Chapter 1 - The ‘Nuts and Bolts’ of Carbohydrates

Abstract

Nowadays, the definition of what is a carbohydrate has been much expanded to include oxidized or reduced molecules and those that contain other types of atoms (often nitrogen). The term sugar is used to describe monosaccharides and the somewhat higher molecular weight di- and trisaccharides. As the name implies, an empirical formula C.H2O (or CH2O) was often encountered, with molecular formulae of C5H10O5 and C6H12O6 being the most common. The appreciable solubility of these molecules in water was commensurate with the presence of hydroxyl groups, and there was often evidence for the carbonyl group of an aldehyde or ketone. These polyhydroxylated aldehydes and ketones were termed aldoses and ketoses, respectively, with the more common members referred to as aldopentoses/aldohexoses and ketopentoses/ketohexoses. Very early on, it became apparent that larger molecules existed that could be converted, by hydrolysis, into smaller and more common units—monosaccharides from polysaccharides. Oxidation was an operationally simple task for the early German chemists. The aldoses, apart from showing the normal attributes of a reducing sugar (forming a beautiful silver mirror when treated with Tollens' reagent or causing the precipitation of brick-red cuprous oxide when subjected to Fehling's solution), were easily oxidized by bromine water to carboxylic acids, termed aldonic acids. (+)-Glucose was a reducing sugar that could be oxidized to gluconic acid with bromine water and to glucaric acid with dilute nitric acid. That the six carbon atoms were in a contiguous chain had been shown by Kiliani: the conversion of (+)-glucose into a mixture of heptonic acids (by conventional Kiliani extension), followed by the treatment of this mixture with red phosphorus and hydrogen iodide (strongly reducing conditions), gave heptanoic acid.