Why Table Sugar Fails the Test That Glucose Passes

Rice, rotis, fruit and even the cotton in your clothes are made mostly of carbohydrates. Some are small, sweet sugars that give quick energy; others are huge polymers that store energy or build plant cell walls.

This part covers classifying carbohydrates, the structures of glucose and fructose, the disaccharides, and the polysaccharides starch, cellulose and glycogen.

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that hydrolyse to them; they are classified by how many units they give on hydrolysis, by whether the carbonyl group is an aldehyde or a ketone, and by whether they reduce Tollens' reagent and Fehling's solution.

By hydrolysis:

- Monosaccharides — cannot be hydrolysed further, such as glucose and fructose
- Oligosaccharides — give two to ten units; disaccharides such as sucrose give two
- Polysaccharides — give very many units, such as starch and cellulose

By carbonyl group:

- Aldoses — an aldehyde group, as in glucose, an aldohexose
- Ketoses — a ketone group, as in fructose, a ketohexose

By reducing power:

- Reducing sugars — have a free aldehyde or ketone group; all monosaccharides, maltose and lactose
- Non-reducing sugars — both carbonyl groups are tied up in the glycosidic bond, as in sucrose

Worked example. A sugar C${}_6$H${}_{12}$O${}_6$ with a ketone group and five –OH groups is a ketohexose, like fructose. Its molar mass is $6(12)+12(1)+6(16)=180$ g mol${}^{-1}$.

An everyday example. Honey is rich in glucose and fructose, both monosaccharides, which is why it supplies energy without needing to be broken down first.

The substance. Not every carbohydrate is sweet — starch and cellulose have no sweet taste at all.

**Glucose is made by hydrolysing sucrose or starch with dilute acid; its open-chain form is a six-carbon aldohexose, but in solution it exists mainly as a six-membered pyranose ring formed when the C5 –OH adds to the aldehyde, giving $\alpha$ and $\beta$ anomers, while fructose forms a five-membered furanose ring.

Preparation:

- From sucrose — boiling with dilute HCl in alcohol gives glucose and fructose
- From starch** — boiling with dilute H${}_2$SO${}_4$ at $393$ K under pressure

Evidence for the open chain:

- HI gives n-hexane — six carbons in a straight chain
- Hydroxylamine gives an oxime — a carbonyl group is present
- Bromine water gives gluconic acid — the carbonyl is an aldehyde
- Acetylation gives a pentaacetate — five –OH groups

Evidence for a ring. Glucose does not give the hydrogensulphite addition product or react with 2,4-DNP, so its aldehyde group is mostly locked in a ring formed by C5 –OH adding to C1 — the pyranose form.

D/L and anomers:

- D means the –OH on the lowest chiral carbon points to the right, as in D-glyceraldehyde
- Anomers differ only at C1: **$\alpha$-D-glucose and $\beta$-D-glucose
- Fructose closes into a furanose ring as its C5 –OH adds to the C2 ketone

Worked example.** Glucose pentaacetate replaces five –OH hydrogens with CH${}_3$CO groups, each adding $42$ g mol${}^{-1}$:

$$180+5\times 42=390{\text{ g mol}}^{-1}$$

An everyday example. Glucose drips in hospitals contain both anomers, which interconvert freely in solution.

The substance. Ring and chain forms of glucose interconvert in water, which is why glucose still reduces Tollens' reagent despite being mostly cyclic.

**Disaccharides join two monosaccharides through a glycosidic linkage: sucrose links C1 of $\alpha$-D-glucose to C2 of $\beta$-D-fructose, tying up both carbonyl carbons so it is non-reducing, while maltose and lactose keep one free anomeric carbon and are reducing sugars.

The three disaccharides:

- Sucrose** — $\alpha$-D-glucose (C1) and $\beta$-D-fructose (C2); non-reducing
- Maltose — two $\alpha$-D-glucose units joined C1 to C4; reducing
- Lactose — $\beta$-D-galactose (C1) and $\beta$-D-glucose (C4); reducing, found in milk

Invert sugar. Sucrose is dextrorotatory, but hydrolysis gives equal amounts of glucose ($+52.5^{\circ }$) and fructose ($-92.4^{\circ }$). Fructose rotates light more strongly, so the mixture is laevorotatory — the sign of rotation has inverted.

Worked example. For an equimolar mixture, the net specific rotation is the average:

$$\frac{52.5+(-92.4)}{2}=-19.95^{\circ }\approx -20^{\circ }$$

so invert sugar is laevorotatory.

An everyday example. Invert sugar syrup stops sweets and jams from crystallising, and honey is largely a natural invert sugar.

The substance. A disaccharide reduces Tollens' reagent only if an anomeric carbon stays free — which sugars it contains does not decide it.

**Starch, cellulose and glycogen are all polymers of glucose, but starch and glycogen use $\alpha$-glycosidic links and serve as energy stores, while cellulose uses $\beta$-links that form straight, strong chains for plant cell walls.

Starch — the main food store of plants:

- Amylose** — about $15$ to $20$ percent of starch; water-soluble, an unbranched chain of $\alpha$-D-glucose joined C1 to C4
- Amylopectin — about $80$ to $85$ percent; insoluble and branched, with C1–C6 branches on C1–C4 chains

Cellulose — the main structural material of plants: straight chains of **$\beta$-D-glucose** joined C1 to C4, packed side by side. Humans lack the enzymes to break $\beta$-links, so we cannot digest cellulose, though cattle can with help from gut microbes.

Glycogen — stored in the liver, muscles and brain; structured like amylopectin but more highly branched.

Importance of carbohydrates:

- Energy — glucose is the body's main fuel
- Storage — starch in plants, glycogen in animals
- Structure — cellulose in plant cell walls, wood and cotton
- Nucleic acids — ribose and deoxyribose sugars in RNA and DNA

Worked example. Each glucose unit in a polysaccharide has lost a water molecule, leaving C${}_6$H${}_{10}$O${}_5$ of mass $162$ g mol${}^{-1}$. A chain of $1000$ units therefore has a molar mass of about $1000\times 162=1.62\times 10^5$ g mol${}^{-1}$.

An everyday example. Cotton clothing is almost pure cellulose.

The substance. **One small change — $\alpha$ versus $\beta$ links — separates a food we digest from a fibre we cannot.**