Organic Chemistry · Biomolecules: Carbohydrates

Reactions of Monosaccharides

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A monosaccharide carries two kinds of reactive sites — the carbonyl (masked as a cyclic hemiacetal) and the hydroxyl groups. Oxidation of the aldehyde gives an and powers the reducing-sugar tests (Tollens', Benedict's, Fehling's); reduction with sodium borohydride gives sugar alcohols (alditols) such as sorbitol; acid-catalyzed reaction of the anomeric OH with an alcohol forms an acetal , the linkage that holds disaccharides and polysaccharides together; and the hydroxyls undergo esterification and ether formation, which protect OH groups and reveal ring sizes.

Why this matters

Reducing-sugar chemistry is a working clinical tool: Benedict's test on urine was for decades the standard screen for glucose in diabetes, and modern test strips still use the same oxidation logic (with glucose oxidase, which is specific for glucose). Sugar alcohols — sorbitol, xylitol, mannitol — sweeten "sugar-free" gum, and mannitol is used intravenously as an osmotic agent. Glycosides matter in pharmacology — many plant drugs, such as digoxin, are glycosides — and the itself is the glue of starch, cellulose, and nucleic acid backbones, making this reaction the foundation of Topics 8 and 9. Glycation, the non-enzymatic reaction of glucose with protein amino groups, is also implicated in diabetes complications, a reminder that sugar reactivity has patient consequences.

The college version

Core Concepts

Oxidation: the reducing-sugar tests

An aldose's aldehyde is easily oxidized to a carboxylic acid. In Tollens' test, silver ion in ammonia is reduced to a silver mirror on the glass; in Benedict's and Fehling's tests, cupric ion Cu²⁺ is reduced to brick-red cuprous oxide Cu₂O. A sugar that gives these tests is a — one whose cyclic hemiacetal can open to an aldehyde. All monosaccharides are reducing, including ketoses: under the basic test conditions a ketose tautomerizes (via an enediol) into an aldose and then reacts. Mild oxidation at C1 gives an aldonic acid (gluconic acid from glucose); strong oxidants such as nitric acid oxidize both ends to an .

Reduction: sugar alcohols

Sodium borohydride reduces the carbonyl to a hydroxyl: D-glucose gives D-glucitol (sorbitol), D-mannose gives mannitol, D-ribose gives ribitol. These alditols have no carbonyl, cannot open to an aldehyde, and are not reducing sugars — which is why sorbitol and xylitol resist fermentation by cavity-causing oral bacteria.

Glycoside formation: acetals at the anomeric carbon

The anomeric OH is a hemiacetal OH. Treating a monosaccharide with an alcohol and a trace of acid makes a full acetal, or glycoside — glucose plus methanol gives methyl α-D-glucopyranoside and methyl β-D-glucopyranoside. Mechanism in words: the alcohol's oxygen attacks the protonated anomeric carbon, water is lost, and the new OR group ends up α or β. Glycosides are stable to base and to mutarotation: the anomeric carbon is locked, so the sugar cannot open to an aldehyde and is not reducing; aqueous acid hydrolyzes it back. The bond from the anomeric carbon to the alcohol is the glycosidic bond — when the alcohol is another sugar, the product is a di- or polysaccharide. The non-sugar part of a glycoside is its (as in digoxigenin).

Esterification and etherification of the hydroxyls

All the OH groups behave like ordinary alcohol OH groups. Acetic anhydride in pyridine acetylates every OH (glucose gives a pentaacetate), and methyl iodide with silver oxide converts every OH to a methyl ether — useful for protecting OH groups and for solubility. These reactions also solve structural puzzles: methylation analysis methylates every free OH and then hydrolyzes the sugar; the OH that was tied up in the ring stays unmethylated, revealing the ring size. Glucose hydrolysis gives 2,3,4,6-tetra-O-methylglucose — four methyl groups — proving a six-membered pyranose ring.

A note on the limits of these reactions

These tests are classic and qualitative; modern diagnostics use enzyme-based strips (glucose oxidase/peroxidase) that are glucose-specific and quantitative. In the lab, prepare Tollens' reagent fresh and use strong oxidants such as nitric acid only with proper ventilation and personal protective equipment — general principles for any oxidizing chemistry.

Common Confusions

Do Not Confuse	With	Difference
Reducing sugar	Sweet sugar	Sweetness is a taste property; reducing ability is chemical. Sucrose is sweet but non-reducing.
Hemiacetal	Acetal (glycoside)	Hemiacetal: one OR + OH, opens, reducing; acetal: two OR groups, locked, non-reducing.
Aldonic acid	Aldaric acid	One end oxidized (mild) vs both ends oxidized (HNO₃).
Glucose test	Any reducing-sugar test	Benedict's detects all reducing sugars; glucose-oxidase strips detect only glucose.
Sorbitol	Mannitol	Reduction products of glucose and mannose, respectively; both are alditols and non-reducing.
Ketose "not reducing"	Ketose reducing	Ketoses DO reduce the classic reagents because base tautomerization converts them to aldoses.

Eli explains

The same idea, in plain words

Explain it like I’m 10

A sugar is a tiny machine with two useful ends. One end (the aldehyde) can give away electrons — that makes it a reducing sugar that turns blue copper test liquid into red mud or makes a silver mirror. The other end (the anomeric carbon) clicks onto other molecules like a Lego brick — that's how sugars chain into starch and cellulose.

Worked example

Example 1: Reducing or not — three cases

Test each sugar's structure. D-Glucose: its hemiacetal opens to an aldehyde → reducing. Methyl α-D-glucopyranoside: the anomeric carbon carries an OCH₃ acetal and cannot open → not reducing. Sucrose: the glucose anomeric carbon is linked to the fructose anomeric carbon, so both anomeric positions are tied up → not reducing, even though both halves are monosaccharides. This is why sucrose fails Benedict's test while a glucose + fructose mixture passes it.

Example 2: Stoichiometry of the Tollens' test (dimensional analysis)

The aldehyde oxidation is a two-electron change, and each Ag⁺ accepts one electron, so each mole of aldose reduces two moles of silver ion. How much silver is produced from 0.180 g of glucose? Write the mole formula first:

$n = m M$

Substitute m = 0.180 g and M = 180.16 g/mol:

$n glucose = 0.180 g 180.16 g/mol = 9.99 \times 10 -4 mol$

Use the 2:1 ratio from the two-electron balance:

$n Ag = 9.99 \times 10 -4 mol glucose \times 2 mol Ag 1 mol glucose = 2.00 \times 10 -3 mol Ag$

So 0.180 g of glucose produces about 2.00 mmol of metallic silver — the classic silver-mirror result. The same ratio explains why one mole of any aldose consumes two moles of cupric ion in Fehling's test.

Example 3: Building a glycoside in words

Dissolve D-glucose in methanol with a trace of HCl. The alcohol attacks the protonated anomeric carbon and water leaves, replacing the C1 OH with OCH₃. Two products form — methyl α- and methyl β-D-glucopyranoside — because the new group can adopt either configuration. The acetal product is stable to base, does not mutarotate, and gives no Tollens' or Benedict's test; aqueous acid hydrolyzes it back to glucose and methanol. If the alcohol is another sugar molecule, the same chemistry builds a disaccharide such as maltose (Topic 8).

Key takeaways

Reducing sugar = has a hemiacetal that can open to an aldehyde; ALL monosaccharides are reducing (ketoses tautomerize first).
Tollens': Ag⁺ → silver mirror. Benedict's/Fehling's: Cu²⁺ → brick-red Cu₂O.
Mild oxidation at C1 → aldonic acid; strong oxidation (HNO₃) → aldaric acid.
NaBH₄ reduction → alditols: glucose → sorbitol, mannose → mannitol, ribose → ribitol.
Glycoside = acetal at the anomeric carbon (sugar + alcohol + acid); locked, non-reducing, stable to base, hydrolyzed by acid.
Glycosidic bonds link monosaccharides in disaccharides and polysaccharides.
Methylation analysis (methylate → hydrolyze → find free OH) reveals ring size.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Why does D-fructose, a ketose, give a positive Benedict's test?
Show answer
Under the basic test conditions, fructose tautomerizes through an enediol to an aldose, which then reduces Cu²⁺.
What product forms when D-glucose is treated with sodium borohydride, and why is it non-reducing?
Show answer
D-Glucitol (sorbitol). The carbonyl is reduced to an OH, so no aldehyde can form and the molecule cannot reduce the test reagents.
Why doesn't methyl α-D-glucopyranoside reduce Tollens' reagent?
Show answer
The anomeric carbon is locked in an acetal (OCH₃) and cannot open to an aldehyde, so there is no reducing group.
Which reagent oxidizes an aldose to an aldaric acid?
Show answer
Nitric acid (HNO₃), which oxidizes both ends.
How many mmol of Ag⁺ are reduced by 0.090 g of glucose (M = 180 g/mol)?
Show answer
n = 0.090/180 = 5.0 × 10^-4 mol glucose; at 2 mol Ag per mol glucose, that is 1.0 × 10^-3 mol = 1.0 mmol of Ag⁺ reduced.
Why is sucrose non-reducing even though it is made of two monosaccharides?
Show answer
The glycosidic bond joins the anomeric carbon of glucose to the anomeric carbon of fructose, so neither sugar has a free anomeric OH that could open to an aldehyde.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Reducing sugar: A sugar whose hemiacetal can open to an aldehyde and reduce Cu²⁺ or Ag⁺.
Aldonic acid: Product of mild oxidation of the aldehyde end (e.g., gluconic acid from glucose).
Aldaric acid: Product of strong oxidation of both ends of an aldose.
Alditol: Sugar alcohol from reduction of the carbonyl (sorbitol, mannitol).
Glycoside: Acetal formed from the anomeric OH and an alcohol.
Glycosidic bond: The acetal linkage from an anomeric carbon to another group.
Aglycone: The non-sugar part of a glycoside.
Tautomerization: Base-catalyzed keto–enol interconversion that lets ketoses act as aldoses.

Sources & references

openstax.org — Organic Chemistry

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