Carbohydrate classification

What counts as a carbohydrate. A carbohydrate is an optically active polyhydroxy aldehyde or polyhydroxy ketone, or a substance that yields such a compound on hydrolysis. Many simple sugars are loosely described by the formula

where: $n$ is the number of carbon atoms, and the formula is only a historical mnemonic (literally "hydrate of carbon"), not a definition — acetic acid, $\text{C}_2\text{H}_4\text{O}_2$, fits the pattern but is not a carbohydrate, while deoxyribose, a genuine carbohydrate, does not fit it exactly. The real test is structural: is there a polyhydroxy aldehyde/ketone, or something that hydrolyses to one?

Sorting by the carbonyl group. Every monosaccharide carries one of two possible carbonyl functions. An aldose carries an aldehyde group, $-\text{CHO}$, as in glucose; a ketose carries a ketone group, $\text{C}=\text{O}$, flanked by carbon on both sides, as in fructose. Combine this with the carbon count and you get compact names: glucose ($\text{C}_6\text{H}_{12}\text{O}_6$, aldehyde) is an aldohexose; fructose (same formula, ketone) is a ketohexose; ribose (5 carbons, aldehyde) is an aldopentose. This carbonyl split is independent of size — it applies only to the free-sugar (open-chain) form and is the first fork in any classification tree.

Sorting by hydrolysis — how many sugar units come out. The second, and more commonly tested, classification asks how many monosaccharide units a carbohydrate breaks into on hydrolysis. A monosaccharide cannot be hydrolysed into a simpler polyhydroxy carbonyl unit — glucose, fructose, and ribose are the standard examples, typically carrying 3 to 7 carbon atoms. An oligosaccharide yields 2 to 10 monosaccharide units, joined by glycosidic linkages: a disaccharide gives 2 (sucrose to glucose + fructose; maltose to glucose + glucose; lactose to glucose + galactose), a trisaccharide gives 3 (raffinose), and so on up to 10 units. A polysaccharide yields a very large number of units — hundreds to thousands — on complete hydrolysis. Starch, cellulose, and glycogen are all polymers of glucose alone, yet they differ sharply in linkage pattern and branching, which is why one is digestible energy storage (starch, glycogen) and another is structural fibre (cellulose). Polysaccharides are, almost without exception, insoluble in water and not sweet — a direct consequence of their enormous size and the way thousands of hydroxyl groups hydrogen-bond within and between chains rather than to surrounding water.

The reducing/non-reducing split is not a separate, arbitrary label — it follows directly from the ring-chain equilibrium every sugar sits in. In solution, a monosaccharide's ring form is in equilibrium with a small amount of open-chain form carrying a genuine free carbonyl group ($-\text{CHO}$ or $-\text{C(=O)}-$) at what is called the anomeric carbon (C1 in an aldose, C2 in a ketose). Mild oxidising agents such as Tollens' reagent ($\text{Ag(NH}_3)_2^+$) and Fehling's solution ($\text{Cu}^{2+}$ in alkaline sodium potassium tartrate) can only be reduced by that free carbonyl form, so any sugar whose anomeric carbon is not locked away gives a positive test — it is a reducing sugar. Every monosaccharide, aldose or ketose, is reducing, because every monosaccharide has a free anomeric carbon by definition (it has not yet formed a glycosidic bond with anything). This is why fructose — a ketone, not an aldehyde — still reduces Fehling's solution: under the alkaline test conditions it isomerises through an enediol intermediate into an aldose-like form that carries the required free carbonyl. For disaccharides, the outcome depends entirely on how many of the two monosaccharide units' anomeric carbons are consumed in forming the glycosidic bond. In maltose and lactose, only one of the two anomeric carbons is used to link the units, leaving the other free — both are reducing sugars. In sucrose, the glycosidic bond forms between C1 of glucose and C2 of fructose — precisely the two anomeric carbons of the two units — so both are tied up and no free carbonyl remains anywhere in the molecule: sucrose is non-reducing. Polysaccharides such as starch technically retain one free reducing end among many thousands of linked units, but that single end is chemically negligible against the bulk of the molecule, so starch is treated as non-reducing for all practical and exam purposes.

Question. Classify glucose, sucrose, and starch by (a) carbonyl type, (b) hydrolysis category, and (c) reducing/non-reducing behaviour, and predict what each shows on heating with Fehling's solution.

Step 1. Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) has a free aldehyde group, so it is an aldose (specifically an aldohexose). It cannot be hydrolysed into a simpler carbonyl unit, so it is a monosaccharide. Its anomeric carbon (C1) is free to open to $-\text{CHO}$, so it is a reducing sugar.

Step 2. Sucrose hydrolyses to one glucose plus one fructose, so it is a disaccharide (oligosaccharide, 2 units). Its glycosidic bond joins glucose C1 to fructose C2 — both anomeric carbons — leaving no free carbonyl anywhere, so it is non-reducing.

Step 3. Starch hydrolyses to a very large number of glucose units, so it is a polysaccharide. With only one free reducing end spread across thousands of units, its reducing power is negligible; it is treated as non-reducing and, unlike glucose or sucrose, it is not sweet.

Step 4. On heating with Fehling's solution ($\text{Cu}^{2+}$, alkaline tartrate, pale blue), glucose gives a brick-red precipitate of $\text{Cu}_2\text{O}$ as its free aldehyde is oxidised to gluconic acid — a clearly positive test. Sucrose and starch, having no accessible free carbonyl, give essentially negative tests — no colour change on heating.

Final answer: glucose is a reducing aldohexose (monosaccharide, positive Fehling's); sucrose is a non-reducing disaccharide (negative Fehling's); starch is a non-reducing polysaccharide (negative Fehling's).

Competitive stretch

If the same three powders were tested with Tollens' reagent instead, the pattern would be identical: glucose deposits a silver mirror (positive), while sucrose and starch show no mirror (negative). This is the standard lab method for distinguishing a reducing sugar from a non-reducing carbohydrate, and it works because Tollens' and Fehling's both require the same free carbonyl group — they simply detect it by different colour changes (silver mirror versus brick-red precipitate).

• Treating $\text{C}_n(\text{H}_2\text{O})_n$ as the definition of a carbohydrate rather than a loose historical mnemonic — acetic acid fits the formula but is not a carbohydrate, and deoxyribose is a carbohydrate that does not fit it.
• Assuming only aldoses can be reducing sugars. Ketoses like fructose are reducing too, because they isomerise to an aldose-like enediol form under the alkaline conditions of Fehling's/Tollens' tests.
• Equating "reducing sugar" with "tastes sweet." Sweetness and reducing power are unrelated properties — sucrose is intensely sweet but non-reducing, while some reducing sugars are only mildly sweet.
• Forgetting that a disaccharide's reducing behaviour depends on whether both anomeric carbons are used in the glycosidic bond (sucrose: both used, non-reducing) or only one (maltose/lactose: one free, reducing) — this is the single fact that decides the whole classification.

Ask the tutor: want to see how the glycosidic bond forms step by step between glucose and fructose to make sucrose, or work through a few more reducing-vs-non-reducing classification examples?