APTER 22 Practice Exercises 22.1 2 2 2 2 2 2 2 2 D-Ribulose L-Ribulose D-Xyulose L-Xyulose (one pair of enantiomers) (a second pair of enantiomers) 22.3 2 Anomeric carbon Glycosidic bond 3 () Methyl -D-mannopyranoside 22.5 Erythritol is achiral because of a mirror plane in the molecule and therefore, the product is optically inactive. NaB 4 2 Mirror plane of symmetry 2 2 D-Erythrose Erythritol (meso) Review Questions 22.1 An aldose is a monosaccharide that has an aldehyde group and a ketose is a monosaccharide with a ketone group. An aldopentose is a five-carbon monosaccharide containing an aldehyde group and a ketopentose is a five-carbon monosaccharide containing a ketone group. 22.3 In an achiral environment D- and L-glyceraldehyde differ only by the sign of their specific rotation of plane-polarised light. Except for this property, enantiomers have identical chemical and physical properties.
22.5 The stereocentres on D-glucose and D-ribose are indicated with an asterisk. D-glucose has four stereocentres for a total of 16 stereoisomers. D-ribose has three stereocentres for a total of eight stereoisomers. 2 D-Glucose 2 D-Ribose 22.7 Each carbon of a monosaccharide has a hydroxyl group that participates both in hydrogen bond accepting and donating with water molecules, making it water soluble. 22.9 The anomeric carbon is the new carbon stereocentre created upon the formation of a cyclic structure. In terms of carbohydrate chemistry, the anomeric carbon is the hemiacetal carbon of the cyclic form of the monosaccharide. 22.11 -D-glucose and -D-glucose are anomers. They only differ in the configuration at the hemiacetal carbon (the anomeric carbon). All of the other carbon stereocentres have the same configuration, so therefore, the anomers are diastereomers, not enantiomers. 22.13 exopyranoses are six-membered rings in the chair conformation. aworth projections represent the six-membered rings as being planar, which they are not. 2 -D-Glucopyranose 2 -D-Glucopyranose 22.15 [α] D = +80.2. Due to mutarotation a solution of β-d-galactopyranose equilibrates to give an equilibrium mixture of the two cyclic forms and the open chain form. This equilibrium mixture is the same regardless of the starting material so the optical activity is the same. 22.17 The specific rotation of -L-glucose changes to 52.7 because -L-glucose and -D-glucose are enantiomers and the only difference between enantiomers in an achiral environment is the sign of their specific rotation of plane-polarised light. 22.19 The alditol of D-glucose is optically active because it is chiral, however the alditol of D-galactose is optically inactive because it possesses a plane of symmetry and is therefore a meso compound.
2 NaB 4 2 2 D-Glucose D-Glucitol (hiral; optically active) 2 NaB 4 Mirror plane of symmetry 2 2 D-Galactose Galactitol (Meso; optically inactive) 22.21 NaB 4 reduction gives two alditols as attack from either side of the carbonyl group is equally likely. 2 2 2 NaB 4 2 2 2 D-Fructose D-Glucitol (D-Sorbitol) D-Mannitol 22.23 A glycosidic bond is the bond from the anomeric carbon of a glycoside to an R group. 22.25 Glycosides do not undergo mutarotation. Glycosides, like acyclic acetals, are stable in aqueous and alkaline conditions, therefore glycosides are not in equilibrium with their open chain forms. 22.27 In order for these disaccharides to react with NaB 4, they must contain at least one carbonyl group that is in equilibrium with the open chain form (a cyclic hemiacetal). Maltose and lactose contain a monosaccharide ring that is a hemiacetal, but sucrose does not.
2 2 Sucrose (no cyclic hemiacetal) Maltose Lactose emiacetal emiacetal 22.29 Three polysaccharides that are composed of D-glucose include cellulose, starch (composed of amylose and amylopectin) and glycogen. ellulose links about 2200 glucose units by -1,4- glycosidic bonds. The amylose in starch is an unbranched polymer with about 4000 glucose units linked by -1,4-glycosidic bonds. Amylopectin contains up to 10,000 glucose units linked by - 1,4-glycosidic bonds, but also contains branching that uses -1,6-glycosidic bonds. Glycogen, like amylopectin, is a highly branched polysaccharide linking 10 6 glucose units with -1,4- and -1,6 glycosidic bonds. ellulose: 1 4-1,4-glycosidic bonds 1 4
Starch (amylose and amylopectin): 1 4-1,4-glycosidic bonds 1 6 1-1,6-glycosidic bond (nly in amylopectin and Glycogen) 4 Review Problems 22.31 2,6-Dideoxy-D-altrose 3 22.33 2 -D-Glucopyranose (a) 2 -D-Mannopyranose (b) 2 -D-Gulopyranose 22.35 In a similar way to the aworth projections, the cyclic hemiacetal is broken, the penultimate carbon rotated and then stretched out vertically.
(a) 2 2 2 D-Galactose (b) 2 2 2 D-Allose 22.37 (a) 2 D-Galactose NaB 4 2 2 2 Galactitol (meso; inactive) Mirror plane of symmetry (b) 2 D-Galactose AgN 3 N 3 ; 2 2 D-Galactonic acid (hiral; optically active)
22.39 2 2 NaB 4 2 2 2 2 2 D-Fructose D-Glucitol (D-Sorbitol) D-Mannitol 22.41 2 1 3 2 pk a 4.10 pk a 11.79 Loss of the more acidic proton results in a conjugate base that is stabilised by three resonance structures delocalising the negative charge over two oxygen atoms and a carbon atom. 2 + + 2 2 2 Loss of the proton with the lesser of the two acidities results in the formation of a conjugate base with only two resonance structures delocalising the charge over an oxygen atom and a carbon atom.
2 2 + + 2 22.43 -D-glucose 22.45 (a)
(b) aworth projection D-galacturonic acid hair onformation Repeating unit of pectin 22.47 Additional Exercises 22.49 The first step involves the tautomerisation of the hydroxy-ketone to the enediol. The enediol then undergoes a second tautomerisation to the hydroxy-aldehyde (keto form).
2 2 P 3 2- Enolisation 2 P 3 2- Enediol intermediate 22.51 (a) There are a number of sets of epimers. The epimers related to D-allose include: D-altrose (differs at 2), D-glucose (differs at 3) and D-gulose (differs at 4). 1 2 3 4 5 2 D-Allose 2 D-Altrose Tautomerisation to the keto form 2 D-Glucose 2 P 3 2-2 D-Gulose (b) Anomer pairs are not epimers and epimers cannot be anomers. A pair of anomers differs in configuration only at the anomeric carbon. Epimers differ in configuration only at a carbon other than the anomeric carbon.