Presentation on theme: "Chapter 16 Carbohydrates"— Presentation transcript:
1 Chapter 16 Carbohydrates Cyclic Structures of MonosaccharidesChemical Properties ofMonosaccharides
2 Cyclic Haworth Structures Stable cyclic hemiacetals form when the C=O group and the-OH are part of the same molecule.For hexoses, the hydroxyl group on C-5 reacts with the aldehyde group or ketone group.The cyclic structure of a D-isomer has the last CH2OH group located above the ring.
3 Formation of Cyclic Glucose The Haworth structure can be written from theFischer Projection.The –OH group on the left (C3) is drawn up.The –OH groups on the right (C2, C4) are drawn down.
4 and Anomers for D-Glucose The new –OH on C1 is drawn down for the anomer, and up for the anomer.-D-Glucose -D-Glucose
5 Mutarotation In solution, -D-glucose is in equilibrium with Mutarotation involves the conversion of the cyclic anomers into the open chain.At any time, there is only a small amount of open chain.-D-glucose D-glucose (open) β-D-glucose(36%) (trace) (64%)
6 Cyclic Structure of Fructose As a ketohexose, fructose forms a cyclic structure when the —OH on C-5 reacts with the C=O on C-2.-D-Fructose-D-Fructose
7 Oxidation of Monosaccharides Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.In the Benedict’s text, D-glucose is oxidized toD-gluconic acid. Glucose is a reducing sugar.
8 Oxidation – Reduction Oxidation (LEO) Loss of electronsGain of OLoss of H (H+ & e)Increase of the oxidation number (state)ReductionGain of electronsLoss of OGain of H (H+ & e)Decrease of the oxidation number (state)
9 Oxidation - ReductionIn simple ionic compounds, the chemical bond is formed by a complete transfer from the more electropositive to the more electronegative element – the actual charge of the ion is equal to its oxidation number (e.g. NaCl – the oxidation number of Na+ is +1 and that of Cl- is -1.In covalent compounds (such as CH4) and in polyatomic ions with covalent bonds (such as SO42-), electrons are shared between bonded atoms. For calculations of oxidation numbers, the electrons are assigned completely to the more electronegative atom (electron hog). In CH4 the oxidation number of C is -4 and that of H is +1. In SO42- the oxidation number of S is +6 and that of O is -2. The sum of the oxidation numbers in the polyatomic ion is equal to the charge of the ion. The sum of the oxidation numbers in the compound is equal to zero. (In most compounds hydrogen is assigned the oxidation number of +1 and oxygen is -2.)For an element (Ag, Na, Cl2, O2, etc.), the oxidation number of each atom is equal to zero.
10 Half – reactions (Reduction) Gain of ElectronsAg+ + e → Ag(+1) (0)Loss of OxygenNO3- + 2H+ + 2e → NO2- + H2O(+5) (+3)Gain of Hydrogen2 CO2 + 2H+ + 2e → H2C2O4(+4) (+3)
11 Oxidation – Reduction in organic chemistry Reduction of an organic molecule usually corresponds to increasing its hydrogen content or to decreasing its oxygen content.Oxidation is the opposite of reduction, thus increasing the oxygen content of organic molecule or decreasing its hydrogen content is an oxidation.
12 Method of assigning an oxidation state to a carbon atom of an organic compound: Base the assignment on the groups attached to carbon.A bond to hydrogen (or anything less electronegative than carbon) makes it -1.A bond to oxygen (or anything more electronegative than carbon, like nitrogen or halogen) makes it +1.A bond to another carbon makes it 0.H|Methane, CH4 (H – C – H) – oxidation state of carbon is -4.Carbon dioxide, CO2 (O=C=O) – oxidation state of carbon is +4.Methanol, CH3OH (H – C – OH) – oxidation state of carbon is3(-1)+1=-2.
13 Oxidation of aldehyde to carboxylic acid R – C = O → R – C = O (oxidation)| |H OH(+1) (+3)Cu2+ + 1e → Cu1+ (reduction)(+2) (+1)
14 Reduction of Monosaccharides The reduction of the carbonyl group produces sugar alcohols, or alditols.D-Glucose is reduced to D-glucitol also called sorbitol.
15 Glycosides and Glycosidic Bonds When a cyclic monosaccharide reacts with an alcohol:A glycoside is produced.The bond is a glycosidic bond.-D-Glucose Methanol Methyl--D-glucoside