Which one among the following chemical tests is used to distinguish monosaccharide from disaccharide?

First, let us recall that a monosaccharide is a simple sugar having the general formula $$\mathrm{C_nH_{2n}O_n}$$ and contains a free, easily oxidisable $$-CHO$$ or $$-CO-$$ group. A disaccharide, on the other hand, consists of two monosaccharide units joined through a glycosidic bond; this linkage generally makes the reducing group less reactive or, if both anomeric centres are involved, completely non-reducing.

To differentiate between these two classes, chemists have designed specific qualitative tests. We need a test whose underlying chemistry is sensitive to the rate at which the sugar can reduce cupric ions or some other oxidant. A reaction that occurs rapidly with monosaccharides but either proceeds slowly or not at all with disaccharides will serve this purpose.

We examine the given options one by one:

We have Seliwanoff’s test. This is based on the dehydration of keto-sugars by concentrated $$H_{2}SO_{4}$$ to form hydroxymethylfurfural, which then condenses with resorcinol to give a cherry-red colour. The test is therefore specific for distinguishing aldo- and keto-hexoses, not mono- versus disaccharides. Hence Seliwanoff’s test is not suitable for our requirement.

Now consider the iodine test. Iodine forms a deep blue complex with polysaccharides such as starch; with simple sugars no characteristic colour is produced because the short chains cannot accommodate iodine within a helical cavity. Therefore the iodine test differentiates polysaccharides from mono- or disaccharides, not monosaccharides from disaccharides. So the iodine test is also unsuitable.

Next we come to Barfoed’s test. First, we state the principle. Barfoed’s reagent is an acidic solution of cupric acetate, $$Cu(CH_{3}COO)2$$, maintained at a pH of about $$4.5$$ with acetic acid. Under these mildly acidic conditions, the following reduction takes place:

$$2Cu^{2+ + R-CHO + 2H2O \longrightarrow Cu2O \downarrow + R-COOH + 4H^+}$$

Because the medium is acidic, only those sugars that are strong reducing agents can reduce $$Cu^{2+}$$ quickly enough to precipitate cuprous oxide $$Cu_{2}O$$ within the usual experimental time (about $$2$$-$$3$$ minutes of boiling). Monosaccharides possess a free carbonyl group and reduce the reagent rapidly, producing a brick-red precipitate. Disaccharides, being less powerful reducers in acid medium, either fail to give a precipitate or do so only after prolonged heating. This distinct difference allows us to distinguish between the two.

Finally, Tollen’s test employs an ammoniacal silver nitrate solution. It detects compounds that can reduce $$Ag^+$$ to metallic silver, giving the famous silver mirror. Many monosaccharides and some reducing disaccharides such as maltose and lactose answer this test. Therefore Tollen’s reagent cannot reliably tell the two classes apart.

From the above discussion, only Barfoed’s test fulfils the criterion of differentiating monosaccharides from disaccharides on the basis of the rate of reduction in an acidic medium.

Hence, the correct answer is Option C.