Carbohydrates

The detailed Organic Chemistry lesson is now available as an individual lesson download!
Click here for more information!

Overview:

Carbohydrates introduces the organic student to the classification of these compounds. Also included are cyclic structures, D & L, d & l designations, reducing sugars, reactions, polysaccharides, and a brief overview of tooth decay.

The unit opens with classifications of carbohydrates and a series of important terms the student must learn. Terms such as erythro, threo, epimers, anomers, Haworth structures, Fischer projections, levulose, dextrose, and reducing sugars are explained. Classification addressed include distinguishing between the characteristic functional groups and the number of carbons involved in these saccharides.

Notations such as D and L, as well as d and l are introduced and explained. Students are then able to make those identifications except where there is the requirement of a polarimeter. Cyclic structures are introduced in a manner that facilitates transitioning from a Fischer projection to a Haworth projection to the chair conformation and knowing where each attached group appears.

The physical properties of alpha-D-glucose and beta-D-glucose are discussed, particularly those structures giving rise to different rotations of polarized light. The concept of mutarotation is presented.

Reactions of monosaccharides is a section that covers epimerization, reduction with agents such as sodium borohydride, oxidation with bromine water, oxidation with nitric acid, and reactions with Tollens, etc. In each of these cases classification and nomenclature of the product are studied. Thus the student becomes familiar with terms such as alditols, aldonic acids, and aldaric acids.

Clarification among the commonly used terms like monosaccharides, disaccharides, oligosaccharides, and polysaccharides is made. The way in which monosaccharides bond to form disaccharides is discussed and the resulting properties when the acetal structure is formed rather than the hemiacetal structure is noted.

Several important polysaccharides are discussed. The structure of cellulose is noted and is accompanied by explanation of why it cannot be digested by humans. Starch with its two forms is noted and the student is presented with the differences between amylose and amylopectin. It is noted that hydrolysis of amylose produces D-glucose as the only monosaccharide and mannose as the only disaccharide. The similarities between glycogen structure and the structure of amylopectin are noted.

A brief section on tooth decay is presented as a way of helping the organic student further see the application of this study to daily life. The breakdown of sucrose in the presence of glucosyl transferase eventually results in the formation of dextran, a sticky polymer that binds lactic acid from sucrose and incoming bacteria into the nefarious dental plaque.