Chemistry
Reactions of Monosaccharides
Monosaccharides undergo various reactions, including oxidation, reduction, and glycosidic bond formation. Oxidation of monosaccharides can lead to the formation of sugar acids or sugar alcohols, while reduction can yield sugar alcohols. Glycosidic bond formation involves the linkage of monosaccharides to form disaccharides or polysaccharides. These reactions play a crucial role in the synthesis and breakdown of carbohydrates.
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9 Key excerpts on "Reactions of Monosaccharides"
eBook - ePub- Robert V. Stick, Spencer Williams(Authors)
- 2010(Publication Date)
- Elsevier Science(Publisher)
1 ]So far, most of the chemical transformations of a monosaccharide discussed in this book have been concerned with the concept of protecting groups. In general, the order of reactivity of the various hydroxyl groups of d-glucopyranose was observed to be O1 (hemiacetal) > O6 (primary) > O2 (adjacent to C1 and therefore more acidic) > O3 > O4; this order, of course, is dictated very much by the nature of the substrate and the reagent, as well as whether an O–H or a C–O bond is being broken. Now, a host of chemical reactions will be discussed that cause functional group transformations within monosaccharides, either for the purpose of elaboration into oligosaccharides or for the synthesis of a derivative of the monosaccharide. Such derivatives include the deoxy and amino deoxy sugars.By the nature of the subject, this discussion will be neither exhaustive nor all-inclusive. It will, however, highlight the methods that work well in synthetic carbohydrate chemistry, particularly those that have evolved over the last few decades.Oxidation
[2–6 ]We have already seen the importance of oxidation to the German chemists of the nineteenth century. Aldoses were oxidized to aldonic acids using bromine water and to aldaric acids using dilute nitric acid; these reagents are still used today.The major advances have been concerned with the discovery or ‘invention’, as some have called it,[a ]of chemoselective reagents for oxidation of the various hydroxyl groups within a carbohydrate. The well-established Jones’ (CrO3 , H2 SO4 )[8 ]and Collins’ (CrO3 , pyridine)[9 ]reagents soon gave way to milder and more easily handled oxidizing agents, namely pyridinium chlorochromate[10 ]and pyridinium dichromate.[11,12
eBook - ePubBiochemistry
An Integrative Approach with Expanded Topics
- John T. Tansey(Author)
- 2022(Publication Date)
- Wiley(Publisher)
G ° values close to zero can be tipped to move in either direction by changing the concentrations of substrates or products.• The synthesis of glucose and the degradation of monosaccharides use similar reactions but different pathways. Key steps are different, and compartmentalization is important. A similar theme is seen in fatty acid metabolism.6.1 Properties, Nomenclature, and Biological Functions of MonosaccharidesCarbohydrates are a group of biological molecules with the basic formula Cx (H2 O)x , hence the name “carbo-hydrate.” At their most basic level, carbohydrates have the equivalent of one molecule of water for every carbon atom. This means that carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, that is, molecules in which one carbon bears a carbonyl group (C=O) and the others all carry hydroxyls ( −OH) . They also contain at least three carbons. We will first discuss the basic structures of carbohydrates and different ways we can represent these molecules on paper, and then we will move into modifications of these molecules.6.1.1 Monosaccharides are the simplest carbohydratesThis section focuses on the simplest carbohydrates, the monosaccharides . Monosaccharides can be linked together to form more complex carbohydrates. Monosaccharides all end in the suffix -ose to designate them as carbohydrates. They can be classified based by the number of carbons in the molecule, the presence of an aldehyde or a ketone, and the stereochemistry at the penultimate carbon (the chiral center farthest from the carbonyl). In terms of size, monosaccharides contain three to nine carbons but most contain five, six, or seven carbons (Table 6.1 ). The reasons for this are rooted in organic chemistry. A numbering system can be combined with the “-ose” suffix to provide a generic name for a group of carbohydrates; for example, five-carbon monosaccharides are pentoses and six-carbon monosaccharides are hexoses . Thanks to the carbonyl moiety, a monosaccharide contains either an aldehyde or a ketone and can therefore be broadly categorized as aldoses or ketoses
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
OH HO OH O HO HO O HO HO OH HO OH 22.4 Reactions of Monosaccharides LEARNING OBJECTIVE 22.4 Describe the chemical Reactions of Monosaccharides. In this section, we discuss the Reactions of Monosaccharides with alcohols, reducing agents and oxidising agents. We also examine how these reactions are useful in our everyday lives. Formation of glycosides (acetals) As we saw in the chapter on aldehydes and ketones, treating one of those compounds with a molecule of an alcohol yields a hemiacetal, and treating the hemiacetal with a molecule of an alcohol yields an acetal. Treating a monosaccharide, all of which exist as cyclic hemiacetals, with an alcohol gives an acetal, as illustrated by the reaction of methanol with -d-glucopyranose ( -d-glucose) drawn in the Haworth projection to emphasise the differences in the structures of the two products. anomeric carbon + CH 3 OH –H 2 O H + H H H H H OH OH OH OH CH 2 OH O β-d-glucopyranose (β-d-glucose) glycosidic bond + H H H H H OH OH OCH 3 OH CH 2 OH O glycosidic bond H H OCH 3 H H OH OH H OH CH 2 OH O methyl-β-d-glucopyranoside (methyl-β-d-glucoside) methyl-α-d-glucopyranoside (methyl-α-d-glucoside) CHAPTER 22 Carbohydrates 1161 A cyclic acetal derived from a monosaccharide is called a glycoside, and the bond from the anomeric carbon to the OR group is called a glycosidic bond. Unlike a hemiacetal, an acetal is no longer in equilibrium with the open-chain carbonyl-containing compound in neutral or alkaline solutions. Like other acetals, glycosides are stable in water and aqueous base, but undergo hydrolysis in aqueous acid to form an alcohol and a monosaccharide. We name glycosides by listing the alkyl or aryl group bonded to oxygen, followed by the name of the carbohydrate with the ending -e replaced with -ide. For example, glycosides derived from -d-glucopyranose are named -d-glucopyranosides; and those derived from -d-ribofuranose are named -d-ribofuranosides.
eBook - PDFFood Carbohydrates
Chemistry, Physical Properties, and Applications
- Steve W. Cui(Author)
- 2005(Publication Date)
- CRC Press(Publisher)
27 1.3.4 Oligosaccharides .............................................................................. 28 1.3.5 Cyclic Oligosaccharides ................................................................. 33 1.4 Reaction of Monosaccharides and Derived Carbohydrate Structures ...................................................................................................... 36 1.4.1 Oxidation and Reduction Reactions ............................................ 36 1.4.2 Deoxy and Amino Sugars .............................................................. 39 1.4.3 Sugar Esters and Ethers ................................................................. 41 1.4.4 Glycosides ......................................................................................... 44 1.4.5 Browning Reactions ........................................................................ 44 1.4.5.1 Maillard Reaction .............................................................. 44 1.4.5.2 Caramelization .................................................................. 47 2 Food Carbohydrates: Chemistry, Physical Properties, and Applications 1.5 Polysaccharides ............................................................................................ 51 1.5.1 General Structures and Classifications ........................................ 51 1.5.2 Factors Affecting Extractability and Solubility of Polysaccharides ................................................................................ 54 1.5.3 Extraction of Polysaccharides ....................................................... 56 1.5.4 Purification and Fractionation of Polysaccharides .................... 61 1.5.5 Criteria of Purity ............................................................................. 63 References ...............................................................................................................
eBook - ePub- Raymond S. Ochs(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
Disaccharides – or any sugar polymer – in our diet must be hydrolyzed to monosaccharides prior to entering the cells lining the small intestine. The reaction is catalyzed by enzymes positioned extracellularly and specific to each digestible disaccharide: i.e., sucrase is required to hydrolyze the glycosidic bond of sucrose, and lactase is required to hydrolyze that of lactose. In some cases, this process is impaired, leading to digestive disorders (Box 4.3).In the disaccharides of Figure 4.9 , one of the sugar units is glucose; the other is glucose, galactose, or fructose. When just one anomeric carbon is involved in glycoside formation, as in maltose and lactose, a free anomeric carbon remains in equilibrium with an open-chain form (see Figure 4.10 for maltose). Even a small amount of this open-chain form in solution allows the carbonyl group to cause the reduction of metal ions in diagnostic tests. The positive result in this experimental test indicates the presence of a reducing sugar. Thus, all monosaccharides are reducing sugars; of the disaccharides displayed in Figure 4.9 , maltose and lactose are reducing sugars, while sucrose and trehalose are nonreducing sugars. An exploration of the term reducing is presented in Box 4.4.4.4 Polysaccharides
When more than two sugars are linked via glycosidic bonds, they are said to be either oligosaccharides (meaning “a few”) or polysaccharides (meaning “many”). The distinction is inexact: some consider chains of up to ten linked sugars to be oligosaccharides. Commonly, polysaccharides have thousands of monosaccharide residues bound together. Such molecules have extremely large molecular weights and have new properties not found in smaller molecules. In general, large molecules that are constructed from small units are called polymers
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
976 CHAPTER 22 CARBOHYDRATES 22.5A Enolization, Tautomerization, and Isomerization Dissolving monosaccharides in aqueous base causes them to undergo a series of enolizations and keto–enol tautomerizations that lead to isomerizations. For example, if a solution of d-glucose containing calcium hydroxide is allowed to stand for several days, a number of products can be isolated, including d-fructose and d-mannose (Fig. 22.5). This type of reaction is called the Lobry de Bruyn–Alberda van Ekenstein transformation after the two Dutch chemists who discovered it in 1895. When carrying out reactions with monosaccharides, it is usually important to prevent these isomerizations and thereby to preserve the stereochemistry at all of the chirality centers. One way to do this is to convert the monosaccharide to the methyl glycoside first. We can then safely carry out reactions in basic media because the aldehyde group has been converted to an acetal and acetals are stable in aqueous base. Preparation of the methyl glycoside serves to “protect” the monosaccharide from undesired reactions that could occur with the anomeric carbon in its hemiacetal form. PRACTICE PROBLEM 22.4 (a) What products would be formed if salicin were treated with dilute aqueous HCl? (b) Outline a mechanism for the reactions involved in their formation. PRACTICE PROBLEM 22.5 How would you convert d-glucose to a mixture of ethyl α-d-glucopyranoside and ethyl β-d-glucopyranoside? Show all steps in the mechanism for their formation.
eBook - PDFBiochemistry
An Integrative Approach
- John T. Tansey(Author)
- 2019(Publication Date)
- Wiley(Publisher)
6.2 Summary • Monosaccharides can be joined to form disaccharides, trisaccharides, oligosaccharides, and polysac- charides. • The acetal or ketal linkage formed between monosaccharide units can be described based on the positions of the carbons involved in the bond. • Polysaccharides often serve as stores of energy or provide structure for an organism. • The properties of complex carbohydrates are based on the monomeric units (glucose or galactose) and the connection between those units (β-1,4). 6.2 Concept Check • Name and describe at the chemical level several different examples of disaccharides, trisaccharides, oligosaccharides, and polysaccharides. • Describe the biochemical function of these molecules. • Explain how the structure of these molecules contributes to their function. • Explain how these molecules are similar to one another or to other biological molecules, and how they are different. 6.3 Glycolysis and an Introduction to Metabolic Pathways This section discusses the breakdown of glucose into pyruvate, a process called glycolysis. In this pathway, as in any metabolic pathway, a small molecule (a metabolite) is transformed into a different molecule through a series of enzymatic reactions (Figure 6.10). This section describes what is meant by a metabolic pathway and some general themes to look for in any pathway, as well as how to apply this information to a discussion of glycolysis. Because glycolysis is a catabolic pathway, one that breaks molecules down, the process is oxidative, and thus the final products are more oxidized than the starting materials. This Having numerous negative charges, alginate will react with cations, particularly Ca 2+ ions. The strands of alginate become linked, resulting in the formation of a gel. The food and beverage industry is now creating pearls or spheres of food or drink encased in a thin layer of alginate: a novel application of polysaccharide chemistry.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
24.6 Reactions of Monosaccharides 1169 CONCEPTUAL CHECKPOINT 24.25 When α-D-galactopyranose is treated with ethanol in the pres- ence of an acid catalyst, such as HCl, two products are formed. Draw both products and account for their formation with a mechanism. 24.26 Methyl α-D-glucopyranoside is a stable compound that does not undergo mutarotation under neutral or basic conditions. However, when subjected to acidic conditions, an equilibrium is established consisting of both methyl α-D-glucopyranoside and methyl β-D-glucopyranoside. Draw a mechanism that accounts for this observation. Epimerization When d-glucose is exposed to strongly basic conditions, it is converted into a mixture containing both d-glucose and d-mannose via the following process: An enediol D-Glucose D-Mannose C OH H HO OH H OH H H H O CH 2 OH C H H HO OH H OH H HO H O CH 2 OH C OH H HO OH H OH H CH 2 OH H H O D-Glucose NaOH, H 2 O NaOH, H 2 O C C OH H HO OH H OH H H OH CH 2 OH + d-Glucose first undergoes base-catalyzed tautomerization to form an enediol. This intermediate can undergo tautomerization once again to revert back to the aldose, but in the process, the configuration at C2 is lost, leading to a mixture of d-glucose and d-mannose. d-Glucose and d-mannose are said to be epimers because they are diastereomers that differ from each other in the configuration of only one chiral center. When either pure d-glucose or pure d-mannose is treated with a strong base, epimeriza- tion occurs, giving a mixture containing both d-glucose and d-mannose. For this reason, chemists generally avoid exposing carbohydrates to strongly basic conditions. CONCEPTUAL CHECKPOINT 24.27 Draw and name the structure of the aldohexose that is epimeric with D-glucose at each of the following positions: (a) C2 (b) C3 (c) C4 Reduction of Monosaccharides The carbonyl group of an aldose or ketose can be reduced upon treatment with sodium borohydride to yield a product called an alditol.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
1 7. 5 What Are Disaccharides and Oligosaccharides? 577 D. Oxidation to Uronic Acids Enzyme‐catalyzed oxidation of the primary alcohol at carbon 6 of a hexose yields a uronic acid. Enzyme‐catalyzed oxidation of d‐glucose, for example, yields d‐glucuronic acid, shown here in both its open‐chain and cyclic hemiacetal forms: O COOH HO HO OH OH Chair conformation Fischer projection D-Glucose D-Glucuronic acid (a uronic acid) enzyme-catalyzed oxidation CHO H OH HO H H OH H OH CH 2 OH CHO H OH HO H H OH H OH COOH d‐Glucuronic acid is widely distributed in both the plant and animal worlds. In humans, it is an important component of the acidic polysaccharides of connective tis- sues. The body also uses it to detoxify foreign phenols and alcohols. In the liver, these compounds are converted to glycosides of glucuronic acid (glucuronides), to be excreted in the urine. The intravenous anesthetic propofol (Problem 10.43), for exam- ple, is converted to the following water‐soluble glucuronide and then excreted in the urine: Propofol A urine-soluble glucuronide HO O COO ‒ HO HO OH O 17.5 What Are Disaccharides and Oligosaccharides? Most carbohydrates in nature contain more than one monosaccharide unit. Those that contain two units are called disaccharides, those that contain three units are called trisaccharides, and so forth. The more general term, oligosaccharide, is often used for carbohydrates that contain from 6 to 10 monosaccharide units. Carbohydrates containing larger numbers of monosaccharide units are called polysaccharides (Section 17.6). In a disaccharide, two monosaccharide units are joined by a glycosidic bond between the anomeric carbon of one unit and an OH of the other. Sucrose, lactose, and maltose are three important disaccharides. A. Sucrose Sucrose (table sugar) is the most abundant disaccharide in the biological world. It is obtained principally from the juice of sugarcane and sugar beets.
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