Dehydration carbohydrates

The wood pyrolysis is attractive because forest and industrial wood residues can be readily converted into liqtrid products. These liqtrids, as erode bio-oil or slurry of charcoal of water or oil, have advantages in transport, storage, combustion, retrofitting and flexibility in production and marketing (Demirbas, 2007). In the first step of pyrolysis of carbohydrates dehydration occtrrs and at low temperatures dehydration predominates. Dehydration is also known as a char-forming reaction. Between 550 and 675 K volatile products, tar, and char are formed. The volatile products are CO, CO, H O, acetals, furfural, aldehydes and ketones. Levoglucosan is the principle component in tar. 

Keywords Carbohydrates, Dehydration, Heterogeneous catalysis. Hydrolysis, Oxidation... 

A third class of solids, which were tested as catalysts for carbohydrate dehydrations are inorganic phosphates. In a comparative study including vanadyl phosphate (VOPO4 2H2O) and partially metal-substituted vanadyl phosphates xPC>4 2H2O, M being Cr, Mn, Fe, A1 and Ga), it was... 

Aniline acetate reaction The solid sample is heated over a flame, and the vapors are allowed to react with aniline acetate impregnated on a filter paper placed on the vapors. A red color will appear in presence of heterocyclic aldehydes produced from carbohydrate dehydration. 

Dehydration is one of the important acid-base-catalyzed reactions of carbohydrates. In alkaline media, a deoxyaldosulose is formed from a carbohydrate molecule after one molecule of water has been split oflF this product then usually undergoes an intramolecular, oxidoreductive disproportionation to give the corresponding saccharinic acid in acidic media (with pentoses and hexoses), up to three molecules of water are split oE per molecule, with formation of 2-furaldehyde or 5-(hydroxymethyl )-2-furaldehyde. In many cases, polarography makes possible the monitoring of carbohydrate dehydration and the determination of its final products. 

This reaction follows the same theoretical principles in which there is formation of furfural from hexoses and hydroxy-methyl-furfural (HMF) and from aldopentoses by add dehydration (Fig. 1). These two products, singly are colorless, however, it is necessary a phenolic compoimd addition in the medium to develop a colored compound, in this case redness. This technique is firstly mentioned by Roe (1934), with some posterior modifications by Roe et al. (1949), becoming a quick reaction and with stable color. The reaction uses the hydrochloride acid (HCl) for carbohydrates dehydration and the resorcinol is the phenolic compoimd that reacts with furfural and HMF. 

Zhang Y, Degirmenci V, Li C, Hensen ElM. Phosphotungstic acid encapsulated in metalorganic framework as catalysts for carbohydrate dehydration to 5-hydroxymethylfurfural. Chem Sus Chem 2011 4 59-64. 

Molisch s Test. Dissolve about 01 g. of the carbohydrate in z ml. of water (for starch use 2 ml. of starch solution ), add 2-3 drops of a 1 % alcoholic solution of i-naphthol (ignoring traces of the latter precipitated by the water) and then carefully pour 2 ml. of cone. H2SO4 down the side of the test-tube so that it forms a heavy layer at the bottom. A deep violet coloration is produced where the liquids meet. This coloration is due apparently to the formation of an unstable condensation product of i-naphthol with furfural (an aldehyde produced by the dehydration of the carbohydrate). 

The principal steps in the mechanism of polyisoprene formation in plants are known and should help to improve the natural production of hydrocarbons. Mevalonic acid, a key intermediate derived from plant carbohydrate via acetylcoen2yme A, is transformed into isopentenyl pyrophosphate (IPP) via phosphorylation, dehydration, and decarboxylation (see Alkaloids). IPP then rearranges to dimethylaHyl pyrophosphate (DMAPP). DMAPP and... 

In the acid hydrolysis process (79—81), wood is treated with concentrated or dilute acid solution to produce a lignin-rich residue and a Hquor containing sugars, organic acids, furfural, and other chemicals. The process is adaptable to all species and all forms of wood waste. The Hquor can be concentrated to a molasses for animal feed (82), used as a substrate for fermentation to ethanol or yeast (82), or dehydrated to furfural and levulinic acid (83—86). Attempts have been made to obtain marketable products from the lignin residue (87) rather than using it as a fuel, but currently only carbohydrate-derived products appear practical. 

Beet juice contains about 80% of fermentable carbohydrates and nitrogenous compounds. To remove these compounds, a yeast fermentation utilising Candida utillis has been suggested (141). By so doiag, a more concentrated form of the dye becomes available. The red dye from beets is sold as beet juice concentrate, as dehydrated beet root, and as a dried powder. 

FIGURE 15.17 Sulfuric acid is a dehydrating agent. When concentrated sulfuric acid is poured on to sucrose (a), the sucrose, a carbohydrate, is dehydrated (b), leaving a frothy black mass of carbon (c). 

Madin, K.A.C. Crowe, J.H. (1975). Anhydrobiosis in nematodes carbohydrate and lipid metabolism during dehydration. Journal of Experimental Zoology, 193, 335-41. 

Harris, J.F. "Acid Hydrolysis and Dehydration Reactions for Utilizing Plant Carbohydrates," Applied Polymer Symposium. 1975, 28, 131-144. 

Mucopolysaccharides (See section IV). In general these compounds have a low but significant protein content and give reactions which are predominantly carbohydrate. Their aqueous solutions show a high viscosity and they tend to become insoluble in water after intensive dehydration. 

Heterogeneous catalysts, particularly zeolites, have been found suitable for performing transformations of biomass carbohydrates for the production of fine and specialty chemicals.123 From these catalytic routes, the hydrolysis of abundant biomass saccharides, such as cellulose or sucrose, is of particular interest. The latter disaccharide constitutes one of the main renewable raw materials employed for the production of biobased products, notably food additives and pharmaceuticals.124 Hydrolysis of sucrose leads to a 1 1 mixture of glucose and fructose, termed invert sugar and, depending on the reaction conditions, the subsequent formation of 5-hydroxymethylfurfural (HMF) as a by-product resulting from dehydration of fructose. HMF is a versatile intermediate used in industry, and can be derivatized to yield a number of polymerizable furanoid monomers. In particular, HMF has been used in the manufacture of special phenolic resins.125... 

M. Kurszewska, E. Skorupowa, J. Madaj, A. Konitz, W. Wojnowski, and A. Wisniewski, The solvent-free thermal dehydration of hexitols on zeolites, Carbohydr. Res., 337 (2002) 1261-1268. 

L. D. S. Yadav, C. Awasthi, V. K. Rai, and A. Rai, A route to functionalized pyrimidines from carbohydrates via amine-driven dehydrative ring transformations, Tetrahedron Lett., 49 (2008) 2377-2380. 

An attractive feature of this dehydrative coupling approach is that it avoids the need for isolation of intermediate glycosyl donors. This can be desirable if a glycosyl donor is not stable to isolation or purification. Moreover, the use of a hemiacetal donor reduces the number of synthetic manipulations of the carbohydrate donor by avoiding hemiacetal derivatization to alternative donor types. In this way, the approach has the potential to streamline time and labor-intensive multiglycosylation sequences. Although there increasingly have been reports of these direct dehydrative... 

Furan Derivatives Catalytic processes used to obtain furan derivatives from carbohydrates and the catalytic routes from furan intermediates to chemicals and polymers have been reviewed by Moreau et al. [27]. Some of the main reactions are summarized in Fig. 3.2. From fructose or carbohydrates based on fructose (sucrose, inulin), the first transformation step is dehydration to 5-hydroxy methylfur-fural (HMF). Fructose dehydration at 165 °C was performed in the presence of... 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

Concentrated sulphuric acid Concentrated acid Spray Dehydration of carbohydrates... 

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