instructor_supplement_24

instructor_supplement_24 - Instructor Supplemental...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 24 Carbohydrates Solutions to In-Text Problems 24.1 (b) The following two of the nine possible Fischer projections of ( S )-2-butanol are related by a cyclic permutation of the ethyl, —OH, and the methyl group. 24.2 (b) Turning either projection 180° in the plane of the page establishes that it is identical to the other projection. 24.4 (a) Use a cyclic permutation to transform the given Fischer projection into a standard form in which all of the backbone carbons are in a vertical line. This will show that the —OH group on carbon-5 is on the right. Thus, this aldose has the D-configuration. 24.6 (b) Recognize that D-mannose is epimeric to D-glucose at carbon-2, and thus convert the b -D-glucopyranose structures on text p. 1180 to b -D-mannopyranose structures by inverting the configuration of carbon-2. (d) Start with the Fischer projection for D-fructose from text p. 1177.
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 24 2 (f) Start with the structures of a -D-glucopyranose in the solution for part (e) (p. 644 of the Study Guide and Solutions Manual) and make the bond to the anomeric carbon a “squiggly bond.” (For the Fischer projection simply do not show the configuration. “Squiggly bonds” are not used in Fischer projections.) 24.7 (b) This is the b -anomer of a D-hexopyranose that differs from D-glucose in its configuration at carbons 3 and 4; therefore, it is b -D-gulopyranose. 24.9 The mechanism for base-catalyzed mutarotation of glucose:
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 24 3 24.11 (a) The easiest way to do this is to consider the relationship of D-allose to D-glucose and modify the conformational representation of b -D-glucopyranose (in which all of the ring substituents are equatorial) accordingly. Thus, Fig. 24.3 on text p. 1174 shows that D-allose is epimeric to D-glucose at carbon-3. Consequently, configurational inversion of the b -D-glucopyranose structure at carbon-3 gives the desired conformational representation of b -D-allopyranose. (b) Follow the procedure in Study Problem 24.2 on text p. 1180. Note that in the second and third Fischer projections the configuration of carbon-5 cannot be represented without violating the Fischer-projection conventions. We must remember that this carbon has the R configuration, and this configuration must be shown explicitly in the conformational representation. 24.13 (b) D-Allose would be transformed by base into a mixture of D-altrose (structure in Fig. 24.3 on text p. 1174) and the ketose D-psicose. 24.15 Because naturally occurring glycosides generally have bonds between an alcohol or phenol oxygen and the anomeric carbon of a carbohydrate, it is reasonable to propose that the b -D-glycoside of vanillin and glucose has the following structure:
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 24 4 24.16 (b) Start with the structure of a -D-galactopyranose, which is the same as
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