Lactose structural formula

Figure 2.2 Structural formulae of a- and /3-lactose, (a) Fischer projection, (b) Haworth projection and (c) conformational formula.

Figure 6.1. Structural formula of a-lactose. (From Herrington 1934. Reprinted with permission from the Journal of Dairy Science 17(7), 533-542.)...

A carbohydrate was isolated from a sample of cow s milk. The substance was found to have a molecular mass of 342. The unknown carbohydrate can be hydrolyzed to form two isomeric compounds, each with a molecular mass of 180. Using the Rule of Thirteen, determine a molecular formula and an index of hydrogen deficiency for the unknown and for the hydrolysis products. Hint Begin by solving the molecular formula for the 180-amu hydrolysis products. These products have one oxygen atom for every carbon atom in the molecular formula. The unknown is lactose. Look up its structure in The Merck Index and confirm its molecular formula.)... 

Due to their ease of isolation and purification, sucrose, lactose (milk sugar), starch, cotton cellulose, glucose, and fructose were among the first to be studied, and their empirical composition was found to correspond to the general formula Cn(H20). Since structural chemistry and the existence of hydroxyl groups and hydrogen as structural elements was unknown at the time, the substances were looked upon quite naturally as compounds of carbon and water, and were termed carbohydrates (French, hydrates de carbone). 

We wll use lactose as an example of how the structure of a disaccharide can be unraveled. The first important observation is that (-l-)-lactose, a disaccharide haring the formula C12H22O11, can be hydrolyzed in acid to a pair of simple aldohexoses D-glucose and D-galactose (Fig. 22.59). This observation identifies the two monosaccharides making up (-l-)-lactose. 

The compound 3,5-dihydroxy-2-methyl-5,6-dihydropyran-4-one (V in Formula 4.67) is also formed from the pyranoid hemiacetals of l-deoxy-2,3-hexodiulose (Formula 4.75). In comparison, maltol is preferentially formed from disaccharides like maltose or lactose (Formula 4.76) and not from dihydroxypyranone by water elimination. The formation of maltol from monosaccharides is negligible. A comparison of the decomposition of 1-deoxyosones from the corresponding cyclic pyranone structure clearly shows (cf. Formula 4.75 and 4.76) that the glyco-sidically bound carbohydrate in the disaccharide directs the course of water elimination in another direction (Formula 4.76). It is the stabilization of the intermediates to quasi-aromatic maltol which makes possible the cleavage of the glycosidic bond with the formation of maltol. Parallel to the formation of maltol, isomaltol derivatives which still contain the second carbohydrate molecule are also formed from disaccharides (Formula 4.77). Indeed, the formation of free isomaltol is possible by the hydrolysis of the... 

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