As carbohydrates is made up of elements that makes them a chemical compound, carbohydrates showed varying chemical properties. Such chemical properties represented by various carbohydrates could serve as the basis for various classifications like reducing and non-reducing sugars and also aldoses and ketoses.
Reactions involving Carbohydrates:
Sugars might be categorized as reducing or non-reducing based on their reactivity by Tollens', Benedict's or Fehling's reagents. Whenever a sugar is oxidized via these reagents it is termed as reducing, as the oxidant (Ag(+) or Cu(+2)) is reduced in the reaction, as evidenced via formation of a silver mirror or precipitation of cuprous oxide. The Tollens' test is generally employed to detect aldehyde functions; and due to the facile Interconversions of ketoses and aldoses under the fundamental conditions of this test, ketoses like fructose as well react and are categorized as reducing sugars.
If the aldehyde function of an aldose is oxidized to a carboxylic acid then the product is termed as an aldonic acid. Due to the 2º hydroxyl functions that are as well present in such compounds, a mild oxidizing agent like hypobromite should be employed for this conversion (equation i). If both the ends of an aldose chain are oxidized to carboxylic acids, then the product is known as an aldaric acid. By transforming an aldose to its corresponding aldaric acid derivative, the ends of the chain becomes similar (that is, this could as well be attained by reducing the aldehyde to CH2OH, as noted). Such an operation will reveal any latent symmetry in the remaining molecule. Therefore, ribose, allose, xylose and galactose yield achiral aldaric acids that are, obviously, not optically active.
The ribose oxidation is illustrated in equation (ii) below.
Fig: Oxidation reactions involving Carbohydrates
The other aldose sugars might give similar chiral aldaric acid products, implying a unique configurational relationship. The illustrations of arabinose and lyxose illustrated in equation (iii) above describe this result.
The Sodium borohydride reduction of an aldose forms the ends of the resultant alditol chain similar, HOCH2(CHOH)nCH2OH, thus achieving the similar configurational change produced via oxidation to an aldaric acid. Therefore, allitol and galactitol from reduction of allose and galactose are achiral, and altrose and talose are reduced to similar chiral alditol. A summary of such redox reactions and derivative nomenclature is represented in the table below.
Derivatives of HOCH2(CHOH)nCHO
HOBr Oxidation → HOCH2(CHOH)nCO2H
An Aldonic Acid
HNO3 Oxidation → H2OC(CHOH)nCO2H
An Aldaric Acid
NaBH4 Reduction → HOCH2(CHOH)nCH2OH
The osazone reaction was expanded and employed by Emil Fischer to recognize aldose sugars differing in configuration merely at the alpha-carbon. The upper equation illustrates the general form of the osazone reaction, which affects an alpha-carbon oxidation by the formation of a bis-phenylhydrazone, termed as an osazone. Application of the osazone reaction to D-glucose and D-mannose illustrate that these compounds vary in configuration only at C-2.
Fig: Osazone Formation
Chain Shortening and Lengthening:
These two methods permit an aldose of a given size to be associated to homologous smaller and larger aldoses. Therefore Ruff degradation of the pentose arabinose provides the tetrose erythrose. Working in the opposite direction, a Kiliani-Fischer synthesis applied to the arabinose provides a mixture of glucose and mannose. The alternative chain shortening method known as the Wohl degradation is necessarily the reverse of the Kiliani-Fischer synthesis.
Fig: Chain Shortening and Lengthening
Summary of these reactions:
A) Ribose and arabinose (that is, two well known pentoses) both gave erythrose on Ruff degradation. As expected, Kiliani-Fischer synthesis exerted to erythrose provides a mixture of ribose and arabinose.
B) Oxidation of erythrose provides an achiral (that is, optically inactive) aldaric acid. This states the configuration of erythrose.
C) Oxidation of ribose provides an achiral (that is, optically inactive) aldaric acid. This states the configuration of both ribose and arabinose.
D) Ruff shortening of glucose provides arabinose and Kiliani-Fischer synthesis applied to the arabinose provides a mixture of glucose and mannose.
E) Glucose and mannose are thus epimers at C-2, a fact verified via the common product from their osazone reactions.
F) A pair of structures for such epimers can be written, however which is glucose and which is mannose?
In order to find out which of such epimers was glucose, Fischer made utilization of the inherent C2 symmetry in the four-carbon dissymmetric core of one epimer (B). This is illustrated in the diagram shown by a red dot where the symmetry axis passes via the projection formula. Due to this symmetry, if the aldehyde and 1º-alcohol functions at the ends of the chain are exchanged, epimer B would be unchanged; while A would be transformed to a different compound.
Fig: Four-carbon Dissymmetric core
The Fischer examined for and discovered a second aldohexose which represented the end group exchange for the epimer lacking the latent C2 symmetry (A). This compound was L-(+)-gulose, and its exchange relationship to D-(+)-glucose was illustrated by oxidation to a common aldaric acid product. The remaining epimer is thus mannose.
Formation of Glycosides:
Acetal derivatives prepared whenever a monosaccharide reacts by an alcohol in the presence of an acid catalyst are termed as glycosides. This reaction is described for glucose and methanol in the diagram shown below. In the naming of glycosides, the 'ose' suffix of the sugar name is substituted by 'oside', and the alcohol group name is positioned first. As is usually true for most acetals, glycoside formation comprises the loss of an equivalent of water. The diether product is stable to base and alkaline oxidants like Tollen's reagent. As acid-catalyzed aldolization is reversible, glycosides might be hydrolyzed back to their alcohol and sugar components via aqueous acid.
The anomeric methyl glucosides are prepared in an equilibrium ratio of around 66% alpha to 34% beta. From the structures in the prior diagram, we observe that pyranose rings prefer chair conformations in which the largest number of substituents is equatorial. In case of glucose, the substituents on the beta-anomer are all equatorial, while the C-1 substituent in the alpha-anomer modifies to axial. As substituents on cyclohexane rings prefer an equatorial position over axial (methoxycyclohexane is 75% equatorial), the choice for alpha-glycopyranoside preparation is unexpected, and is termed to as the anomeric effect.
Fig: Formation of Glycosides
The Glycosides abound in biological systems. By attaching a sugar moiety to the lipid or benzenoid structure, the solubility and other properties of the compound might be modified substantially. Due to the significant modifying affect of such derivatization, plentiful enzyme systems, termed as glycosidases, have evolved for the attachment and elimination of sugars from alcohols, phenols and amines. Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol component as the aglycon.
Two illustrations of naturally occurring glycosides and one illustration of an amino derivative are presented. Salicin, one of the oldest herbal remedies known, was the model for the synthetic analgesic aspirin. Large classes of hydroxylated, aromatic oxonium cations termed as anthocyanins give the red, purple and blue colors of numerous flowers, fruits and some vegetables. Peonin is one illustration of this class of natural pigments that show pronounced pH color dependence. The oxonium moiety is merely stable in acidic atmospheres and the color changes or disappears whenever base is added. The complex changes which take place if wine is fermented and stored are in part related with glycosides of anthocyanins. Ultimately, amino derivatives of ribose, like cytidine play significant roles in the biological phosphorylating agents, coenzymes and information transport and the storage materials.
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