1

16.5 Cyclic Structures of Monosaccharides16.6 Chemical Properties of

Monosaccharides

Chapter 16 Carbohydrates

o

OH

OH

CH2OH

OH

OH

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.

oCH2OH

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

OCH2OH

OHOH

OH

OH

OCH2OH

OHOH

OH

OH

5

Mutarotation In solution, -D-glucose is in equilibrium with

β-D-glucose. 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.

CH2OH

C

C

C

C

CH2OH

O

HO H

OHH

OHH

D-Fructose

CH2OH

OH

OH

OH

CH2OHO

CH2OH

OH

OH

CH2OH

OHO

-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 to

D-gluconic acid. Glucose is a reducing sugar.

C

C

C

C

C

CH2OH

HO

OHH

HO H

OHH

OHH

C

C

C

C

C

CH2OH

OHO

OHH

HO H

OHH

OHH

+ Cu2+

D-Glucose D-Gluconic acid

+ Cu2O(s)

8

Oxidation – Reduction Oxidation (LEO)

Loss of electrons Gain of O Loss of H (H+ & e)

Increase of the oxidation number (state)

Reduction Gain of electrons Loss of O Gain of H (H+ & e)

Decrease of the oxidation number (state)

9

Oxidation - Reduction In 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 SO4

2-), 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 SO4

2- 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 Electrons

Ag+ + e → Ag(+1) (0)

Loss of OxygenNO3

- + 2H+ + 2e → NO2- + H2O

(+5) (+3) Gain of Hydrogen

2 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. |

H Carbon dioxide, CO2 (O=C=O) – oxidation state of carbon is +4.

H |

Methanol, CH3OH (H – C – OH) – oxidation state of carbon is | H

3(-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

glycosidic bond

O

OH

OH

CH2OH

O

OH

CH3

HOCH3+

O

OH

OH

CH2OH

OH

OH