Aldonic acids aqueous bromine solution

When D-glucose is treated with an aqueous bromine solution (buffered to a pH of 6), an aldonic acid is formed called D-gluconic acid. Treatment of D-gluconic acid with an acid catalyst produces a lactone (cyclic ester) with a six-membered ring. 

Aldoses can be distinguished from ketoses by observing what happens to the color of an aqueous solution of bromine when it is added to the sugar. Br2 is a mild oxidizing agent and easily oxidizes the aldehyde group, but it cannot oxidize ketones or alcohols. Consequently, if a small amount of an aqueous solution of Br2 is added to an unknown monosaccharide, the reddish-brown color of Br2 will disappear if the monosaccharide is an aldose, but will persist if the monosaccharide is a ketose. The product of the oxidation reaction is an aldonic acid. 

The calcium salt of the aldonic acid necessary for the Ruff degradation is easily obtained by oxidizing an aldose with an aqueous solution of bromine and then adding calcium hydroxide to the reaction mixture. 

The most commonly used method for preparing aldonic acids is by oxidation with bromine in aqueous solution. The species that is oxidized is a furanose or pyranose form of the carbohydrate. 

Aldonic acids are made on a preparative scale by oxidation of aldoses with bromine in buffered aqueous solution (pH = 5-6). For example, o-mannose yields D-mannonic acid in this way. Upon subsequent evaporation of solvent from the aqueous solution of the aldonic acid, the y-lactone (Section 20-4) forms spontaneously. 

The aldehydic or hemiacetal group is the most easily oxidised portion of the monosaccharide molecule. All aldopyranoses (CLVIII) are oxidised with aqueous buffered bromine solution (pH 5.6). The initial product is the 1,5-lactone (CLIX) which then hydrolyses to the aldonic acid (CLX). Evaporation of the aqueous solution yields the y-lactone (CLXI), but from certain solvents such as p uidine which inhibit lactone formation the... 

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