction of carbon dioxide to carbohydrate and the formation of oxygen. If the carbohydrate formed is cellulose, then the reaction in effect is the reverse of the burning of wood, and obviously requires considerable energy input.
• 20.11: The Generation of Energy from Carbohydrate Metabolism
This section is concerned mainly with the pathway by which glucose is metabolized by the process known as glycolysis. Initially, the storage fuels or foodstuffs (fats, carbohydrates, and proteins) are hydrolyzed into smaller components (fatty acids and glycerol, glucose and other simple sugars, and amino acids). In the next stage, these simple fuels are degraded further to two-carbon fragments.
• 20.E: Carbohydrates (Exercises)
These are the homework exercises to accompany Chapter 20 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).