SUGAR, in chemistry, the generic name for a certain series of carbohydrates, i.e. substances of the general formula C n (H2O) m . Formerly the name was given to compounds having a sweet taste, e.g. sugar of lead, but it is now restricted to certain oxyaldehydes and oxy-ketones, which occur in the vegetable and animal kingdoms either free or in combination as glucosides (q.v.) and to artificial preparations of similar chemical structure. Cane sugar has been known for many centuries; milk sugar was obtained by Fabrizio Bartoletti in 1615; and in the middle of the 18th century Marggraf found that the sugars yielded by the beet, carrot and other roots were identical with cane sugar. The sugars obtained from honey were investigated by Lowitz and Proust, and the latter decided on three species: (i) cane sugar, (2) grape sugar, and (3) fruit sugar; the first has the formula CuHaOii, the others C 6 Hi 2 O s . This list has been considerably developed by the discovery of natural as well as of synthetic sugars.
It is convenient to divide the sugars into two main groups: monosaccharoses (formerly glucoses) and disaccharoses (formerly saccharoses). The first term includes simple sugars containing two to nine atoms of carbon, which are known severally as bioses, trioses, tetroses, pentoses, hexoses, etc. ; whilst those of the second group have the formula CuHaOu and are characterized by yielding two monosaccharose molecules on hydrolysis. In addition trisaccharoses are known of the formula CiaHszOu ; these on hydrolysis yield one molecule of a monosaccharose and one of a disaccharose, or three of a monosaccharose. It is found also that some monosaccharoses behave as aldehydes whilst others contain a keto group ; those having the first character are called aldoses, and the others ketoses. All sugars are colourless solids or syrups, which char on strong heating; they are soluble in water, forming sweet solutions but difficultly soluble in alcohol. Their solutions are optically active, i.e. they rotate the plane of polarized light; the amount of the rotation being dependent upon the concentration, temperature, and, in some cases, on the age of the solution (cf. GLUCOSE). The rotation serves for the estimation of sugar solutions (saccharimetry). They are neutral to litmus and do not combine with dilute acids or bases; strong bases, such as lime and baryta, yield saccharates, whilst, under certain conditions, acids and acid anhydrides may yield esters. Sugars are also liable to fermentation. 1 Our knowledge of the chemical structure of the monosaccharoses may be regarded as dating from 1880, when Zincke suspected some to be ketone alcohols, for it was known that glucose and fructose, for example, yielded penta-acetates, and on reduction gave hexahydric alcohols, which, when reduced by hydriodic acid, gave normal and secondary hexyliodide. The facts suggested that the six carbon atoms formed a chain, and that a hydroxy group was attached to five of them, for it is very rare for two hydroxy groups to be attached to the same carbon atom. The remaining oxygen atom is aldehydic or ketonic, for the sugars combine with hydrocyanic acid, hydroxylamine and phenylhydrazine. The correctness of this view was settled by Kiliani in 1885. He prepared the cyanhydrins of glucose and fructose, hydrolysed them to the corresponding oxy-acids, from which the hydroxy groups were split out by reduction; it was found that glucose yielded normal heptylic acid and fructose methylbutylacetic acid; hence glucose is an aldehyde alcohol, CH 2 OH-(CH-OH) 4 -CHO, whilst fructose is a ketone alcohol CH 2 OH- (CH-OH),-CO-CH 2 OH. 2 Kiliani also showed that arabinose, CjHi 2 Os, a sugar found in cherry gum, was an aldopentose, and thus indicated an extension of the idea of a " sugar."
Before proceeding to the actual synthesis of the sugars, it is advisable to discuss their decompositions and transformations.
1. Cyanhydrins. The cyanhydrins on hydrolysis give monocarboxylic acids, which yield lactones; these compounds when reduced by sodium amalgam in sulphuric acid solution yield a sugar containing one more carbon atom. This permits the formation of a higher from a lower sugar (E. Fischer)
CH 2 OH CH 2 OH CHjOH CH 2 OH CH-OH CH-OH /CH CH-OH (CH-OH)j -> (CH-OH) 2 -/ (CH-OH), -> (CH-OH) 2 CHO CH-OH \ CH-OH CH-OH CN \CO CHO Pentose > Cyanhydrin > Lactone > Hexose.
2. Oximes. The oximes permit the reverse change, i.e. the passage from a higher to a lower sugar. Wohl forms the oxime and converts it into an acetylated nitrile by means of acetic anhydride and sodium acetate; ammoniacal silver nitrate solution removes hydrocyanic acid and the resulting acetate is hydrolysed by acting with ammonia to form an amide, which is finally decomposed with sulphuric acid.
CH 2 OH CH 2 OH CH 2 OH CH 2 OH (CH-OH), - (CH-OH), - (CH-OH), - (CH-OH), CH-OH CH-OH CH-OH CHO CHO CH:NOH CN Hexose Oxime > Nitrile Pentose.
Ruff effects the same change by oxidizing the sugar to the oxy-acid, 'See FERMENTATION; and for the relation of this property to structure see STEREOISOMERISM.
2 These formulae, however, require modification in accordance with the views of Lowry and E. F. Armstrong, which postulate a y oxidic structure (see GLUCOSE). This, however, does not disturb the tenor of the following arguments.
XXVI. 2 and then further oxidizing this with Fenton's reagent, i.e. hydrogen peroxide and a trace of a ferrous salt :
C4H,04(CH-OH)-CHO->C < H,04(CH-OH)-C0 2 H- > C,H,0 4 -CHO Hexose - Acid - Pentose.
3. Phenylhydrazine Derivatives. Fischer found that if one molecule of phenylhydrazine acted upon one molecule of an aldose or ketose a hydrazone resulted which in most cases was very soluble in water, but if three molecules of the hydrazine reacted (one of which is reduced to ammonia and aniline) insoluble crystalline substances resulted, termed osazones, which readily characterized the sugar from which it was obtained.
R R R CH-OH - CH-OH -+ C-.N-NHPh CHO CH:N-NHPh. CH:N-NHPh.
Aldose > Hydrazone Osazone; R R R CO - C:N-NHPh. -> C:N-NHPh CH 2 OH CH 2 OH CH:N-NHPh.
Ketose Hydrazone Osazone.
On warming the osazone with hydrochloric acid the phenylhydrazine residues are removed and an osone results, which on reduction with zinc and acetic acid gives a ketose.
R R R C:N-NHPh. -> CO - CO CH:N-NHPh. CHO CH 2 OH Osazone > Osone Ketose.
A ketose may also be obtained by reducing the osazone with zinc and acetic to an osamine, which with nitrous acid gives the ketose:
R R R C:N-NHPh. -* CO -> CO CH:N-NHPh. CH 2 NH 2 CH,OH.
Osazone Osamine Ketose.
These reactions permit the transformation of an aldose into a ketose; the reverse change can only be brought about by reducing the ketose to an alcohol, and oxidizing this compound to an aldehyde. It is seen that aldoses and ketoses which differ stereochemically in only the two final carbon atoms must yield the same osazone; and since d-mannose, d-glucose, and d-fructose do form the same osazone (d-glucosazone) differences either structural or stereochemical must be placed in the two final carbon atoms. 3 It may here be noticed that in the sugars there are asymmetric carbon atoms, and consequently optical isomers are to be expected. Thus glucose, containing four such atoms, can exist in 16 forms; and the realization of many of these isomers by E. Fischer may be regarded as one of the most brilliant achievements in modern chemistry. The general principles of stereochemistry being discussed in Stereoisomerism (?..), we proceed to the synthesis of glucose and fructose and then to the derivation of their configurations.
In 1861 Butlerow obtained a sugar-like substance, methylenitan, by digesting trioxymethylene, the solid polymer of formaldehyde, with lime. The work was repeated by O. Loew, who prepared in 1885 a sweet, unfermentable syrup, which he named formose, CeHiiOt and, later, by using magnesia instead of lime, he obtained the fermentable methose. Fischer showed that methose was identical with the a-acrose obtained by himself and Tafel in 1887 by decomposing acrolein dibromide with baryta, and subsequently prepared by oxidizing glycerin with bromine in alkaline solution, and treating the product with dilute alkali at o. Glycerin appears to yield, on mild oxidation, an aldehyde, CH 2 OH-CH(OH)-CHO, and a ketone, CH 2 OH-CO-CH 2 OH, and these condense as shown in the equation :
CH 2 OH-CH(OH)-CHO+CH 2 OH-CO-CH ! OH = CH 2 OH-CH(OH)-CH(OH)-CH(OH)-CO.CH,OH+H 2 O.
The osazone prepared from a-acrose resembled most closely the glucosazone yielded by glucose, mannose, and fructose, but it was optically inactive; also the ketose which it gave after treatment with hydrochloric acid and reduction of the osone was like ordinary fructose except that it was inactive. It was surmised that o-acrpse was a mixture of dextro and laevo fructose, a supposition which was proved correct by an indirect method. The starting point was ordinary (<f)mannite (mannitolJ.QHuOe.a naturally occurring hexahydric alcohcrt, which only differed from o-acntol, the alcohol obtained by reducing a-acrose, with regard to optical activity. Mannite on oxidation yields an aldose, mannose, CgHuOt, which ' To distinguish the isomerides of opposite optical activity, it is usual to prefix the letters d- and /-, but these are used only to indicate the genetic relationship, and not the character of the optical activity; ordinary fructose, for example, being represented as d-fructose although it exercises a laevorotatory power because it is derived from d-glucose.
on further oxidation gives a mannonic acid, ("J IJ< il I j.,-(( >_1 1 ; this acid readily yields a lactone. Also Kiliani found that the lactone derived from the cyanhydrin of natural arabinose (laevo) was identical with the previous lactone except that its rotation was equal and opposite. On mixing the eslactones and reducing (a +/)-mnanitol was obtained, identical with o-acritol. A separation of u-acrose was made by acting with beer yeast, which destroyed the ordinary fructose and left /-fructose which was isolated as its osazone. Also (</ + /) mannonic acid can be split into the d and / acids by fractional crystallization of the strychnine or brucine salts. The acid yields, on appropriate treatment, (/-mannose and (/-mannite. Similarly the / acid yields the laevo derivatives.
The next step was to prepare glucose. This was effected indirectly. The identity of the formulae and osazones of (/-mannose and (/-glucose showed that the stereochemical differences were situated at the carbon atom adjacent to the aldehyde group. Fischer applied a method indicated by Pasteur in converting dextro into laevo-tartaric acid; he found that both (/-mannonic and (/-gluconic acids (the latter is yielded by glucose on oxidation) were mutually convertible by heating with quinoline under pressure at 140. It was then found that on reducing the lactone of the acid obtained from (/-mannonic acid, ordinary glucose resulted.
Fischer's o-acrose therefore led to the synthesis of the dextro and laevo forms of mannose, glucose and fructose; and these substances have been connected synthetically with many other sugars by means of his cyanhydrin process, leading to higher sugars, and Wohl and Ruff's processes, leading to lower sugars. Certain of these relations are here summarized (the starting substance is in italics) :
/-Glucose 4 l-arabinose > /-mannose > /-mannoheptose; glucononose 4 o-gluco-octose 4 a-glucoheptose 4 d-glucose > 0-glucoheptose 7 /8-gluco-octose ; A-mannose> (/-mannoheptose ^manno-octose~>mannononose; d-glucose $ (/-arabinose > (/-erythrose. l-glucose $ 6-arabinose ^ /-erythrose.
Their number is further increased by spatial inversion of the dicarboxylic acids formed on oxidation, followed by reduction; for example: d- and /-glucose yield (/-and /-gulose; and also by Lobry de Bruyn and Van Ekenstein's discovery that hexoses are transformed into mixtures of their isomers when treated with alkalis, alkaline earths, lead oxide, etc.
Biose. The onl\ ily possible biose is glycollic aldehyde, CHO-CHjOH, obtained impure by Fischer from bromacetaldehyde and baryta water, and crystalline by Fenton by heating dihydroxymaleic acid with water to 60. It polymerizes to a tetrose under the action of sodium hydroxide.
Triases. The trioses are the aldehyde and ketone mentioned above as oxidation products of glycerin. Glyceric aldehyde CH 2 OH-CH(OH)-CHO, was obtained pure by Wohlon oxidizing acrolein acetal, CH 2 -CH(OC 2 H 6 ) 2> and hydrolysing. Although containing an asymmetric carbon atom it has not been resolved The ketone, dihydroxyacetone, CH 2 OH-CO-CH 2 OH, was obtained by Piloty by condensing formaldehyde with nitromethane, reducing to a hydroxylamino compound, which is oxidized to the oxime of dihydroxyacetone ; the ketone is liberated by oxidation with bromine water:
* (CH 2 OH) 3 C-NO 2 -> (CH 2 OH) 3 C-NH OH -> (CH 2 OH) 2 C:NOH-(CH 2 OH) 2 CO. The ketone is also obtained when Bertrand's sorbose bacterium acts on glycerol ; this medium also acts on other alcohols to yield ketoses ; for example: erythrite gives erythrulose, arabite arabinulose, mannitol fructose, etc.
Tetroses. Four active tetroses are possible, and three have been obtained by Ruff and Wohl from the pentoses. Thus Wohl prepared /-threose from /-xylose and /-erythrose from /-arabinose, and Ruff obtained d- and /-erythrose from d- and /-arabonic acids, the oxidation products of d- and /-arabinoses. Impure inactive forms result on the polymerization of glycollic aldehyde and also on the oxidation of erythrite, a tetrahydric alcohol found in some lichens. d-Erythrulose is a ketose of this series.
Pentoses. Eight stereoisomeric pentaldoses are possible, and six are known : d- and /-arabinose, d- and /-xylose, /-ribose, and o-lyxose. Scheibler discovered /-arabinose in 1869, and regarded it as a glucose; in 1887 Kiliani proved it to be a pentose. (/-Arabinose is obtained from (/-glucose by Wohl's method. /-Xylose was discovered by Koch in 1886; its enantiomorph is prepared from (/-gulose by Wohl's method. /-Ribose and (jflyxose are prepared by inversion from /-arabinose and /-xylose; the latter has also been obtained from (/-galactose. We may notice that the pentoses differ from other sugars by yielding furfurol when boiled with hydrochloric acid. Rhamnose or isodulcite, a component of certain glucosides, fucose, found combined in seaweeds and chinovose, present as its ethyl ester, chinovite, in varieties of quina-bark, are methyl pentoses. /-Arabinulose obtained from arabite and Bertrand's sorbium bacterium is a ketose.
Hexoses. The hexoses may be regarded as the most important sub-division of the monosaccharoses. The reader is referred to GLUCOSE and FRUCTOSE for an account of these substances. The next important aldose is mannose. (/-Mamiose, first prepared by oxidizing (/-mannite, found in plants and manna-ash (Fraxinus ornus), was obtained by Tollens and Gans on hydrolysing cellulose and by Reis from seminine (reserve cellulose), found in certain plant seeds, e.g. vegetable ivory. /-Mannose is obtained from /-mannonic acid. Other forms are: d- and /-gulose, prepared from the lactones of the corresponding gulonic acids, which are obtained from d- and /-glucose by oxidation and inversion; d- and /-idose, obtained by inverting with pyridine d- and /-gulonic acids, and reducing the resulting idionic acids; d- and /-galactose, the first being obtained by hydrolysing milk _sugar with dilute sulphuric acid, and the second by fermenting inactive galactose (from the reduction of the lactone of d, /-gaiactonic acid) with yeast; and (/- and /-talose obtained by inverting the gaiactonic acids by pyridine into d- and /-talonic acids and reduction. Of the ketoses, we notice (/-sorbose, found in the berries of mountain-ash, and (/-tagatose, obtained by Lobry de Bruyn and van Ekenstein on treating galactose with dilute alkalis, talose and /-sorbose being formed at the same time. The higher sugars call for no special notice.
Configuration of the Hexaldoses. 1 The plane projection of molecular structures which differ stereochemically is discussed under STEREO- ISOMERISM; in this place it suffices to say that, since the terminal groups of the hexaldose molecule are different and four asymmetric carbon atoms are present, sixteen hexaldoses are possible; and for the hexahydric alcohols which they yield on reduction, and the tetrahydric dicarboxylic acids which they give on oxidation, only ten forms are possible. Employing the notation in which the molecule is represented vertically with the aldehyde group at the bottom, and calling a carbon atom+or according as the hydrogen atom is to the left or right, the possible configurations are shown in the diagram. The grouping of the forms 5 to to with 1 1 to 16 is designed to show that the pairs 5, 1 1 for example become identical when the terminal groups are the same.
13 14 15 16 We can now proceed to the derivation of the structure of glucose. Since both (/-glucose and (/-gulose yield the same active (d) saccharic acid on oxidation, the configuration of this and the corresponding /-acid must be sought from among those numbered 5-10 in the above table. Nos. 7 and 8 can be at once ruled out, however, as acids so constituted would be optically inactive and the saccharic acids are active. If the configuration of (/-saccharic acid were given by either 6 or 10, bearing in mind the relation of mannose to glucose, it would then be necessary to represent (/-mannosaccharic acid by either 7 or 8 as the forms 6 and 10 pass into 7 and 8 on changing the sign of a terminal group; but this cannot be done as mannosaccharic acid is optically active. Nos. 6 and 10 must, in consequence, also be ruled out. No. 5, therefore, represents the configuration of one of the saccharic acids, and No. 9, that of the isomeride of ' equal opposite rotatory power. As there is no means of distinguishing between the configuration of a dextro- and laevo-modification, an arbitrary assumption must be made. No. 5 may therefore be assigned to the d- and No. 9 to the /-acid. It then follows that (/-mannose is represented by No. I, and /-mannose by No. 4, as mannose is produced by reversing the sign of the asymmetric system adjoining the terminal COH group.
It remains to distinguish between 5 and 11,9 and 1 5 as representing glucose and gulose. To settle this point it is necessary to consider the configuration of the isomeric pentoses arabinose and xylose from which they may be prepared. Arabinose being convertible into /-glucose and xylose into /-gulose, the alternative formulae to be considered are CH 2 (OH) +COH CH 2 (OH)+-M COH.
1 The following account is mainly from H. E. Armstrong's article CHEMISTRY in the loth edition of this Encyclopaedia ; the representation differs from the projection of Meyer and Jacobsen.
If the asymmetric system adjoining the COH group, which is that introduced in synthesizing the hexose from the pentose, be eliminated the formulae at disposal for the two pentoses are CH,(OH) --- COH CH,(OH)H --- COH.
When such compounds are converted into corresponding dibasic acids, CO 2 H.[CH(OH)].CO 2 H, the number of asymmetric carbon atoms becomes reduced from three to two, as the central carbon atom is then no longer associated with four, but with only three different radicles. Hence it follows that the " optical " formulae of the acids derived from two pentoses having the configuration given above will be CO 2 H-0-CO a H COjH+O-COaH, and that consequently only one of the acids will be optically active. As a matter of fact, only arabinose gives an active product on oxidation; it is therefore to be supposed that arabinose is the --- compound, and consequently CH,(OH) --- + COH = /-glucose CH 2 (OH) H ---- COH = /-gulose.
When xylose is combined with hydrocyanic acid and the cyanide is hydrolysed, together with /-gulonic acid, a second isomeric acid, /-idonic acid, is produced, which on reduction yields the hexaldose /-idose. When /-gulonic acid is heated with pyridine, it is converted into /-idonic acid, and vice versa; and d-gulonic acid may in a similar manner be converted into d-idonic acid, from which it is possible to prepare d-idpse. It follows from the manner in which /-idose is produced that its configuration is CH 2 (OH) -\ ---- j-COH. The remaining aldohexoses discovered by Fischer are derived from d-galactose from milk-sugar. When oxidized this aldohexose is first converted into the monobasic galactonic acid, and then into dibasic mucic acid; the latter is optically inactive, so that its configuration must be one of those given in the sixth and seventh columns of the table. On reduction it yields an inactive mixture of galactonic acids, some molecules being attacked at one end, as it were, and an equal number of others at the other. On reducing the lactone prepared from the inactive acid an inactive galactose is obtained from which /-galactose may be separated by fermentation. Lastly, when </-galactpnic acid is heated with pyridine, it is converted into talonic acid, which is reducible to talose, an isomeride bearing to galactose the same relation that mannose bears to glucose. It can be shown that d-galactose is CH 2 (OH) H --- h -COH, and hence (f-talose is CH S (OH) + + + COH.
The configurations of the penta-and tetra-aldoses have been determined by similar arguments; and those of the ketoses can be deduced from the aldoses.
The disaccharoses have the formula C^HaOu and are characterized by yielding under suitable conditions two molecules of a hexose : CiiHaOii+HjO = CeHijOe+CeHuOc. The hexoses so obtained are not necessarily identical : thus cane sugar yields d-glucose and d-fructose (invert sugar) ; milk sugar and melibiose give d-glucose and (/-galactose, whilst maltose yields only glucose. Chemically they appear to be ether anhydrides of the hexoses, the union being effected by the aldehyde or alcohol groups, and in consequence they are related to the ethers of glucose and other hexoses, i.e. to the alkyl glucosides. Cane sugar has no reducing power and does not fprm an hydrazone or osazone; the other varieties, however, reduce Fehling's solution and form hydrazones and osazones, behaving as aldoses, i.e. as containing the group -CH(OH)-CHO. The relation of the disaccharoses to the a- and /3-glucosides was established by E. F. Armstrong (Journ. Chem. Soc., 1903, 85, 1305), who showed that cane sugar and maltose were a-glucosides, and raffinose an a-glucoside of melibiose. These and other considerations have led to the proposal of an alkylen oxide formula for glucose, first proposed by Tollens ; this view, which has been mainly developed by Armstrong and Fischer, has attained general acceptance (see GLUCOSE and GLUCOSJDE). Fischer has proposed formulae for the important disaccharoses, and in conjunction with Armstrong devised a method for determining how the molecule was built up, by forming the osone of the sugar and hydrolysing, whereupon the hexosone obtained indicates the aldose part of the molecule. Lactose is thus found to be glucosido-galactose and melibiose a galactosido-glucose.
Several disaccharoses have been synthesized. By acting with hydrochloric acid on glucose Fischer obtained isomaltose, a disaccharose very similar to maltose but differing in being amorphous and unfermentable by yeast. Also Marchlewski (in 1899) synthesized cane sugar from potassium fructosate and acetochloroglucose; and after Fischer discovered that acetochlorohexoses readily resulted from the interaction of the hexose penta-acetates and liquid hydrogen chloride, several others have been obtained.
Cane sugar, saccharose or saccharobiose, is the most important sugar; its manufacture is treated below. When slowly crystallized t forms large monoclinic prisms which are readily soluble in water but difficultly soluble in alcohol. It melts at 160, and on cooling solidifies to a glassy mass, which on standing gradually becomes opaque and crystalline. When heated to about 200 it yields a brown amorphous substance, named caramel, used in colouring liquors, etc. Concentrated sulphuric acid gives a black carbonaceous mass; boiling nitric acid oxidizes it to d-saccharic, tartaric and oxalic acids; and when heated to 160 with acetic anhydride an octa-acetyl ester is produced. Like glucose it gives saccharates with lime, baryta and strontia.
Milk sugar, lactose, lactobiose, CuHnOu, found in the milk of mammals, in the amniotic liquid of cows, and as a pathological secretion, is prepared by evaporating whey and purifying the sugar which separates by crystallization. It forms hard white rhombic prisms (with 1H 2 O), which become anhydrous at 140 and melt with decomposition at 205. It reduces ammoniacal silver solutions in the cold, and alkaline copper solutions on boiling. Its aqueous solution has a faint sweet taste, and is dextro-rotatory, the rotation of a fresh solution being about twice that of an old one. It is difficultly fermented by yeast, but readily by the lactic acid bacillus. It is oxidized by nitric acid to d-saccharic and mucic acids ; and acetic anhydride gives an octa-acetate.
Maltose, malt-sugar, maltobiose, Ci 2 H M Ou, is formed, together with dextrine, by the action of malt diastase on starch, and as an intermediate product in the decomposition of starch by sulphuric acid, and of glycogen by ferments. It forms hard crystalline crusts (with IHjO) made up of hard white needles.
Less important disaccharoses are : Trehalose or mycose, Ci 2 H2jOii-2H 2 O, found in various fungi, e.g. Boletus edulis, in the Oriental Trehala and in ergot of rye; melibiose, CuHzjOu, formed, with fructose, on hydrolysing the tnsaccharose melitose (or raffinose), CisHnOu-S^O, which occurs in Australian manna and in the molasses of sugar manufacture; touranose, C u H a Ou, formed with a-glucose and galactose on hydrolysing another trisaccharose, rnehzitose, C, 8 H32Oi 6 -2H 2 O, which occurs in Pinus larix and in Persian manna; and agavose, Ci 2 H n On, found in the stalks of Agave amencana. (X.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)