Reducing Reactions of Carbohydrates

Aldehydes in general will reduce an ammoniacal solution of silver ion to metallic silver (Tollen s test). Most carbohydrates are aldehydes or ketones. The free aldehyde or ketone usually exists in very small amounts. It is the aldo or keto group, however, that usually reacts, and as it reacts, the other forms (hemiacetals or hemiketals) are converted into the free aldehyde or ketone forms. The conversion is catalyzed by alkali. The oxidation of the aldehyde is conducted under alkaline conditions of pH 10-11 and is catalyzed by metal ions such as Ag+, Cu, or Fe .  

Several macro or titration methods involve the heating (usually in a boiling water bath) of an aqueous solution of the carbohydrate for a specified period of time with an alkaline solution of complexed cupric ion. After cooling, an excess amount of sodium iodide solution is added, which is oxidized to iodine by the unreacted cupric ion. The iodine is titrated with a standard solution of sodium thiosulfate (Schoorl method [19]). 

Name of test Acid Phenol Carbohydrate determined Type of test Sensitivity Color and absorbance Ref. 

Molisch HjSO, a-Naphthol All carbohydrates Qualitative 10 xg/mL Purple [9] 

Anthrone HjSO, Anthrone All carbohydrates Quantitative 50 xg/mL Blue-green (600 nm) [13] 

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