708 CARBOHYDRATES Only the configuration of the hemiacetal function is affected in this process. The a configuration of the glycosidic linkage remains unchanged 25.13 Write the chemical equation so that you can clearly relate the product to the starting material CHO CH-OH H NabH H CHOH CHOH D-Ribose Ribitol is a meso form; it is achiral and thus not optically active. a plane of symmetry passin through C-3 bisects the molecule 25. 14(b) Arabinose is a reducing sugar; it will give a positive test with Benedict's reagent, because its open-chain form has a free aldehyde group capable of being oxidized by copper(l ion (c) Benedicts reagent reacts with a-hydroxy ketones by way of an isomerization process involv- ing an enediol intermediate Benedict CHOH tive test: cu,O formed CHOH CHOH CHO 1.3-Dihydroxyacetone Enedio Glyceraldehyde 1, 3-Dihydroxyacetone gives a positive test with Benedicts reagent. (d)D-Fructose is an a-hydroxy ketone and will give a positive test with Benedict's reagent CHOH H Benedicts H OH positive test; Cu,O formed H CH OH CHOH (e) Lactose is a disaccharide and will give a positive test with Benedict's reagent by way of an open-chain isomer of one of the rings. Lactose is a reducing sugar. OCH HOCH CHo Benedict s reagent, positive test; Cu,O formed OH Open-chain form (structure presented in Section 25. 14) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE WebsiteOnly the configuration of the hemiacetal function is affected in this process. The configuration of the glycosidic linkage remains unchanged. 25.13 Write the chemical equation so that you can clearly relate the product to the starting material. Ribitol is a meso form; it is achiral and thus not optically active. A plane of symmetry passing through C-3 bisects the molecule. 25.14 (b) Arabinose is a reducing sugar; it will give a positive test with Benedict’s reagent, because its open-chain form has a free aldehyde group capable of being oxidized by copper(II) ion. (c) Benedict’s reagent reacts with -hydroxy ketones by way of an isomerization process involv￾ing an enediol intermediate. 1,3-Dihydroxyacetone gives a positive test with Benedict’s reagent. (d) D-Fructose is an -hydroxy ketone and will give a positive test with Benedict’s reagent. (e) Lactose is a disaccharide and will give a positive test with Benedict’s reagent by way of an open-chain isomer of one of the rings. Lactose is a reducing sugar. HOCH2 HO O O HO OH HOCH2 O OH OH HO HOCH2 HO O O HO OH HOCH2 OH OH CHO HO positive test; Cu2O formed Lactose (structure presented in Section 25.14) Open-chain form Benedict’s reagent positive test; Cu2O formed HO H H OH CH2OH CHOH H OH d-Fructose HO H H OH CH2OH O CH2OH C H OH Benedict’s reagent C H O C O CH2OH CH2OH 1,3-Dihydroxyacetone Enediol Glyceraldehyde C CH2OH H OH C OH CHOH CH2OH Benedict’s reagent positive test; Cu2O formed C H O CHO CH2OH H H H OH OH OH d-Ribose NaBH4 H2O Plane of symmetry Ribitol CH2OH CH2OH H H H OH OH OH 708 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website