Animated Solution for Chemistry - Organic Chemistry: Fructose and glucose can be distinguished by

Imagine you are standing in a chemistry lab with two unlabelled test tubes. One contains glucose, and the other contains fructose. Both are simple sugars, both are sweet, and both are vital for life. But structurally, they have a key difference: glucose is an aldohexose (containing an aldehyde group), while fructose is a ketohexose (containing a ketone group). The challenge is to find a chemical test that can reliably tell them apart.

This is where many students fall into a classic trap. You might immediately think of reaching for Fehling's solution, Benedict's reagent, or Barfoed's reagent. However, there is a catch here. Both glucose and fructose are reducing sugars.

Even though fructose is a ketone, under the alkaline conditions of Fehling's and Benedict's tests, it undergoes a process called tautomerization (via an enediol intermediate) to form glucose and mannose. As a result, both test tubes will happily reduce the $C{u}^{2+}$ ions to $C{u}^{+}$, yielding a beautiful brick-red precipitate of cuprous oxide ($C{u}_{2}O$). These tests confirm that you have reducing sugars, but they completely fail to distinguish between the two.

The Ketose Detector

To solve this mystery, we need a test that specifically targets the structural difference between an aldose and a ketose. Enter Seliwanoff's test.

Seliwanoff's reagent is a mixture of resorcinol (a phenol derivative) dissolved in concentrated hydrochloric acid ($HCl$). This test does not rely on the reducing property of the sugars; instead, it exploits their dehydration kinetics.

When we add Seliwanoff's reagent to our test tubes and apply heat, the concentrated $HCl$ acts as a powerful dehydrating agent. It strips water molecules away from the sugar molecules.

Here is the beautiful part: ketoses dehydrate much more rapidly than aldoses. Fructose quickly loses water to form a compound called hydroxymethylfurfural. This furfural derivative then rapidly condenses with the resorcinol present in the reagent to form a deep, vibrant cherry-red complex.

Because the dehydration of an aldose like glucose is a much slower and more difficult process, the glucose test tube will remain clear or only develop a faint pink tint after prolonged boiling. Fructose, on the other hand, will flash a brilliant red almost immediately upon heating.

Thus, Seliwanoff's test elegantly and definitively distinguishes fructose from glucose. As a bonus thought experiment, you could also use bromine water ($B{r}_2/H_{2}O$), a mild oxidizing agent that oxidizes the aldehyde group of glucose to gluconic acid but leaves the ketone group of fructose completely untouched!