CARBOHYDRATES, a group of substances which includes the sugars, starches, and celluloses, along with the many varied products, either found in nature or prepared in the laboratory, which are closely related to them chemically. This group of compounds is comparable in importance with the proteins and fats. Its members play an all-important part in the lives of plants and animals, as structural elements and in the maintenance of functional activity. Cane sugar, glucose, fructose, and the vari ous forms of starch and cellulose may be cited as typical rep resentatives. Their industrial importance may be estimated from the fact that amongst the undertakings directly dependent on carbohydrate materials are the cotton industry, the manufac ture of paper, the artificial silk industries, certain branches of the explosives industry, brewing and the manufacture of power alcohol.
Chemically, the carbohydrates are characterised by great reactivity. As found in nature they are almost invariably optically active (see STEREOCHEMISTRY). They reveal exceed ingly complex possibilities of isomerism and many of them occur in amorphous colloidal forms. For these reasons, their study has provided problems of exceptional interest and difficulty, and in spite of the long-continued efforts of chemists, it is only within recent years that definite knowledge has been obtained concern ing the internal structure of even the simplest members. Formal dehyde undergoes polymerization (q.v.) in presence of alkali to give a product resembling fructose, and since formaldehyde is itself obtainable by reduction of carbonic acid, the synthetic processes which go on in the leaf of the plant with the formation of carbohydrates are partly explained.
The simplest carbohydrates function as polyhydroxyaldehydes or ketones containing from 3 to 9 carbon atoms, although, as will be seen later, these assume a cyclic character. With few exceptions they correspond to the formula They are soluble in water and possess a sweet taste. These simpler mem bers, the sugars, may be regarded as the units of which the more complex carbohydrates are built up. The simplest sugars are termed monosaccharides and their nomenclature is based on the value of n in the above formula. If n = 3 the substance is said to be a triose, if n=6 a hexose and so on. The commonest and most important representatives of the two classes of sugars are glucose (an aldohexose) and fructose (a ketohexose) and the reactions of these two will now be considered in greater detail. In essential respects the chemistry of the other monosaccharides may be taken to be similar either to that of glucose or to that of fructose.
Glucose.—Glucose, dextrose or grape sugar, is widely distributed in plants and animals. It occurs alone or combined with other sugars as in cane sugar, milk sugar, etc., or combined with alcohols of various types to form glucosides (q.v.). Glucose may be prepared readily from starch or from cane sugar by the action of dilute acids. It is very soluble in water and is easily fermented by yeast to give alcohol. From its aqueous solution it separates in warty masses containing one molecule of water of crystallization, m.p. 86°. It may be obtained from other solvents in two distinct forms (see below). In its chemical properties glucose resembles an aldehyde and is a strong reducing agent, quickly precipitating gold and silver from warm solutions of their salts. Solutions of glucose do not, however, restore the colour to Schiff's reagent. The reaction between glucose and an alkaline solution of copper hydroxide (Fehling's solution), which results in the precipitation of cuprous oxide, is used to estimate glucose in aqueous solution. In the absence of other sugars, glucose may be estimated by observing the extent to which it rotates the plane of polarized light since a given weight of pure glucose has a fixed rotation. This is known as its specific rotation.
The aldehydic functions disappear on reduction of glucose with nascent hydrogen, and a hexahydric alcohol, sorbitol (II.), is formed which, when further reduced with hydrogen iodide, yields a derivative of normal hexane, This proves that the arrangement of the six carbon atoms of glucose does not involve a branched chain of carbon atoms. That there are five hydroxyl groups present in the glucose mole cule is shown by the fact that glucose gives penta-acetyl and penta-methyl derivatives and on the basis of the facts so far considered the simplest formula for glucose would be (I.). Furthermore, oxidation with bromine water yields gluconic acid (III.) which, even in aqueous solution, is transformed partly into the 7-lactone, [4+68°, m.p. 130-35° (IV.). Reduction of gluconolactone in aqueous solution with sodium amalgam and dilute acid gives glucose, whilst on oxidation with nitric acid both glucose and its lactone yield saccharic acid (V.).
The aldehydic functions are further illustrated by the action of such reagents as hydroxylamine and phenylhydrazine, which give respectively glucose-oxime and glucose-phenylhydrazone. Excess of the latter reagent causes oxidation, with the produc tion of glucose-phenylosazone (m.p. a substance of char acteristic appearance which is often used as a qualitative test for the presence of glucose. The oxidation can be shown to involve the second carbon atom and the formation of the osazone may be summarized thus: Glycuronic Acid. — The reactive grouping (No. i in formula I.) which functions as an aldehydic group in glucose can be masked (as in the glucosides), and then oxidation of the terminal primary alcoholic group (6) may be effected, and in this way it is possible to obtain glycuronic acid (VI.), which is of special importance in metabolism, since many objectionable substances are removed from the body in the form of their condensation products with it in the form of glucosides (see below).
The aldehydic formula for glucose explains satisfactorily most of the observations mentioned above, but it is impossible to account on this basis for the existence of two isomeric crystalline a- and 13-forms of the sugar or of its simple derivatives.
Stereochemistry of the Sugars.—To visualize the arrange ment in space of the sugar molecule, it is imperative to know how a simple chain of six carbon atoms is disposed. It has been shown that three carbon atoms are linked together in such a manner that they are inclined at an angle of 109° 28', thus: If six such atoms are joined then the fifth and sixth will approach near in space to the first, that is, the chain of atoms curls round on itself. Even more important, is the recognition that when two other atoms are attached to each of these carbon atoms, these also will make an angle of 1o9° 28'. Hence to por tray such a figure on a plane surface, such as this page, is impos sible, and some pictorial convention must be devised. Thus in representing the H and OH attached to a carbon atom it is always assumed that the reader sees each individual carbon atom projected into the plane of the printed page, but the H and OH group attached to this carbon atom will emerge upwards from the page at the above angle.
Only thus is it possible to realize that the expression Otherwise it may be erroneously considered that the former can be made to be identical with the latter by merely revolving the second carbon atom round the first. In the study of sugar chemistry, spherical models of the atoms are essential aids to the realization of these factors of the distribution of groups in three dimensional space.
In the following expressions it is assumed: (a) that the chain of carbon atoms has been uncurled and placed in a straight line; (b) that the two or more addenda which point outwards from this plane are projected downwards into the plane of the paper.
Reference to the aldehydic formula (VI., see below) for glucose shows that the second, third, fourth, and fifth carbon atoms are asymmetric (see STEREOCHEMISTRY), and accordingly there exist or 16 isomeric aldohexoses differing only in the stereochemical arrangement of the —H and —OH groups round the asymmetric carbon atoms. The 16 are arranged in 8 pairs, the two members of each pair being identical except that one form has a dextro configuration (d-series) and the other a laevo-configuration (/-series). All eight aldohexoses are known. Similarly, on theo retical principles there could exist or 8 aldopentoses which may be arranged in 4 pairs. Corresponding to these there exist d- and /-modifications of xylose, ribose, arabinose, and lyxose. The naturally occurring dextrorotatory glucose is genetically related to the dextrorotatory form of glyceraldehyde, to which is assigned the projection formula (A), and the sugars are there fore classified into two series, called the d- and the /-series re spectively, according as they may be built up by synthetical means from d- or l-glyceraldehyde (Rosanoff). On this basis the four d-forms of the aldopentoses can be represented by the pro jection formulae shown below.
(V. and VI.) have been assigned to the latter, the a- and 0-forms of glucose may similarly be represented by formulae (I.) and (II.). Hence the formulation of glucose as an aldehyde must now be superseded by these more precise representations which admit of the explanation of all the aldehydic functions of glucose (as also for other sugars) whilst serving the better to elucidate the very special properties of sugars as modified aldehydes. It is seen that the —OH on the fifth carbon atom of the hexose chain is spatially near to the aldehyde group in the transitory phase, and by interaction the CHO deprives the OH of its hydrogen, thus: The physical and chemical behaviour of these sugars renders possible the accurate diagnosis of the above configurations and it is on this basis that the formulae are assigned. Similarly there exist 16 aldohexoses-8 of the d-series, and 8 of the /-series--the chief representatives in the d-series being those formulated below. The corresponding formulae for those in the 1-series are the mirror images of the formulae now given.