Why Glucose and Fructose Share a Formula but Fold Into Different Rings

Rice, roti, sugar, fruit and even the cotton in a shirt are made largely of carbohydrates — polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis. They fuel the body, store energy in plants and build plant cell walls.

This lesson covers how carbohydrates are classified, the structures of glucose and fructose, and the difference between reducing and non-reducing sugars.

Carbohydrates are classified by how many simple sugar units they give on hydrolysis: monosaccharides cannot be hydrolysed further, disaccharides give two monosaccharide units, and polysaccharides give many.

Monosaccharides:

- The simplest sugars, which cannot be broken into smaller carbohydrates
- Grouped by carbonyl type into aldoses, such as glucose, and ketoses, such as fructose
- Grouped by number of carbons into trioses, pentoses such as ribose, and hexoses such as glucose, fructose and galactose, all ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$

Disaccharides — two monosaccharides joined by a glycosidic linkage, with loss of water:

- Sucrose: glucose + fructose, ordinary table sugar
- Maltose: glucose + glucose, formed when starch is digested
- Lactose: glucose + galactose, the sugar in milk

Polysaccharides — long chains of monosaccharide units:

- Starch — the storage carbohydrate of plants, made of unbranched amylose and branched amylopectin
- Glycogen — the highly branched storage carbohydrate of animals, kept in the liver and muscles
- Cellulose — unbranched chains of beta-glucose that form plant cell walls

Monosaccharides and disaccharides are sweet, crystalline and soluble, so they are called sugars; polysaccharides are tasteless and mostly insoluble.

Worked example — hydrolysing sucrose:

$${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$$

So 342 g of sucrose and 18 g of water give 180 g of glucose and 180 g of fructose.

An everyday example. Cotton grown in Gujarat and Maharashtra is mostly cellulose, built from the same glucose units as the starch in rice but joined in a different way.

The substance. Starch and cellulose are both made of glucose, yet humans digest only starch — our enzymes break the alpha links of starch but not the beta links of cellulose.

**Glucose is an aldohexose and fructose a ketohexose with the same formula, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$; both exist mainly as rings in solution, and any sugar with a free or potentially free aldehyde or ketone group is a reducing sugar.

Open-chain structure of glucose — six carbons in a straight chain, with an aldehyde at carbon 1 and -OH groups on carbons 2 to 6. The evidence:

- Heating with HI gives n-hexane, so the chain is unbranched
- It forms an oxime and a cyanohydrin, so it has a carbonyl group
- Bromine water oxidises it to gluconic acid, so the carbonyl is an aldehyde
- Acetylation gives a pentaacetate, so it has five -OH groups

Why a ring is needed. Glucose does not give Schiff's test or a hydrogensulphite adduct, showing that its aldehyde group is mostly tied up.

Ring structures:

- In glucose, the -OH on carbon 5 adds to the aldehyde, forming a six-membered pyranose ring
- Carbon 1 becomes a new chiral centre, giving two anomers, alpha and beta, which interconvert in water by mutarotation
- In fructose, a ketohexose with its ketone at carbon 2, the -OH on carbon 5 adds to the ketone, forming a five-membered furanose ring

Reducing and non-reducing sugars:

- Reducing sugars reduce Tollens' and Fehling's reagents: all monosaccharides, maltose and lactose
- Non-reducing sugars have no free anomeric carbon: sucrose, whose glycosidic link joins carbon 1 of glucose to carbon 2 of fructose

Worked example — invert sugar.** Hydrolysed sucrose gives equal amounts of glucose, $+52.5^{\circ }$, and fructose, $-92.4^{\circ }$. The mixture's rotation is about $\frac{52.5+(-92.4)}{2}=-20^{\circ }$, turning from right to left — hence the name invert sugar.

An everyday example. Mithai makers add a little lemon juice when boiling sugar syrup, because the acid partly hydrolyses sucrose into invert sugar, which keeps the syrup from crystallising.

The substance. Sucrose is non-reducing because of how its units join, not because of what they are — glucose and fructose on their own are both reducing sugars.