XtraEdge for IIT-JEE 34 JANUARY 2011 D-(+)-Glucose has the cyclic structure represented
crudely by IIa and IIIa, more accurately by IIb and IIIb.
H H HO H H
OH OH H OH 1
2 3 4 5
6 CH2OH
H OH
OH OH
H H
H H
HO
CH2OH O
1 3 2
4 5 6
IIa IIb
α-D-(+)-Glucose (m.p. 146 ºC, [α] = +112º) O
HO H HO H H
H OH H OH 1
2 3 4 5
6 CH2OH
H OH
OH H
H H
OH H
HO
CH2OH O
1 2 3 4
5 6
IIIa IIIb
β-D-(+)-Glucose (m.p. 150 ºC, [α] = +19º) O
D-(+)-Glucose is the hemiacetal corresponding to reaction between the aldehyde group and the C-5 hydroxyl group of the open-chain structure. It has a cyclic structure simply because aldehyde and alcohol are part of the same molecule.
There are two isomeric forms of D-(+)-glucose because this cyclic structure has one more chiral centre than Fisher’s original open-chain structure. α-D-(+)-Glucose and β-D-(+)-glucose are diastereomers, differing in configuration about C-1.
Such a pair of distereomers are called anomers.
As hemiacetals, α-and β-D-(+)- glucose are readily hydrolyzed by water. In aqueous solution either anomer is converted –via the open-chain form–into an equilibrium mixture containing both cyclic isomers. This mutarotation results from the ready opening and closing of the hemiacetal ring.
The typical aldehyde reactions of D-(+)-glucose – osazone formation, and perhaps reduction of Tollens’
and Fehling’s reagents– are presumably due to a small amount of open-chain compound, which is replenished as fast as it is consumed. The concentration of this open-chain structure, however, is too low (less than 0.5%) for certain easily reversible aldehyde reactions like bisulfite addition and the Schiff test.
Disaccharides :
Disaccharides are carbohydrates that are made up of two monosaccharide units. On hydrolysis a molecule of disaccharide yields two molecules of monosaccharide.
We shall study four disaccharides : (+)-maltose (malt sugar), (+)-cellobiose, (+)-lactose (milk sugar), and (+)-sucrose (cane or beet sugar).
(+)-Maltose :
(+)-Maltose can be obtained, among other products, by partial hydrolysis of starch in aqueous acid. (+)-Maltose is also formed in one stage of the fermentation of starch to ethyl alcohol; here hydrolysis is catalyzed by the enzyme diastase, which is present in malt (sprouted barley).
Let us look at some of the facts from which the structure of (+)-maltose has been deduced.
(+)-Maltose has the molecular formula C12H22O11. It reduces Tollens’ and Fehling’s reagents and hence is a reducing sugar. It reacts with phenylhydrazine to yield an osazone, C12H20O9(=NNHC6H5)2. It is oxidized by bromide water to a monocarboxylic acid, (C11H21O10)COOH, maltobionic acid. (+)-Maltose exists in alpha ([α] = + 168º) and beta ([α] = + 112º) forms which undergo mutarotation in solution (equilibrium [α] = + 136º).
(+)-Cellobiose :
When cellulose (cotton fibers) is treated for several days with sulfuric acid and acetic anhydride, a combination of acetylation and hydrolysis takes place; there is obtained the octaacetate of (+)-cellobiose. Alkaline hydrolysis of the octaacetate yields (+)-cellobiose itself.
Like (+)-maltose, (+)-cellobiose has the molecular formula C12H22O11, is a reducing sugar, forms an osazone, exists in alpha and beta forms that undergo mutarotation, and can be hydrolyzed to two molecules of D-(+)-glucose. The sequence of oxidation, methylation, and hydrolysis (as described for (+)-maltose) shows that (+)-cellobiose contains two pyranose rings and glucoside linkage to an –OH group on C–4.
(+)-Cellobiose differs from (+)-maltose in one respect : it is hydrolyzed by the enzyme emulsin (from bitter almonds), not by maltase. Since emulsin is known to hydrolyze only β-glucoside linkages.
(+)-Lactose :
(+)-Lactose makes up about 5% of human milk and of cow’s milk. It is obtained commercially as a by-product of cheese manufacture, being found in the whey, the aqueous solution that remains after the milk proteins have been coagulated. Milk sours when lactose is converted into lactic acid (sour, like all acids) by bacterial action (e.g., by Lactobacillus bulgaricus).
XtraEdge for IIT-JEE 35 JANUARY 2011 (+)-Lactose has the molecular formula C12H22O11, is a
reducing sugar, forms an osazone, and exists in alpha and beta forms which undergo mutarotation. Acidic hydrolysis or treatment with emulsin (which splits β linkages only) converts (+)-lactose into equal amounts of D-glucose and D-galactose. (+)-Lactose is evidently a β-glycoside formed by the union of a molecule of D-(+)glucose and a molecule of D-(+)-galactose.
(+)-Sucrose :
(+)-Sucrose is our common table sugar, obtained from sugar cane and sugar beets. Of organic chemicals, it is the one produced in the largest amount in pure form.
(+)-Sucrose has the molecular formula C12H22O11. It does not reduce Tollen’s or Fehling’s reagent. It is a non-reducing sugar, and in this respect it differs from the other disaccharides we have studied. Moreover, (+)-sucrose does not form an osazone, does not exist in anomeric forms, and does not show mutarotation in solution. All these facts indicate that (+)-sucrose does not contain a “free”aldehyde or ketone group.
(+)-Sucrose is made up of a glucose unit and a D-fructose unit; since there is no “free” carbonyl group, if must be both a D-glucoside and a D-fructoside.
Polysaccharides :
Polysaccharides are compounds made up of many-hundreds or even thousands-monosaccharide units per molecule.
Polysaccharides are naturally occurring polymers, which can be considered as derived from aldoses or ketoses by polymerization with loss of water. A polysaccharide derived from hexoses, for example, has the general formula (C6H10O5)n.
The most important polysaccharides are cellulose and starch. Both are produced in plants from carbon dioxide and water by the process of photosynthesis.
Starch :
Starch occurs as granules whose size and shape are characteristic of the plant from which the starch is obtained. When intact, starch granules are insoluble in cold water; if the outer membrane has been broken by grinding, the granules swell in cold water and form a gel.
In general, starch contains about 20% of a soluble fraction called amylose, and 80% of a water-insoluble fraction called amylopectin. These two fractions appear to correspond to different carbohydrates of high molecular weight and formula (C6H10O5)n. Upon treatment with acid or under the
influence of enzymes, the components of starch are hydrolyzed progressively to dextrin (a mixture of low-molecular-weight polysaccharides), (+)-maltose, and finally D-(+)-glucose. (A mixture of all these is found in corn sirup, for example.) Both amylose and amylopectin are made up of D-(+)-glucose units, but differ in molecular size and shape.
Cellulose :
Cellulose is the chief component of wood and plant fibers; cotton, for instance, is nearly pure cellulose. It is insoluble in water and tasteless; it is a non-reducing carbohydrate. These properties, in part at least, are due to its extremely high molecular weight.
Cellulose has the formula (C6H10O5)n. Complete hydrolysis by acid yields D-(+)-glucose as the only monosaccharide. Hydrolysis of completely methylated cellulose gives a high yield of 2, 3, 6-tri-O-methyl-D-glucose. Like starch, therefore, cellulose is made up of chains of D-glucose units, each unit joined by a glycoside linkage to C–4 of the next.
Cellulose differs from starch, however, in the configuration of the glycoside linkage. Upon treatment with acetic anhydride and sulfuric acid, cellulose yields octa-O-acetylcellobiose.
Reactions of cellulose :
Like any alcohol, cellulose form esters. Treatment with a mixture of nitric and sulfuric acid converts cellulose into cellulose nitrate. The properties and uses of the product depend upon the extent of nitration.
In the presence of acetic anhydride, acetic acid, and a little sulfuric acid, cellulose is converted into the triacetate. Partial hydrolysis removes some of the acetate groups, degrades the chains to smaller fragments (of 200–300 units each), and yields the vastly important commercial cellulose acetate (roughly a diacetate). Cellulose acetate is less flammable than cellulose nitrate and has replaced the nitrate in many of its applications, in safety-type photographic film, for example. When a solution of cellulose acetate in acetone is forced through the fine holes of a spinnerette, the solvent evaporates and leaves solid filaments. Threads from these filaments make up the material known as acetate rayon.
Industrially, cellulose is alkylated to ethers by action of alkyl chlorides (cheaper than sulfates) in the presence of alkali. Considerable degradation of the long chains is unavoidable in these reactions. Methyl, ethyl, and benzyl ethers of cellulose are important in the production of textiles, films, and various plastic objects.
XtraEdge for IIT-JEE 36 JANUARY 2011