Aldose vs Ketose
Aldose and ketose are two types of monosaccharides, which are simple sugars. The main difference between them lies in their functional group: aldoses have an aldehyde group, while ketoses have a ketone group. This structural difference affects their chemical properties and reactivity. Both aldoses and ketoses play important roles in biological processes and are fundamental building blocks of carbohydrates.
7 Key excerpts on "Aldose vs Ketose"
eBook - ePub- Jose Perez-Castineira(Author)
- 2020(Publication Date)
- De Gruyter(Publisher)
...Their chemical formulae are often depicted in the two-dimensional Fischer projection (Figures 3.1 to 3.3), in which the carbon atoms are aligned vertically and numbered from top to bottom following the rules of Organic Chemistry nomenclature (the carbon of the carbonyl group is C-1 in aldoses and C-2 in ketoses). This projection was originally proposed by Emil Fischer in the late nineteenth century for monosaccharides and, although problematic for many organic molecules, it remains acceptable nowadays for carbohydrates and their derivatives [ 7 ]. Figure 3.1: Chemical structures of glyceraldehyde and dihidroxyacetone. Glyceraldehyde is the simplest aldose and dihydroxyacetone is the simplest ketose (Figure 3.1). Addition of successive hydroxymethylene units to these molecules after the carbonyl group can be used to formulate the aldoses and ketoses families, respectively (Figure 3.2). From the biological point of view, the most prominent monosaccharides are those containing between three and seven carbon atoms, but, in nutritional terms, the most important and abundant are those that have six [(CH 2 O) 6 or C 6 H 12 O 6 ], the so-called hexoses (Table 3.1). Hexoses containing an aldehyde group are known as aldohexoses and those containing an α-hydroxy ketone group, ketohexoses or hexuloses. Figure 3.2: The D-aldoses (A) and D-ketoses (B) families of monosaccharides. Figure 3.2: (continued). With the only exception of dihydroxyacetone, all monosaccharides present a number of chiral carbons (linked to four different atoms or atom groups) equal to that of hydromethylene units: aldoses of n carbon atoms contain n−2 chiral centers, whereas ketoses contain n−3. Molecules that differ only in the configuration of one or more chiral carbons are known as stereoisomers...
eBook - ePub- Raymond S. Ochs(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
...All remaining carbons have an attached hydroxyl group. There are two classes of monosaccharides: aldoses and ketoses. Aldoses (aldehydes) have the carbonyl at carbon one, whereas ketoses (ketones) have the carbonyl at carbon two. The “-ose” suffix (as in aldose and ketose) usually indicates that a compound is a sugar. The general category name for sugars is shown in Table 4.1, starting with the smallest, the triose. Exceptions to this naming rule occur in the simplest monosaccharides: dihydroxyacetone and glyceraldehyde. Table 4.1 General Nomenclature of Sugars Carbons Category Name 3 Triose 4 Tetrose 5 Pentose 6 Hexose 7 Heptose 8 Octose Dihydroxyacetone is a symmetrical molecule because a plane of symmetry can be drawn through its center. However, glyceraldehyde is asymmetric: there are two distinct arrangements of the four groups attached to its central carbon. These forms are illustrated in Figure 4.1 using a Fischer projection to represent three-dimensional molecules in a plane. The two-dimensional Fischer projection convention is that the horizontal attachments project above the plane, and the vertical attachments project behind the plane. A further requirement is that the first carbon – the most oxidized – is shown on top. Another representation of the central carbon and its attached groups is a virtual tetrahedron, as shown in Figure 4.2. FIGURE 4.1 d -and l -glyceraldehyde. These mirror images are shown as Fischer projections and three-dimensional views. Asterisks indicate chiral carbons. FIGURE 4.2 Tetrahedral carbon. The four groups attached to the central carbon are as far apart from one another as possible, forming a tetrahedron in space. The two forms of glyceraldehyde are called stereoisomers, and molecules that display this form of stereoisomerism are known as enantiomers. The central carbon to which the four distinct groups are attached is the chiral carbon ; any molecule containing such a carbon is known as a chiral molecule...
eBook - ePub- David Hames, Nigel Hooper(Authors)
- 2011(Publication Date)
- Taylor & Francis(Publisher)
...SECTION J – CARBOHYDRATE METABOLISM J1 Monosaccharides and disaccharides Key Notes Aldoses and ketoses A monosaccharide has the general formula (CH 2 O) n and contains either an aldehyde group (an aldose) or a ketone group (a ketose). The free aldehyde or ketone group can reduce cupric ions (Cu 2+) to cuprous ions (Cu +) and hence such a monosaccharide is called a reducing sugar. Stereoisomers The D and L stereoisomers of sugars refer to the configuration of the asymmetric carbon atom furthest from the aldehyde or ketone group. The sugar is said to be a D isomer if the configuration of the atoms bonded to this carbon atom is the same as for the asymmetric carbon in D -glyceraldehyde. Ring structures Tetroses and larger sugars can cyclize by reaction of the aldehyde or ketone group with a hydroxyl group on another carbon atom of the sugar. Glucose mainly cyclizes to form a six-membered pyranose ring whilst other sugars form five-membered furanose rings. Two forms (anomers) of D -glucopyranose exist, depending on whether the hydroxyl group attached to the anomeric carbon atom (C-1) lies below the plane of the ring (the α form) or above the plane of the ring (β form). In solution, the α and β forms interconvert via the open-chain form (mutarotation). The pyranose ring can exist in either boat or chair conformations but the chair form predominates since the side groups, which are usually OH groups, are less sterically hindered in this conformation. Disaccharides A disaccharide is formed when two monosaccharides become joined by a glycosidic bond. The bond may be an α- or β-bond depending on the configuration of the anomeric carbon atom involved in the bond. Usually the anomeric carbon atom of only one of the two monosaccharides is involved in the bond so that the disaccharide still has one free aldehyde or ketone group and is still reducing...
eBook - ePub- S. P. Bhutani(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...Examples of aldose monosaccharides are arabinose, glucose and mannose. The most common ketose is fructose. Monosaccharides are further characterised in terms of the number of carbon atoms present in an aldose or a ketose. Thus, pentoses and hexoses contain five and six carbons respectively. For example, glucose is a hexose but it is completely characterised as an aldohexose, which means it is a six carbon sugar containing an aldehyde group. Similarly, fructose is a ketohexose and arabinose an aldopentose. In aldoses, the aldehyde group is at the top of the chain and the aldehyde carbon is considered as carbon number 1. All naturally occurring ketoses have the keto group on the second carbon in the chain. The simplest sugars have three carbon atoms and are called trioses. The simplest aldose is glyceraldehyde, an aldotriose, and the simplest ketose is dihydroxy acetone, a ketotriose. Addition of an extra carbon atom to a triose gives successively a tetrose, a pentose, a hexose and so on. Most of the naturally occurring monosaccharides are either pentoses or hexoses. Trioses and tetroses do not occur in nature. The most abundant pentoses are L -arabinose, D -ribose, 2-deoxy- D -ribose, and D -xylose, which are all aldopentoses (Table 1.2). The common hexoses are D -glucose, D -fructose, D -mannose and D -galactose. D -Fructose is a ketohexose whereas all others are aldohexoses (Table 1.3). B. Oligosaccharides These are low molecular weight condensation polymers containing 2-9 monosaccharide units. More often the monosaccharide units are hexoses. Oligosaccharides can be further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharide units obtained on hydrolysis. i. Disaccharides A sugar that yields two monosaccharide units on hydrolysis is known as a disaccharide. For example, sucrose, the common table sugar is a disaccharide because on hydrolysis it produces one mole of glucose and one mole of fructose...
eBook - ePub- James N. BeMiller(Author)
- 2018(Publication Date)
...It and all sugars containing an aldehydic group are classified as aldoses (Table 1.1). The prefix ald - indicates that they are aldehydes; the suffix - ose usually (but not always as you will find out) signifies a nonpolymeric carbohydrate (that is, a monosaccharide). D -Glucose contains six carbon atoms, making it a hexose (Table 1.1); more specifically, it is an aldohexose. When the structure of D -glucose is written in a vertical straight-chain fashion (termed an acyclic or open-chain structure) with the aldehydic group (position 1 [C1]) at the top and the carbon atom with the primary hydroxyl group attached to it at the bottom (at position 6 [that is, on C6]), it can be seen that all secondary hydroxyl groups are on carbon atoms C2, C3, C4, and C5. To make the determination/assignment of which side of the carbon chain the hydroxyl groups are on, a convention for orientation of the carbon chain is used. In this convention, the carbon chain is oriented so that each vertical (carbon-to-carbon) bond projects into the plane of the page and each horizontal bond projects outward from the plane of the page as in Fig. 1.1 (although in solution there is rotation about the vertical bonds that allows a hydroxyl group to be in any position with respect to the one above [or below] it, so that the molecules can actually assume a large number of different conformations [shapes]). Each of the four carbon atoms that have a secondary hydroxyl group attached to it (C2, C3, C4, C5) are chiral carbon atoms because each has four different substituents attached to it...
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...13 Carbohydrates Another important class of molecules that are critical to an understanding of biological processes are carbohydrates, commonly known as sugars. Carbohydrates are key components of glycosides and cells, and they comprise the backbone of nucleotides, including DNA and RNA. This chapter will introduce the fundamentals of carbohydrate structure and nomenclature. 13.1 (Poly)hydroxy Carbonyl Compounds Most carbohydrates are (poly)hydroxylated aldehydes or ketones, although there are other (poly)hydroxylated derivatives. There are two fundamental ways to classify carbohydrates. The first is based on the type of functional group that accompanies the hydroxyl units. The second is based on the number individual units that make up the carbohydrate. Carbohydrates are classified by the nature of the functional groups X 1 and X 2. In a carbohydrate there are several repeating CHOH units defined by the integer “n,” where n = 3, 5, or 6, and so on. For example, when n = 3 in a carbohydrate, the structure is X 1 —CHOH-CHOH-CHOH—X 2. Most carbohydrates are defined by making X 1 and/or X 2 = CH 2 OH, CHO, COR (a ketone), or COOH. A glycose has a CH 2 OH and an aldehyde or ketone unit and a glycitol has two CH 2 OH units (sometimes called an alditol). When one group is a carboxylic acid and the other is CH 2 OH, it is a glyconic acid (sometimes called an aldonic acid). A glycaric acid has two carboxyl units and is a hydroxy-dioic acid (sometimes called an aldaric acid), and a uronic acid has a carboxyl group and an aldehyde group. The other way to categorize carbohydrates is by the number of sugar units the carbohydrate contains. The general carbohydrate structure shown above contains one carbohydrate unit and it is categorized as a monosaccharide. If two monosaccharides are coupled together, the resulting molecule is a disaccharide and a molecule with three monosaccharide units is a trisaccharide...
eBook - ePubFood Chemistry
A Laboratory Manual
- Dennis D. Miller, C. K. Yeung(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
...3 Properties of Sugars 3.1 Learning Outcomes After completing this exercise, students will be able to: Draw structures of common reducing and nonreducing sugars. Explain the difference between a hemiacetal and an acetal. Distinguish between reducing and nonreducing sugars experimentally. 3.2 Introduction Sugars are polyhydroxylated aldehydes or polyhydroxylated ketones (Figure 3.1). Thus, they participate in reactions characteristic of alcohols and aldehydes or ketones. Please review the sections in your organic chemistry textbook that describe reactions for alcohols, aldehydes, and ketones. Recall that alcohols react reversibly with aldehydes or ketones to form hemiacetals or hemiketals. When the alcohol and carbonyl groups are on the same molecule, as is the case with sugars, a cyclic or ring structure is formed (Figure 3.2). Note that hemiacetals contain a carbon atom bonded to an –OH group and an –O–R group. Hemiacetals are relatively unstable. In aqueous solution, the open and closed ring forms are both present in equilibrium. Thus, sugars like glucose participate in reactions characteristic of aldehydes even though the predominant form is the hemiacetal. Sugars containing the hemiacetal group are called reducing sugars because they are capable of reducing various oxidizing agents. Several well‐known assays, based on this tendency to oxidize, have been developed for detecting reducing sugars. These include the Tollen's test (sugars are mixed with Ag + in aqueous ammonia solution), the Fehling's test (sugars are mixed with Cu 2+ in aqueous tartrate solution), and the Benedict's test (sugars are mixed with Cu 2+ in aqueous citrate solution). When mixed with these solutions, reducing sugars are oxidized causing a reduction in the valence of the metal ion. In the Tollen's test, a shiny mirror of elemental silver (Ag 0) forms on the inside surface of the test tube...
