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Organic Chemistry
Table of Contents
Unit 25 Overview: Biomolecules
25.1 Classification of Carbohydrates
25.2 Representing Carbohydrate Stereochemistry: Fischer Projections
25.3 D,L Sugars
25.4 Configurations of the Aldoses
25.5 Cyclic Structures of Monosaccharides: Anomers
25.6 Reactions of Monosaccharides
25.7 The Eight Essential Monosaccharides
25.8 Disaccharides
25.9 Polysaccharides and Their Synthesis
25.10 Some Other Important Carbohydrates

Monosaccharides are versatile molecules that undergo various reactions. They can form esters and ethers, create glycosidic bonds, and participate in oxidation and reduction processes. These reactions are crucial for energy storage, cell signaling, and structural roles in living organisms.

Understanding the structural representations of monosaccharides is key to grasping their behavior. Fischer and Haworth projections help visualize these molecules, while processes like mutarotation and epimerization showcase their dynamic nature in solution.

Reactions of Monosaccharides

Formation of esters and ethers

  • Esterification of monosaccharides occurs when a monosaccharide reacts with an alcohol or carboxylic acid
    • Hydroxyl groups of the monosaccharide react to form ester linkages (glucose reacts with acetic acid to form glucose pentaacetate)
  • Etherification of monosaccharides happens when a monosaccharide reacts with an alkyl halide or alcohol
    • Hydroxyl groups of the monosaccharide are converted to ether linkages (glucose reacts with methyl iodide to form methyl glucoside)

Process of glycoside formation

  • Glycoside formation involves the reaction between a monosaccharide and another molecule (aglycon)
    • Hemiacetal or hemiketal group of the monosaccharide reacts with the hydroxyl group of the aglycon
    • Results in the formation of an acetal or ketal linkage, known as a glycosidic bond (glucose reacts with methanol to form methyl $\alpha$-D-glucopyranoside and methyl $\beta$-D-glucopyranoside)
  • Glycosidic bonds are present in many biologically important molecules
    • Disaccharides (maltose, lactose)
    • Polysaccharides (starch, cellulose)
    • Glycoproteins and glycolipids
  • Glycosidic bonds are hydrolyzed by specific enzymes called glycosidases
  • Glycosidic bond formation and hydrolysis are crucial for energy storage, cell signaling, and structural roles in living organisms

Oxidation vs reduction of monosaccharides

  • Oxidation reactions of monosaccharides
    1. Aldoses can be oxidized to form aldonic acids (glucose is oxidized to gluconic acid by glucose oxidase)
      • Terminal carbon (C-1) of an aldose is oxidized to a carboxylic acid group
    2. Ketoses can be oxidized to form aldoses and ketoacids (fructose is oxidized to gluconic acid and 2-keto-gluconic acid)
  • Reduction reactions of monosaccharides
    1. Aldoses can be reduced to form alditols or sugar alcohols (glucose is reduced to sorbitol by aldose reductase)
      • Carbonyl group (C-1) of an aldose is reduced to a hydroxyl group
    2. Ketoses can be reduced to form alditols (fructose is reduced to a mixture of sorbitol and mannitol)
  • Biological significance of oxidation and reduction reactions
    • Oxidation reactions are involved in the catabolism of monosaccharides (glucose oxidation is a key step in the pentose phosphate pathway)
    • Reduction reactions produce sugar alcohols, which are used as low-calorie sweeteners and in the production of pharmaceuticals and cosmetics (xylitol, erythritol)

Structural representations and isomerization

  • Fischer projection is a two-dimensional representation of three-dimensional monosaccharide structures
  • Haworth projection is a cyclic representation of monosaccharides, showing the ring structure
  • Mutarotation is the process of interconversion between α and β anomers in solution
  • Anomers are stereoisomers that differ in configuration at the hemiacetal or hemiketal carbon
  • Epimerization occurs when two monosaccharides differ only in the configuration of one specific stereocenter

Key Terms to Review (47)

Wohl degradation: Wohl degradation is a chemical reaction that involves the transformation of aldoses into smaller chain aldoses by the removal of one carbon atom from the carbohydrate molecule. This process uses chemicals like hydrogen cyanide (HCN) and sodium amalgam as reagents to achieve the reduction and rearrangement of the sugar molecule.
Kiliani–Fischer synthesis: The Kiliani–Fischer synthesis is a chemical reaction used to elongate the carbon chain of monosaccharides by one carbon atom, utilizing cyanohydrin formation followed by hydrolysis and hydrogenation. It allows for the systematic exploration of stereochemistry at the newly created chiral center.
Koenigs–Knorr reaction: The Koenigs–Knorr reaction is a glycosylation process used in organic chemistry for the synthesis of glycosides from halogenated sugars and alcohols, typically using a silver salt catalyst. It is particularly significant in the modification of carbohydrates to form various glycosidic linkages.
Glycoconjugate: A glycoconjugate is a compound where one or more sugar molecules (monosaccharides) are covalently bonded to another type of molecule, such as proteins or lipids. These conjugates play crucial roles in biological processes, including cell-cell recognition and signaling.
Tollens’ reagent: Tollens’ reagent is a chemical solution used to detect the presence of aldehyde functional groups in organic compounds, indicating their ability to be oxidized. It consists of silver nitrate in ammonia, which reacts with aldehydes to form a silver mirror on the reaction container.
Uronic acid: Uronic acids are a class of sugar acids derived from monosaccharides, where the terminal carbon's hydroxyl group is oxidized to a carboxylic acid. They play a crucial role in the metabolism of carbohydrates and in the formation of polysaccharides such as pectin and heparin.
Alditol: Alditols are sugar alcohols formed by the reduction of the aldehyde group in monosaccharides to an alcohol. This process preserves the carbon chain but changes the functional group at the end, affecting its reactivity and properties.
Esterification: Esterification is a chemical reaction that involves the formation of an ester compound from the reaction between a carboxylic acid and an alcohol. This process is crucial in various areas of organic chemistry, including the properties of functional groups, polar reactions, the behavior of alcohols and phenols, the reactions of carboxylic acids, the chemistry of esters, and the reactions of monosaccharides.
Fischer Projection: A Fischer projection is a way of representing the three-dimensional structure of a molecule, particularly organic compounds with tetrahedral carbon centers, on a two-dimensional plane. It is used to depict the relative orientation of substituents around a carbon atom and is crucial for understanding concepts such as enantiomers, diastereomers, and the configuration of sugars.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
Reduction: Reduction is a chemical process that involves the gain of electrons by a molecule or atom, resulting in a decrease in its oxidation state. This term is particularly important in the context of various organic chemistry reactions and transformations.
Glycosidic Bond: A glycosidic bond is a covalent bond that connects a carbohydrate (sugar) molecule to another molecule, such as another carbohydrate, a lipid, or a protein. This bond is formed when the hydroxyl group of one molecule reacts with the anomeric carbon of a monosaccharide, creating a new compound with unique properties and functions.
Hemiacetal: A hemiacetal is a type of functional group formed by the addition of an alcohol to the carbonyl carbon of an aldehyde or ketone, resulting in a cyclic structure with an ether and a hydroxyl group. This term is particularly relevant in the contexts of nucleophilic addition reactions, the cyclic structures of monosaccharides, reactions of monosaccharides, and the formation of disaccharides.
Disaccharides: Disaccharides are a class of carbohydrates composed of two monosaccharide units joined together through a glycosidic bond. They are an important part of the classification of carbohydrates and play a role in the reactions of monosaccharides as well as the formation of disaccharides.
Ketoses: Ketoses are a class of monosaccharides, the simplest form of carbohydrates, where the carbonyl carbon (C=O) is located at the terminal end of the carbon chain. This structural feature distinguishes ketoses from aldoses, another class of monosaccharides where the carbonyl carbon is located at the non-terminal end of the carbon chain.
Pentose Phosphate Pathway: The pentose phosphate pathway, also known as the hexose monophosphate shunt, is an alternative metabolic pathway to glycolysis that generates NADPH and pentose sugars. It plays a crucial role in the context of 23.13 Some Biological Carbonyl Condensation Reactions and 25.6 Reactions of Monosaccharides by providing reducing power and precursors for biosynthetic processes.
Mutarotation: Mutarotation is the spontaneous interconversion between the '$\alpha$-' and '$\beta$-'anomeric forms of a monosaccharide in aqueous solution. This process occurs as the monosaccharide forms a cyclic structure and the orientation of the hydroxyl group on the anomeric carbon changes.
Lactose: Lactose is a disaccharide sugar found in milk and dairy products. It is composed of two monosaccharides, glucose and galactose, linked together. Lactose plays an important role in the classification of carbohydrates, the reactions of monosaccharides, and the formation of disaccharides.
Aldoses: Aldoses are a class of monosaccharides, the simplest form of carbohydrates, that have an aldehyde group (CHO) at the first carbon atom. They are one of the two main types of monosaccharides, the other being ketoses, which have a ketone group (C=O) at the second carbon atom.
Anomers: Anomers are the two possible stereoisomers that can exist at the anomeric carbon of a monosaccharide, where the hydroxyl group can be in either the alpha (α) or beta (β) configuration. This concept is crucial in understanding the classification of carbohydrates, their stereochemical representation, and the reactions they undergo.
Polysaccharides: Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units, such as glucose, connected by glycosidic bonds. They serve various structural and functional roles in living organisms and are an important class of biomolecules discussed in the context of carbohydrate chemistry and metabolism.
Starch: Starch is a complex carbohydrate that serves as the primary energy storage molecule in many plants. It is a polysaccharide composed of long chains of glucose units and plays a crucial role in the context of carbohydrate classification, monosaccharide reactions, and polysaccharide synthesis.
Cellulose: Cellulose is a complex carbohydrate and the primary structural component of the cell walls of plants. It is the most abundant organic polymer on Earth and is an essential material for various industries, including paper, textiles, and biofuels.
Haworth Projection: The Haworth projection is a two-dimensional representation of the cyclic structure of monosaccharides, which helps visualize the stereochemistry and orientation of the sugar ring and its substituents. This projection is particularly useful in understanding the anomeric configuration and reactions of carbohydrates.
Etherification: Etherification is a chemical reaction in which an alcohol group reacts with another alcohol group or a halide to form an ether compound. This process is particularly relevant in the context of carbohydrate chemistry, specifically the reactions of monosaccharides.
Glycoside: A glycoside is a compound in which a sugar molecule is bonded to a non-sugar molecule, typically through an oxygen atom. Glycosides are important in the context of the reactions of monosaccharides, as they represent one of the key functional groups that monosaccharides can form.
Methyl α-D-glucopyranoside: Methyl α-D-glucopyranoside is a monosaccharide derivative formed by the addition of a methyl group to the anomeric carbon of the glucose molecule in the alpha configuration. It is an important compound in the study of carbohydrate chemistry and its reactions.
Glycoproteins: Glycoproteins are complex molecules composed of a protein backbone with covalently attached carbohydrate (glycan) side chains. They play crucial roles in various biological processes, including cell-cell recognition, cell signaling, and immune function.
Alditols: Alditols are polyhydric alcohols derived from the reduction of monosaccharides, where the carbonyl group (C=O) is reduced to a hydroxyl group (-OH). They are important intermediates in the reactions of monosaccharides and have various applications in the food and pharmaceutical industries.
Maltose: Maltose, also known as malt sugar, is a disaccharide composed of two glucose molecules linked together. It is an important intermediate in the breakdown of starch and plays a crucial role in the production of fermented beverages like beer and wine.
Mannitol: Mannitol is a sugar alcohol that is commonly found in various fruits and vegetables. It is an important compound in the context of the reactions of monosaccharides, as it can be derived from the reduction of the monosaccharide mannose.
Hemiketal: A hemiketal is a cyclic structure that forms when a carbonyl carbon of a monosaccharide reacts with a hydroxyl group on the same molecule, creating a new ring-like structure. This structural feature is crucial in understanding the cyclic nature of monosaccharides and their subsequent reactions.
Glycosidases: Glycosidases are enzymes that catalyze the hydrolysis of glycosidic bonds, breaking down complex carbohydrates into simpler sugar units. They play a crucial role in the metabolism and processing of monosaccharides, which is the focus of the 25.6 Reactions of Monosaccharides chapter.
Glucose pentaacetate: Glucose pentaacetate is a derivative of the monosaccharide glucose, where all five hydroxyl groups on the glucose molecule have been replaced with acetyl groups. This modification alters the chemical and physical properties of glucose, making it useful in various organic chemistry reactions and applications.
Glycolipids: Glycolipids are a class of lipids that contain carbohydrate (glyco-) moieties covalently attached to a lipid component. These molecules are important structural components of cell membranes and play crucial roles in various cellular processes related to the reactions of monosaccharides.
Xylitol: Xylitol is a naturally occurring sugar alcohol that is commonly used as a sugar substitute. It is found in various fruits and vegetables, and is often used in sugar-free gum, candies, and other food products due to its sweetness and potential health benefits.
Epimerization: Epimerization is a chemical reaction in which a molecule with multiple stereogenic centers is converted to an epimer, a stereoisomer that differs in the configuration of only one stereogenic center. This process is particularly relevant in the context of monosaccharide chemistry and the interconversion of various sugar forms.
Methyl β-D-glucopyranoside: Methyl β-D-glucopyranoside is a monosaccharide derivative formed by the condensation of glucose with methanol. It is a common model compound used to study the reactions and properties of monosaccharides, as outlined in the topic 25.6 Reactions of Monosaccharides.
Glucose Oxidase: Glucose oxidase is an enzyme that catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide. It is commonly found in various organisms, including fungi and bacteria, and plays a crucial role in the context of the reactions of monosaccharides.
Erythritol: Erythritol is a naturally occurring sugar alcohol found in various fruits, fermented foods, and the human body. It is a small, four-carbon polyol that has unique properties relevant to the reactions of monosaccharides in organic chemistry.
Methyl Glucoside: A methyl glucoside is a glycoside in which a methyl group is attached to the anomeric carbon of a glucose molecule. These compounds are commonly used as models for studying the reactions and properties of monosaccharides.
Sugar Alcohols: Sugar alcohols, also known as polyols, are a class of carbohydrates that resemble the chemical structure of both sugars and alcohols. They are commonly used as sweeteners and found in various food and pharmaceutical products.
Sorbitol: Sorbitol is a sugar alcohol that is naturally found in many fruits and berries. It is commonly used as a sweetener and humectant in various food and pharmaceutical products due to its low glycemic index and ability to retain moisture.
Aldose Reductase: Aldose reductase is an enzyme that catalyzes the reduction of aldoses, such as glucose, to their corresponding sugar alcohols, known as polyols. This process is a crucial step in the polyol pathway, which is involved in the metabolism of carbohydrates and the development of certain diabetic complications.
Aldonic acids: Aldonic acids are a class of carbohydrate derivatives formed by the oxidation of the terminal aldehyde group of a monosaccharide to a carboxylic acid group. They are closely related to the reactions of monosaccharides discussed in Chapter 25.6.
Ketoacids: Ketoacids are organic compounds that contain both a carboxyl group (-COOH) and a ketone group (-C=O) within their molecular structure. They are important intermediates in various metabolic pathways, particularly in the context of carbohydrate and lipid metabolism.
Aglycon: The aglycon, also known as the aglycone, is the non-sugar portion of a glycoside molecule. It is the part of the molecule that is not a carbohydrate and is responsible for the characteristic properties and biological activities of the glycoside.