Carbohydrates — Biomolecules

Carbohydrates are where the carbonyl chemistry of Module 11 stops being an exercise. A sugar is a polyhydroxy aldehyde or ketone, and almost everything distinctive about one — the ring, the two forms it exists in, whether it reduces Tollens' reagent — follows from a carbonyl reacting with an alcohol that happens to be attached to the same molecule.

The vocabulary, quickly

An aldose has an aldehyde; a ketose has a ketone. Combined with the carbon count: glucose is an aldohexose, fructose a ketohexose.
A monosaccharide is one sugar unit, a disaccharide two joined together, a polysaccharide many.
D and L describe the configuration at the stereocenter furthest from the carbonyl. Drawn as a Fischer projection with the carbonyl at the top, OH on the right at that carbon means D. Essentially every naturally occurring sugar is D.

D and L are not the same as (+)/(−) or R/S. D says only where one particular OH sits in a Fischer projection; it does not tell you which way the compound rotates plane-polarized light, and you cannot infer it. D-glucose and D-fructose are both D and rotate light in opposite directions.

The ring is a hemiacetal, and that explains nearly everything

A sugar in solution is overwhelmingly cyclic, and the ring is not a new kind of bond. It is the intramolecular hemiacetal from Module 11: one of the molecule's own OH groups attacks its own carbonyl carbon. Five- and six-membered rings form because those are the ring sizes that close without strain.

A six-membered ring is a pyranose; a five-membered ring is a furanose.
The former carbonyl carbon becomes the anomeric carbon — the one bearing both an OR and an OH.

Closing the ring creates a new stereocenter at that carbon, which did not exist in the open-chain form. The two resulting diastereomers are called anomers and are labeled α and β. In a standard Haworth projection of a D-sugar, the α anomer has the anomeric OH pointing down, on the opposite side from the CH₂OH; β has it up, on the same side.

Mutarotation is the observation that a freshly dissolved pure anomer slowly changes its optical rotation until it reaches a fixed value. The explanation is that the ring is opening and reclosing through the open-chain aldehyde, so the two anomers interconvert until they reach equilibrium. It is direct evidence that the open form is present, even though it is a tiny fraction of the mixture.

Hemiacetal or acetal: the distinction that decides reducing sugars

Module 11 made the point that a hemiacetal can revert to the carbonyl and an acetal cannot without acid. In sugar chemistry that is the whole difference between a reducing and a non-reducing sugar.

A free anomeric OH means the ring is a hemiacetal, so it can open to the aldehyde. That aldehyde is oxidizable, so the sugar is a reducing sugar — it gives a positive Tollens' or Benedict's test.
If the anomeric position is tied up as an acetal — a glycosidic bond to another alcohol — the ring cannot open, no aldehyde appears, and the sugar is non-reducing.

So the test is mechanical: look at the anomeric carbon and ask whether it carries an OH or an OR.

Worked example — why sucrose does not reduce and maltose does

Maltose joins two glucose units through the anomeric carbon of one and an ordinary OH of the other. That leaves the second unit's anomeric carbon free as a hemiacetal, so it can still open. Maltose is a reducing sugar.

Sucrose joins glucose to fructose through both of their anomeric carbons. Neither ring can open, so no aldehyde is ever available. Sucrose is non-reducing, which is unusual among common disaccharides and is the standard exam example.

The reasoning never mentions sweetness, size or source. It is only ever: is there a free anomeric OH.

The polysaccharides worth knowing

The anomeric carbon, with and without a free OH. On the left it is a hemiacetal: the ring opens back to the open-chain aldehyde, and that aldehyde is what a reducing-sugar test oxidizes. On the right a second alcohol has capped it, making a full acetal — stable to base, so the ring never opens and no aldehyde is ever available.This is why sucrose is the standard exception. Its glycosidic bond runs anomeric carbon to anomeric carbon, so both rings are acetals at once and neither end can open; maltose commits only one, which leaves the other free and keeps maltose reducing.

Cellulose — glucose joined β-1,4. The β linkage makes a straight chain that stacks into fibers, and human enzymes cannot hydrolyze it.
Starch (amylose) — glucose joined α-1,4. The α linkage makes a helical chain, and we digest it easily.

The two polymers are made of the same monomer and differ only in the configuration at one carbon. That single stereochemical difference is why one is food and the other is wood, which is as sharp a demonstration of why stereochemistry matters as this course contains.

What carries forward

Nothing in this section is new chemistry. It is hemiacetal and acetal formation, Fischer projections and diastereomers, applied to molecules that happen to be biological. If a carbohydrate question looks unfamiliar, the useful move is to find the carbonyl — or the carbon that used to be one — and ask what has added to it.