JEE Main · 2021 · 27 Jul 2021 · Shift 1 · Biomolecules

Barfoed's reagent is copper(II) acetate in dilute acetic acid, and the mildly acidic medium slows the reduction down enough for the rate to become diagnostic. A monosaccharide is a strong enough reducing sugar to throw down red $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ within about half a minute, whereas a reducing disaccharide takes several minutes because it must first hydrolyse in the acidic medium. Tollens' reagent is reduced by both classes, Seliwanoff's test separates ketoses from aldoses and the iodine test detects polysaccharides, so none of these three tells a monosaccharide from a disaccharide. The test used for this distinction is Barfoed's test.