Creative Chemistry: Carbohydrates

December 22, 2018

Carbohydrates

Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. Though often maligned in trendy diets,carbohydrates — one of the basic food groups — are important to a healthy life.

Classification of Carbohydrates
There are three categories of carbohydrates. The illustration below shows the three categories.

1.Monosaccharides:Monosaccharides, also called simple sugars, are the simplest form of sugar and the most basic units of carbohydrates. They cannot be further hydrolyzed to simpler chemical compounds

2.Disaccharides: A disaccharide (also called a double sugar or bivose) is the sugar formed when two monosaccharides (simple sugars) are joined by glycosidic linkage. Like monosaccharides, disaccharides are soluble in water. Three common examples are sucrose, lactose, and maltose.

3.Polysaccharides:a carbohydrate (e.g. starch, cellulose, or glycogen) whose molecules consist of a number of sugar molecules bonded together.

Carbohydrates and Biochemistry

Carbohydrates are compounds of tremendous biological importance:
They provide energy through oxidation
They supply carbon for the synthesis of cell components
They serve as a form of stored chemical energy
They form part of the structures of some cells and tissues
Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomoleculesbecause they are closely associated with living organisms.

The Stereochemistryof Carbohydrates

Two Forms of Glyceraldehyde.Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other.

Stereoisomers

• These forms are stereoisomers of each other.

• Glyceraldehyde is a chiral molecule — it cannot be superimposed on its mirror image. The two mirrorimage forms of glyceraldehyde are enantiomers of each other.

Chirality and Handedness

Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror.

Chiral Carbons

•Chiral objects cannot be superimposed on their mirror images —e.g., hands, gloves, and shoes.

•Achiralobjects can be superimposed on the mirror images —e.g., drinking glasses, spheres, and cubes.

•Any carbon atom which is connected to four different groupswill be chiral, and will have two nonsuperimposable mirror images; it is a chiral carbonor a center of chirality.

–If anyof the two groups on the carbon are the same, the carbon atom cannot be chiral.

•Many organic compounds, including carbohydrates, contain more than one chiral carbon.

Examples: Chiral Carbon

Identify the chiral carbons (if any) in the following carbohydrates:

Fischer Projections

• Fischer projections are a convenient way to represent mirror images in two dimensions.

• Place the carbonyl group at or near the top and the last achiral CH2OH at the bottom.

Naming Stereoisomers

When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest fromthe carbonyl group: if the hydroxy group points to right when the carbonyl is ―up‖ it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer.

A levorotatory(–) substance rotates polarized light to the left [e.g., l-glucose; (-)-glucose].
A dextrorotatory(+) substance rotates polarized light to the right [e.g., d-glucose; (+)-glucose].
Molecules which rotate the plane of polarized light are optically active.

Classification of Monosaccharides

The monosaccharides are the simplest of the carbohydrates, since they contain only one polyhydroxy aldehyde or ketone unit.
Monosaccharides are classified according to the number of carbon atoms they contain:

No.of carbons Classof Monosaccharide

3 triose

4 tetrose

5 pentose

6 hexose