Presentation on theme: "17.5 Properties of Monosaccharides"— Presentation transcript:
1 17.5 Properties of Monosaccharides Physical propertiesChemical propertiesOxidationReductionFormation of ring (hemiacetal) structuresGlycoside (acetal) formation
2 Physical Properties Taste = sweet Solids at room temperature Water soluble (H-bonding)
3 Chemical Properties Review: Reactions of aldehydes and ketones Oxidation (aldehydes) to form carboxylic acidsReduction to form alcoholsFormation of hemiacetal/hemiketalHemiacetal/hemiketal + alcohol → acetal/ketalNow we’ll see all of these with monosaccharides…
4 1. Oxidation of Monosaccharides Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acidsBenedict’s reagent can oxidize aldehydes adjacent to -OH groupIn the Benedict’s text, D-glucose is oxidized to D-gluconic acid. Glucose is a reducing sugar.
5 2. Reduction of Monosaccharides The reduction of the carbonyl group produces sugar alcohols, or alditolsD-Glucose is reduced to D-glucitol (also called sorbitol)Used in sugar-free products
6 Learning CheckWrite the products of the oxidation and reduction of D-mannose.
7 3. Monosaccharides + alcohol Formation of hemiacetal/hemiketal (cyclic structures)
8 Hemiacetal Review What is a hemiacetal? How is a hemiacetal formed? What if the alcohol and carbonyl are attached?
9 Hexose hemiacetals Favor formation of 5- or 6-membered rings Hydroxyl group on C5 reacts with the aldehyde or ketoneThe Haworth structure can be written from the Fischer projectionThe D-isomer has the last CH2OH group located above the ringThe –OH group on the left (C3) is drawn upThe –OH groups on the right (C2, C4) are drawn downCarbonyl C becomes chiral (anomeric carbon); drawn on the right
10 and Anomers for D-Glucose Anomers are isomers which differ in placement of hydroxyl on anomeric CThe –OH is drawn down for the -anomer, and up for the -anomer-D-Glucose -D-GlucoseMashed potatoes or mashed paper?
12 MutarotationIn solution, -D-glucose is in equilibrium with β-D-glucoseMutarotation involves the conversion of the cyclic anomers into the open chainAt any time, there is only a small amount of open chainaldehyde
13 Cyclic Structure of Fructose As a ketohexose, fructose forms a cyclic structure when the —OH on C5 reacts with the ketone on C2Result is 5-atom ringAnomeric carbon is C2-D-Fructose-D-Fructose
15 Learning CheckWrite the cyclic form of -D-galactose:
16 4. Monosaccharide hemiacetals/ hemiketals + alcohol When a cyclic monosaccharide reacts with an alcohol:A glycoside is produced (acetal/ketal)The bond is a glycosidic bond or glycosidic linkageGlycosides do not exhibit open chain formsGlycosides are not reducing sugars
17 17.7 Disaccharides A disaccharide consists of two monosaccharides Disaccharide MonosaccharidesMaltose + H2O Glucose + GlucoseLactose + H2O Glucose + GalactoseSucrose + H2O Glucose + FructoseH+
18 Maltose Malt sugar Obtained from starch Used in cereals, candies, and brewingA disaccharide in which two D-glucose molecules are joined by an (1 4)glycosidic bondA reducing sugar (has a hemiacetal)
19 LactoseMilk sugarComposed of galactose and glucose linked by a (1 4) glycosidic bondLactose intoleranceA reducing sugar
20 SucroseTable sugarComposed of glucose and fructose joined by 1 2-glycosidic bondHas no isomers because mutarotation is blockedNot a reducing sugar (no hemiacetal)
21 Benedict’s Test on Disaccharides From left to right, the four testTubes contain Benedict’s reagent,2% maltose solution, 2% sucrosesolution, and 2% lactose solution.Both maltose and lactose arereducing sugars. The sucrose hasremained unreacted.
23 Learning CheckIdentify the monosaccharides in lactose, maltose, and sucrose as glucose, fructose, and/or galactose:A. LactoseB. MaltoseC. Sucrose
24 17.8 Polysaccharides Polysaccharides are polymers of monosaccharides “Complex” carbohydratesImportant polysaccharides of D-glucose are:Starch (Amylose and Amylopectin)GlycogenCellulose
25 Starch and Glycogen Storage polysaccharides Starch Glycogen Form monosaccharides used for energyStarchPlantsAmylose is a continuous chain of D-glucose molecules linked by (1 4)-glycosidic bonds.Amylopectin is a branched chain of D-glucose molecules linked by (1 4)- and (1 6)-glycosidic bonds.GlycogenHumans, animalsSimilar to amylopectin,but more highly branched.
28 Cellulose Structural polysaccharide Plant cell walls (cellulose) and animal exoskeletons (chitin)Cellulose is a linear polymer of glucose molecules linked by (1 4)-glycosidic bondsEnzymes in saliva can hydrolyze (1 4)-glycosidic bonds in starch, but not (1 4)-glycosidic bonds in cellulose
29 Learning Check Identify the types of glycosidic bonds in: 1) Amylose 2) Glycogen3) Cellulose