ORGANIC CHEMISTRY II CHEM 2325

CARBOHYDRATES

INTRODUCTION 

Carbohydrates are essentially carbon + water ..... C_n(H₂O)_n

Examples of carbohydrates are sugars, starch, cellulose, and glycogen produced in nature by photosynthesis.

Carbohydrates in organic terms are polyhydroxy aldehydes and ketones (or compounds that can be converted into one of these by hydrolysis).

CLASSIFICATIONS

1. Number of aldose/ketose produced on complete hydrolysis.

2. Number of carbons

3. Configuration of -OH attached to last chiral carbon.

HYDROLYSIS PRODUCTS (splitting with H₂O)

monosaccharides (simple sugars): single unit cannot be hydrolyzed

disaccharides: yield two monosaccharides on hydrolysis

oligosaccharides: 3 to 10 monosacharides on hydrolysis

 

polysaccharides: more than 10 monosacharides on hydrolysis

Number of carbon atoms in monosaccharides

In aldoses, the carbonyl group is the number 1 carbon.

In ketoses, the carbonyl group is the number 2 carbon.

Configurational Isomers: (D and L)

D conformation: the chiral -OH points right

L conformation: the chiral -OH points left

the chiral carbon that determines the D or L configuration is the last chiral carbon furthest from the carbonyl group.

PHYSICAL PROPERTIES OF MONOSACCHARIDES

• Highly soluble in H₂O

• Higher boiling points (-OH bond strongly) (normally will decompose at 760 mm pressure)

• Sweet and white

CYCLIC FORMS OF GLUCOSE

Fischer Representation

D and L have different physical properties but similiar chemical properties (except toward enzymes).

Haworth Representation

All -OH to right of chiral center in a Fischer representation point down in the Haworth representation and all to the left in the Fischer point up in the Haworth.

The -CH₂OH is up in the D configuration and down in the L configuration in a Haworth representation.

Five membered Sugar Rings

Glycosides

DISACCHARIDES

Disaccharides are classified using three characteristics:

• monosaccharide components

• glyosodic linkage

• whether it is a reducing sugar* or not

* a sugar on a diet (reference: Margaret Stafford Wheet, BS,MS,RD)

POLYSACCHARIDES

Cellulose is a major component of the cell walls of plants and is a linear b (1, 4) glucose based polysaccharide.

Starch consists of 20% amylose and 80% amylopectin.

Amylose is a linear a (1,4) glucose polysaccharide.

Amylopectin is a branched a (1,6) glucose polysaccharide which branches every 25 - 30 units.

Glycogen is a a (1,6) glucose polysaccharide similiar to amylopectin but branches every 8 - 10 units.