Question

why won't glyceraldehyde, ribose, or fructose react with lugol's?

Answer 1

Glyceraldehyde, ribose and fructose are monosaccharides. Both monosaccharides and disaccharides does not give any colour change with lugol's solution (Iodine in potassium iodide solution).

Lugol's solution is a test for polysaccharides. When lugol's solution is added to polysaccharides like starch, the iodine forms a coordinate complex with the helically coiled chain. The resulting complex of polyiodide in the central cavity of helix results in dark blue colour (in case of starch). The colour formed varies according to the length of the chain. For example glycogen gives reddish brown colour with lugol's.