Carbohydrate chemistry studies the properties, reactions, and structures of compounds made up of carbon, hydrogen, and oxygen. Carbohydrates include sugars, starch, and cellulose, and are an essential energy source for animals. Monosaccharides are simple carbohydrates, while complex carbohydrates are made up of many monosaccharide units. Different carbohydrates can have the same formula but different structures, and one type of carbohydrate can exist in different forms. Carbohydrates can form the basis of other biologically important compounds, such as nucleic acids.
Carbohydrate chemistry describes the structure, properties and reactions of compounds of carbon, hydrogen and oxygen known as carbohydrates. These compounds have the general formula C(H2O)n, where n can be any number from three or higher. It can be seen that carbohydrates always contain hydrogen and oxygen in the proportions of water (H2O), hence the “hydrate” part of the name. Carbohydrates include sugars, starch, cellulose and many other common substances. They are produced from carbon dioxide and water by photosynthesis in plants and are an essential energy source in the diet of all animals.
The simplest carbohydrates are trioses, with only three carbon atoms. The chemistry of carbohydrates can, however, be quite complex, as a number of small carbohydrate molecules can be joined together to form much larger structures. Simple carbohydrates, such as glucose, are known as monosaccharides. Carbohydrate chemistry advanced significantly when the detailed structures of many monosaccharides were first elucidated by Emil Fischer, a German chemist, in the late 19th century. Complex carbohydrates are made up of monosaccharide units bonded together.
Molecules made up of two monosaccharides are known as disaccharides; a common example is sucrose, better known as table sugar, which consists of the monosaccharides glucose and fructose. Oligosaccharides have several monosaccharide units, and polysaccharides are made up of long chains of these units, sometimes numbering in the thousands; examples are starch in its various forms and cellulose. Each molecular unit in the chain is joined to its neighbor by a glycosidic bond, which is formed by the removal of hydrogen (H) and hydroxyl (OH) groups – forming water – from adjacent monosaccharide molecules.
The structures of carbohydrates are such that different molecules can have the same overall formula, with the atoms arranged differently. Monosaccharides, for example, can be divided into aldoses, which have an aldehyde group, and ketoses, which have a keto group, a carbon-oxygen double bond also known as a carbonyl group. Although glucose and fructose have the same chemical formula (C6H12O6), they are structured differently: glucose is an aldose and fructose is a ketose. This is a common feature of carbohydrate chemistry.
It is also possible for one type of carbohydrate to exist in different forms. Glucose can take a linear form, with its six carbon atoms forming a short chain; the carbon atoms may be numbered C1-C6, with C1 forming the aldehyde group and C6, at the other end, bonded to two hydrogen atoms and a hydroxyl group. The four carbon atoms in the middle have a hydrogen atom on one side and a hydroxyl group on the other. There are two forms of glucose, D-glucose and L-glucose, differing only in that the former has only one of its hydroxyl groups on the same side of the oxygen molecule as the aldehyde group, while in the latter this arrangement it is exactly reversed. This is true for many monosaccharides, with the “D” forms predominating among natural sugars.
In solution, D-glucose tends to form a ring structure, with C6 pushed to one side and the hydroxyl group on C5 reacting with the aldehyde group on C1 to form a six-membered ring with 5 carbon atoms and one oxygen atom. This is known as the glucopyranose ring. The ring can take two different shapes, depending on the position of the hydroxyl group on C1. If it is in the same plane as the ring, the compound is known as D-glucopyranose, but if it lies perpendicular to the plane of the ring, it is known as α D-glucopyranose. The two different forms are known as anomers and the C1 carbon atom is known as anomeric carbon.
The β form appears to be structurally more stable, and in the case of D-glucopyranose it is, but in some monosaccharides the α form is more common. This is because in these compounds the electrostatic repulsion between the electron pairs in the anomeric hydroxyl group and those of the oxygen atom in the ring can overcome the greater structural stability of the shape, a phenomenon known as the anomeric effect. Which form is more stable depends not only on the compound, but also on the solvent and temperature.
The hydroxyl, aldehyde and ketone groups in carbohydrates can be replaced by other groups, allowing for a wide variety of reactions. Carbohydrates form the basis of many other biologically important compounds. For example, ribose and deoxyribose, a related compound, are the building blocks from which the nucleic acids DNA and RNA are formed. Glycosides are formed from carbohydrates and alcohols; Fischer glycosidation, named after Emil Fischer, involves the use of a catalyst to form methylcoside glucoside from glucose and methanol. Another route for the production of glycosides is the Koenigs-Knorr reaction, which combines a glycosyl halide with an alcohol to form the glycoside.
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