Chemical Properties

CHEMICAL PROPERTIES 1. Glycoside formation Carbohydrate acetals, also call glycosides are formed when monosaccahrides react with alcohol (symbol as ROH) in gaseous hydrogen chloride. The reaction results in the formation of anomeric R acetals. For example, when we put methanol into a solution of D-(+)-glucose with hydrogen chloride, it leads to the formation of anomeric methyl actals.

FIGURE : The formation of methyl α-D-Glucopyranoside (source : http://www.ncbi.nlm.nih.gov/) An acetal of glucose is called glucoside, acetal of fructose is fructoside, and so on. Beacsue glucosides are acetals, they are stable in basic solution. However, in acidic solution, they undergo hydrolysis to produce a sugar and an alcohol. 2. Oxidation reactions a. With Benedict’s reagent Benedict’s reagent is a basic solution that contain a cupric citrate complex ion. When we oxidized an aldose in Benedict’s solution, the reaction forms a brick-red precipitates of Cu2O. In basic solution, ketoses are converted to aldoses, so ketoses are also oxidized by Benedict’s solution.

b. Tollens’ silver mirror test Mixing aqueous silver nitrate with aqueous ammonia produces a solution called Tollen’s reagent. AgNO3 + 3NH3 + H2O  [Ag(NH3)2]OH + NH4NO3 Although the ion Ag(NH3)2+ is a very weak oxidizing agent, it oxidizes aldehydes group of one monosaccharides to carboxylate anions. Silver is reduced from the +1 oxidation to metallic silver.

FIGURE : The reaction of a monosaccharide with Tollens’ reagent. (http://www.uni-r.de/) Sugars that give positive tests with Tollens’ or Benedict’s solutions are called reducing sugar. All carbohydrates that contain a hemiacetal of hemiketal group give positive test. c. With bromine water

Bromine water is general reagent that oxidizes the aldehyde group of sugar to carboxyl group. In other words, the reaction converts an aldose to an an aldonic acid.

FIGURE : The formation gluconic acid from the reaction of glucose and Bromine water ( source: http://www.grossmont.net/tomolmstead/) d. With nitric acid Since nitric acid is an oxidizing agent which stronger than bromine water, it oxidizes both the –CHO group and the terminal –CHO group of an aldose to –COOH groups. The reactions converts an aldose to aldaric acid.

FIGURE : The reaction between aldose and HNO3 to form aldaric acid ( source: http://www.grossmont.net/tomolmstead/) 3. Reduction reactions Alsoes and ketoses can be reduced with sodium borohydride or hydrogen with catalized platinum/nickel to compounds called alditols. For example, when D-glucose is reduced, we have D-glucitol or common name as sorbitol.

/ FIGURE : The formation of sorbitol (source:http://www.grossmont.net/tomolmstead/) 4. Esters formation Monosaccharides can react with acetic anhydride and a catalyzed weak base ( i.e pyrimidine or sodium acetate). It convets all the hydroxyl groups to ester groups. For example, β-D-glucopyranose is converted into its pentaacetate by treating with acetic anhydride in pyridine solution.

FIGURE: The formation of pentaacetate. (source: McMurry - Organic Chemistry 7ed) e. Fermentation Fermentation involves microorganisms which consume sugars as a food source. Glucose fermentation results in four major products: additional yeast cells, ethanol, carbon dioxide and heat. e → C6H12O6  En zym 2C2H5OH + 2CO2