Haworth Projection

5 Key excerpts on "Haworth Projection"

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  eBook - PDF

  The Carbohydrates Volume 1A

  Chemistry and Biochemistry

  • W.W. Pigman(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Six of them have clockwise numbering and are generated by rotating the three planes simultaneously through intervals of one-sixth of a revolution. The other six are generated in the same way, but by starting from the molecule drawn with counterclockwise numbering. There are also 372 additional variations, all of which represent other configurations of the parent structure. Because of the possibilities for confusion, it has, as already mentioned (see p. 42), become customary to adopt a standard ring orientation, in which the reducing carbon atom is on the extreme right and the ring is numbered clockwise. A simple paper model (Fig. 1) provides a convenient XH 2 OH vOH Topside view Underside view Model for Haworth representation Model for conformational representation (two chairlike conformations, both viewed from the top side) FIG. 1. Cut-out paper or cardboard models for Haworth perspective and conformational structures. The example given is for a-D-glucopyranose. device for easy recognition of the various correct orientations of the groups in a Haworth perspective formula. Models of this type, advocated by C. S. Hudson, consist of paper cut-outs of the ring system. After the ring-oxygen atom has been marked on one corner, on the top, and on the underside of the model, the ring is held in the conventional orientation and the various groups 1. STEREOCHEMISTRY OF THE MONOSACCHARIDES 59 on the left of the modified Fischer projection are marked on the appropriate corners on the top of the model, and those groups on the right side of the modified Fischer projection are marked on the corresponding corners of the underside. The model can then be turned to any of the twelve possible orientations, and the Haworth structure having the correct orientations of all substituents can readily be drawn.

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  eBook - ePub

  Handbook of Carbohydrate-Modifying Biocatalysts

  • Peter Grunwald(Author)
  • 2016(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  1 conformation of the β-anomer preponderates that of the α-anomer. This inversely proportional dependence of the strength of the anomeric effect on the dielectric constant of the solvent is in agreement with the idea that the effect is caused by electrostatic interactions. Yet, the anomeric effect can come into play if the solvent is non-polar or if the anomeric HO group is replaced by a bulky residue such as acetate.

  The five-membered furanose ring is uneven, too, as shown in Fig. 1.6 , with ribose as an example. A plane is formed by three C atoms and the ring oxygen, and either C2 or C3 is out of plane and points to the same direction as the -CH2 OH group does.

  Figure 1.6 β-D -Ribose in the

  C3 -endo

  (left) and the

  C3 -exo

  configuration, in which this moiety is mainly found in bio-molecules.

  In the literature, sometimes the Haworth Projection is used to illustrate the structure of a cyclic sugar. In a Haworth Projection of a D -pyranose, the six-membered ring is represented as flat and is viewed edge on. The ring oxygen is always placed in the back right-hand corner of the ring, with the anomeric carbon C1 on the right hand side and the primary alcohol group drawn up from the back left corner C5. Groups on the right in a Fischer projection are pointing down in a Haworth Projection, while groups on the left in a Fischer projection are directed upward in a Haworth Projection. These two rules can lead to didactic disasters according to the present author’s experience: if students have to convert the Haworth Projection formulae to the realist chair configuration, they often misinterpret the Haworth rules and assume an all-axial direction of the HO groups in glucose. Because of this didactic misunderstanding, the present author warns about applying the Haworth Projection in basic courses of organic chemistry and refrains from showing them in this chapter.

  1.2.2 Disaccharides

  Further reaction of the hemiacetal group of a cyclic monosaccharide with another alcohol group of a second monosaccharide leads to an acetal group, in this case referred to as a glycoside. Thus disaccharides consist of two monosaccharides subunits that are linked by a glycosidic bonding. For example, α-maltose contains two α-D -glucose subunits that are hooked together between C1 of one sugar and C4 of the second one (Fig. 1.7a ). The brief description of this kind of acetal bonding is α-1,4’-glycosidic linkage.

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  eBook - ePub

  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  Section 3.6 ). Any hydroxyl group (or other group) that is up in the Haworth Projection is also up in the chair conformation. However, “up” and “down” do not correspond to axial and equatorial, respectively. Each carbon atom must be individually examined. On one set of alternating carbon atoms, an “up” substituent is axial; on the intervening carbon atoms, an “up” substituent is equatorial.

  Haworth Projection formulas are converted into chair representations by “moving” two carbon atoms. The anomeric carbon atom is lowered below the plane of the ring, and the C-4 atom is raised above the plane of the ring. The remaining four atoms—three carbon atoms and the ring oxygen atom—are unchanged. This process is shown in Figure 13.3 for both α-D -glucopyranose and β-D -glucopyranose. Both the hydrogen atoms and the hydroxyl groups can be shown. However, a more condensed form that eliminates the C—H bonds is often used.

  Figure 13.3 Conversion of Haworth Projections to Chair Conformations

  Note the changes in the locations of the hydroxyl groups in the Haworth Projection compared to those in the chair conformation. Although the hydroxyl groups were both up and down in the Haworth Projection formula, all hydroxyl groups are equatorial in β-D -glucopyranose. This anomer is the more stable based on the position of equilibrium observed in mutarotation experiments. β-D -Glucopyranose, the most abundant aldohexose, is the only aldohexose that has all of its hydroxyl groups in equatorial positions.

  13.5 Reduction of Monosaccharides

  Although five- and six-carbon monosaccharides exist predominantly as hemiacetals and hemiketals, they undergo the characteristic reactions of simple aldehydes and ketones. One such reaction is reduction. Treating an aldose or ketose with sodium borohydride reduces it to a polyalcohol called an alditol . The reduction reaction occurs via the aldehyde group in the small amount of the open-chain form of the aldose in equilibrium with its cyclic hemiacetal. As the aldehyde is reduced, the equilibrium shifts to produce more aldehyde until eventually all of the monosaccharide is reduced. The alditol derived from D -glucose is called D -glucitol. D

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  eBook - ePub

  Understanding the Science of Food

  From molecules to mouthfeel

  • Sharon Croxford, Emma Stirling(Authors)
  • 2020(Publication Date)
  • Routledge

    (Publisher)

  Figure 2.17 . Perspective is gained through the use of a thicker line on the bond closest to the viewer. In monosaccharides, the hydrogen and hydroxyl groups to the right on a Fischer projection are shown below the plane in a Haworth Projection. Carbon 1 is referred to as the anomeric carbon, and the direction of the hydroxyl group on this carbon determines whether it is α or β. If the hydroxyl group on the anomeric carbon of a molecule in a Fischer projection is on the same side as the oxygen on the second-bottom carbon (the carbon that determines whether it is in D or L formation) in a Haworth Projection, the formation is α-anomeric. A β-anomer forms when the hydroxyl group is on the opposite side of the oxygen on the second-bottom carbon.

  Figure 2.16

  Formation of α-D-fructose and β-D-fructose

  Reducing sugars

  Reducing sugars are carbohydrates that contain an aldehyde group or a potential aldehyde group. The aldehyde monosaccharides (glucose and maltose) are reducing sugars; however, the ketone fructose needs to undergo tautomerisation (change in isomeric form) to an aldose form before it can be oxidised, or reduce an oxidising agent. Sucrose is not a reducing sugar, but other disaccharides are. Sucrose does not have a hemiacetal group, which provides the ability for either of its two rings to open and produce an aldehyde for reaction. The ability of sugars to act as strong reducing agents is important in food chemistry, particularly in caramelisation and Maillard reactions (see Chapter 7 ). Sucrose slowly dissociates to glucose and fructose in solution, and the addition of acid accelerates this reaction.

  Figure 2.17

  Haworth Projections of (a) α-D-glucose and (b) β-D-glucose

  Monosaccharides and disaccharides

  D-glucose, D-fructose and D-galactose are important natural monosaccharides. Both glucose and galactose are aldohexoses, and fructose is an aldoketose. Disaccharides arise when two monosaccharides undergo condensation or dehydration. They are referred to as glycosides, due to the formation of a glycosidic bond, a type of functional group that joins sugar molecules (see Figure 2.18

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  Organic Chemistry

  • John M. McIntosh(Author)
  • 2018(Publication Date)
  • De Gruyter

    (Publisher)

  3.2 can become very complex when more than two or three carbon atoms are involved. To simplify this situation, a new type of drawing must be introduced: the so-called projection drawings . We will see different applications of these, but in general, they serve to convert three-dimensional drawings into two-dimensional ones. To allow this, certain restrictions must be applied, the violation of which may lead to erroneous conclusions. Thus, it is very important to fully under-stand what is involved in making a projection drawing and what the restrictions are for each type. C C H H H H H H H H H H H H front carbon rear carbon Fig. 3.3 26 | 3 The Shapes of Organic Molecules – Stereochemistry 1 If a two-carbon fragment is turned so that the two carbons under consideration are colinear with the line of sight and then a light beam in projected down this axis, the molecule will cast a shadow which is the two-dimensional projection of the actual molecule (Fig. 3.3). This shadow shows the front carbon atom as a dot in the middle and three of the four bonds attached to it are now oriented at 120° to each other. (The fourth C − C bond is behind the front atom and therefore not visible.) The rear carbon is represented by the larger circle. Of the four bonds attached to it, three are oriented at 120° to each other. Since these are behind the rear carbon atom, they cannot be seen until they extend beyond the radius of the carbon. Consequently, these bonds begin at the circumference of the circle representing the carbon. This type of projec-tion drawing is called a Newman projection . This type of drawing can be used for larger molecules. However, only one C − C bond can be shown at a time. The Newman projec-tions of some simple alkanes are shown in Fig. 3.4. H H H H H H H H CH 3 H H H CH 3 H H CH 3 H H H H H CH 2 CH 3 H H ethane (C 2 H 6 ) propane (C 3 H 8 ) butane (C 4 H 10 ) butane (C 4 H 10 ) viewed down the C2-C3 bond viewed down the C1-C2 bond Fig.

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