Hydrogenation carbohydrates

As recently reviewed by Abbadi and van Bekkum,[51] the most important carbohydrate hydrogenation reaction is the transformation of glucose into sorbitol (Scheme 7.4). The world production of sorbitol is around 650 000 tons year-1.[52] Sorbitol is used in many fields, pharmaceuticals, foods, cosmetics, chemical industry, and is the starting material for the preparation of vitamin C. 

The crystal structure of 5-chloro-2-deoxyuridine [CLDOUR] has a very simple pattern with two infinite chains (Fig. 17.21). One is through the O -H groups, and the other through the minor component of a three-center bond formed by 0(30-H. It strongly resembles a carbohydrate hydrogen-bonding pattern. A very simple scheme is also formed by 2-chloro-2 -deoxyuridine [CDURID] with a short finite chain and a separate link from the primary alcohol (Fig. 17.22). 

Industrially the most important carbohydrate hydrogenation product is sorbitol (o-glucitol), which is obtained by reduction of the carbonyl group of o-glucose [4-7]. Worldwide production is estimated to be 650 000 tons p. a. Sorbitol is used in numerous cosmetic, food and drinks formulations, and as a starting material e. g. in the manufacture of ascorbic acid (vitamin C). 

Fig. 15 Modified van Krevelen plot for cellulose hydrolysis, carbohydrate hydrogenation and...

Carbohydrate Force Fields Carbohydrates Hydrogen Bonding 2 Molecular Dynamics DNA Molecular Dynamics Simulations of Nucleic Acids Molecular Dynamics Techniques and Applications to Proteins Solvation Modeling Water Clusters. 

Crystallizes from water in large colourless prisms containing 2H2O. It is poisonous, causing paralysis of the nervous system m.p. 101 C (hydrate), 189°C (anhydrous), sublimes 157°C. It occurs as the free acid in beet leaves, and as potassium hydrogen oxalate in wood sorrel and rhubarb. Commercially, oxalic acid is made from sodium methanoate. This is obtained from anhydrous NaOH with CO at 150-200°C and 7-10 atm. At lower pressure sodium oxalate formed from the sodium salt the acid is readily liberated by sulphuric acid. Oxalic acid is also obtained as a by-product in the manufacture of citric acid and by the oxidation of carbohydrates with nitric acid in presence of V2O5. 

A commonplace variation on the general pattern seen m carbohydrate structure is the replacement of one or more of the hydroxyl substituents by some other atom or group In deoxy sugars the hydroxyl group is replaced by hydrogen Two examples of deoxy sugars are 2 deoxy d nbose and l rhamnose... 

The carbonyl group of carbohydrates can be reduced to an alcohol function Typi cal procedures include catalytic hydrogenation and sodium borohydnde reduction Lithium aluminum hydride is not suitable because it is not compatible with the solvents (water alcohols) that are required to dissolve carbohydrates The products of carbohydrate reduc tion are called alditols Because these alditols lack a carbonyl group they are of course incapable of forming cyclic hemiacetals and exist exclusively m noncyclic forms... 

Section 28 8 The most common form of DNA is B DNA which exists as a right handed double helix The carbohydrate-phosphate backbone lies on the outside the punne and pyrimidine bases on the inside The double helix IS stabilized by complementary hydrogen bonding (base pairing) between adenine (A) and thymine (T) and guanine (G) and cytosine (C)... 

Branched chain carbohydrate (Section 25 12) Carbohydrate in which the main carbon chain bears a carbon substituent in place of a hydrogen or hydroxyl group Bromohydrin (Section 6 17) A halohydnn in which the halo gen IS bromine (see halohydnn)... 

Deoxy sugar (Section 25 10) A carbohydrate in which one of the hydroxyl groups has been replaced by a hydrogen... 

The earliest examples of analytical methods based on chemical kinetics, which date from the late nineteenth century, took advantage of the catalytic activity of enzymes. Typically, the enzyme was added to a solution containing a suitable substrate, and the reaction between the two was monitored for a fixed time. The enzyme s activity was determined by measuring the amount of substrate that had reacted. Enzymes also were used in procedures for the quantitative analysis of hydrogen peroxide and carbohydrates. The application of catalytic reactions continued in the first half of the twentieth century, and developments included the use of nonenzymatic catalysts, noncatalytic reactions, and differences in reaction rates when analyzing samples with several analytes. 

Other potential synthetic methods include fermentation (qv) of certain carbohydrates (qv), oxidation of propane, hydrogenation of acetone, and hydrolysis of isopropyl acetate. The hydrogenation of by-product acetone is the only method practiced commercially. 

Sugar analysis by hplc has advanced greatly as a result of the development of columns specifically designed for carbohydrate separation. These columns fall into several categories. (/) Aminopropyl-bonded siHca used in reverse-phase mode with acetonitrile—water as the eluent. (2) Ion-moderated cation-exchange resins using water as the eluent. Efficiency of these columns is enhanced at elevated temperature, ca 80—90°C. Calcium is the usual counterion for carbohydrate analysis, but lead, silver, hydrogen, sodium, and potassium are used to confer specific selectivities for mono-, di-, and... 

Modification of the Carbohydrate Ring Substituents. Replacement of hncomycin s 2-hydroxy group by methoxy or by hydrogen (74), or inversion of the configuration of the hydroxy group at C-4 or at C-2 destroys activity (75). 

The basic carbohydrate molecule possesses an aldehyde or ketone group and a hydroxyl group on every carbon atom except the one involved in the carbonyl group. As a result, carbohydrates are defined as aldehyde or ketone derivatives of polyhydroxy alcohols and their reaction products. A look at the formula for glucose shows that it contains hydrogen and oxygen atoms in the ratio in which they are found in water. The name carbohydrate... 

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