Organic Chemistry · Biomolecules: Carbohydrates
Disaccharides
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A Disaccharide Two monosaccharides joined by one glycosidic bond Full entry → is a carbohydrate made of two monosaccharide units joined by a Glycosidic bond Acetal C–O–C link between two sugars, formed with loss of water Full entry → — an acetal-like link between the Anomeric carbon The carbonyl carbon of the cyclic sugar; the one that forms the glycosidic bond Full entry → of one sugar and a hydroxyl of the other, formed with loss of water. The four most important are maltose, cellobiose, lactose, and sucrose. All share the formula C12H22O11 (two hexoses minus one water), yet behave very differently because their linkages join different carbons with α or β configuration. The key classification is reducing vs. non-reducing: reducing if at least one anomeric carbon remains free as a hemiacetal (maltose, cellobiose, lactose), non-reducing if both anomeric carbons are in the bond (sucrose). This topic covers glycosidic bond formation, linkage notation, and each disaccharide's biological role.
Why this matters
- Sucrose is table sugar. The most consumed pure organic compound on Earth is the α-1,2-linked glucose–fructose disaccharide from cane and beet.
- Lactose intolerance is a disaccharide problem. Lactose must be split by the enzyme lactase; when lactase is low or absent, undigested lactose ferments in the gut.
- Maltose and cellobiose show how digestion depends on linkage. Maltose is the α-linked product of starch breakdown; cellobiose the β-linked product of cellulose breakdown. Humans hydrolyze maltose but not cellobiose — one configuration at one carbon.
- Reducing-sugar tests are clinical and lab tools. Urine glucose tests (and Tollens'/Fehling's tests) detect free hemiacetal anomeric carbons; this explains why sucrose gives a negative test.
- Exam relevance. Naming glycosidic linkages (α-1,4 vs. β-1,4), predicting reducing behavior, and writing Hydrolysis Cleavage of a glycosidic bond by addition of water Full entry → products are favorite test questions.
The college version
Core Concepts
How a glycosidic bond forms
A monosaccharide in its cyclic form has an anomeric carbon (C1 for aldoses, C2 for ketoses) bearing an -OH from ring closure. When that anomeric -OH reacts with a hydroxyl on a second sugar, water is lost and an acetal link — the glycosidic bond — forms:
sugar-OH + HO-sugar → sugar-O-sugar + H2O
The bond is named by the anomeric configuration (α or β) and the carbon numbers joined, e.g., α(1 → 4). Because bond formation consumes the anomeric hydroxyl, that carbon can no longer equilibrate between α and β — it is locked.
Maltose: α(1 → 4) glucose–glucose
Maltose is two D-glucose units joined α(1 → 4). It forms during starch digestion and is the "malt sugar" of germinating grain. The second glucose's anomeric carbon is free, so maltose is a Reducing sugar Sugar with a free anomeric carbon that can reduce Tollens'/Benedict's reagent Full entry → — it mutarotates and reduces Tollens' or Benedict's reagent. Complete hydrolysis gives two molecules of glucose:
C12H22O11 + H2O → 2 C6H12O6
Cellobiose: β(1 → 4) glucose–glucose
Cellobiose is the same two glucose units joined β(1 → 4). It is cellulose's repeating unit, produced by partial hydrolysis of cellulose. Humans lack a β-glucosidase, so we cannot digest cellobiose or cellulose; it, too, is reducing (free anomeric carbon on the second unit).
Lactose: β(1 → 4) galactose–glucose
Lactose is D-galactose linked β(1 → 4) to D-glucose — the sugar of mammalian milk (~5% by mass in cow's milk). The galactose unit contributes the anomeric carbon (linkage Gal(β1 → 4)Glc). The glucose end keeps a free anomeric carbon, making lactose reducing; lactase hydrolyzes it to galactose + glucose, and too little lactase causes lactose intolerance.
Sucrose: α(1 → 2) glucose–fructose
Sucrose joins glucose α(1 → 2) to fructose, and fructose's anomeric carbon (C2) is involved — so both anomeric carbons are committed to the bond. No free hemiacetal remains: sucrose is non-reducing (negative Tollens'/Benedict's tests, no mutarotation). Hydrolysis (sucrase or acid) gives one glucose plus one fructose — Invert sugar Equimolar glucose + fructose mixture from sucrose hydrolysis Full entry →, sweeter than sucrose itself.
Reducing vs. non-reducing: the rule
A sugar is reducing if any anomeric carbon exists as a free hemiacetal. Check the glycosidic bond: if one sugar's anomeric carbon is not in the linkage, the disaccharide is reducing. Maltose, cellobiose, and lactose are reducing; sucrose is not — the classic exam discriminator, and why sucrose gives no positive Benedict's test until hydrolyzed.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Reducing sugar | Sweet-tasting sugar | "Reducing" is a structural property (free anomeric carbon), not a taste or health property; sucrose is sweet but non-reducing. |
| Maltose vs. cellobiose | Same disaccharide | Both are Glc–Glc, but maltose is α(1 → 4) and cellobiose is β(1 → 4); only maltose is digestible by humans. |
| α vs. β in a glycosidic bond | D vs. L sugar | α/β describes the configuration at the anomeric carbon of the linkage; D/L describes the whole sugar's reference carbon. |
| Sucrose hydrolysis products | Two glucoses | Sucrose gives glucose + fructose; maltose/cellobiose give two glucoses; lactose gives galactose + glucose. |
| Glycosidic bond | Hydrogen bond | The glycosidic bond is a covalent acetal C–O–C link; hydrogen bonds are much weaker noncovalent interactions. |
| "Invert sugar" | Sucrose | Invert sugar is the hydrolyzed glucose + fructose mixture; it is sweeter and reducing. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A disaccharide is two sugar bricks glued together — the glue is a squeezed-out water molecule. The joint has a name ("1-to-4 glue" or "1-to-2 glue") and can point up (alpha) or down (beta). Sucrose is special: both bricks are glued at their "special ends," so no free end remains to grab — it doesn't react in sugar tests, while maltose, cellobiose, and lactose still have one free end and do react.
Worked example
Example 1: Molar mass of sucrose
Calculate the molar mass of sucrose, C12H22O11 (C = 12.011, H = 1.008, O = 15.999 g/mol).
Formula first:
M = 12M(C) + 22M(H) + 11M(O)
Substitute:
M = 12(12.011) + 22(1.008) + 11(15.999) = 144.132 + 22.176 + 175.989 = 342.30 g/mol
Answer: Sucrose is 342.30 g/mol. Check: two hexoses are 2 × 180.16 = 360.32 g/mol; minus one water (18.02) gives 342.30. A teaspoon of sugar (~4.2 g) is 4.2/342.30 = 0.0123 mol.
Example 2: Mass of glucose from lactose hydrolysis
A glass of milk contains 12.0 g of lactose. How many grams of glucose are released if all the lactose is hydrolyzed?
Reaction first: lactose + water → galactose + glucose. One mole of lactose (342.30 g/mol) yields one mole of glucose (180.16 g/mol).
Convert moles of lactose:
12.0 g × 1 mol342.30 g = 0.03506 mol lactose
Convert to glucose mass:
0.03506 mol × 180.16 g1 mol = 6.32 g glucose
Answer: About 6.32 g of glucose (and the same mass of galactose, since products are 1:1); roughly half the lactose mass becomes glucose — a useful sanity check.
Example 3: Predicting reducing behavior from structure
Which of the following give a positive Tollens' test: maltose, cellobiose, lactose, sucrose?
Reasoning walkthrough: locate each glycosidic bond and ask whether an anomeric carbon remains free.
- Maltose α(1 → 4): second glucose's C1 is free → reducing.
- Cellobiose β(1 → 4): same bonding geometry; second glucose's C1 is free → reducing.
- Lactose β(1 → 4): glucose's C1 is free → reducing.
- Sucrose α(1 → 2): glucose C1 and fructose C2 are both in the bond → no free hemiacetal → non-reducing.
Answer: Maltose, cellobiose, and lactose are reducing; sucrose is not. Lactose intolerance, cellobiose indigestibility, and sucrose's negative sugar test all follow from these linkage structures — one rule, three applications.
Key takeaways
- Disaccharides = two monosaccharides joined by a glycosidic (acetal) bond, formed by condensation with loss of H2O.
- All four common hexose disaccharides share the formula C12H22O11.
- Maltose: Glc α(1 → 4) Glc — from starch digestion; reducing.
- Cellobiose: Glc β(1 → 4) Glc — cellulose repeating unit; reducing; humans cannot digest it.
- Lactose: Gal β(1 → 4) Glc — milk sugar; reducing; hydrolyzed by lactase.
- Sucrose: Glc α(1 → 2) Fru — table sugar; non-reducing because both anomeric carbons are in the bond.
- Reducing sugar = has a free anomeric (hemiacetal) carbon; reduces Tollens'/Benedict's reagent.
- Hydrolysis of any hexose disaccharide: C12H22O11 + H2O → 2 C6H12O6 (sucrose → glucose + fructose).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Write the hydrolysis reaction of maltose and name the products.
Show answer
C12H22O11 + H2O → 2 C6H12O6; two molecules of D-glucose.
Why is sucrose classified as a non-reducing sugar?
Show answer
Both anomeric carbons (glucose C1 and fructose C2) are involved in the glycosidic bond, so no free hemiacetal exists to reduce the reagent.
What is the molecular formula shared by maltose, cellobiose, lactose, and sucrose, and why?
Show answer
C12H22O11 — two hexoses (C6H12O6 each) minus one water (H2O).
Which disaccharide is cellulose's repeating unit, and what is its linkage?
Show answer
Cellobiose, with a β(1 → 4) linkage between two glucose units.
A patient's urine gives a negative Benedict's test after a sucrose-sweetened drink. Explain why, and predict the result after sucrase treatment.
Show answer
Sucrose is non-reducing, so intact sucrose gives a negative test; after sucrase hydrolysis, the glucose product (a reducing sugar) gives a positive test.
What is the molar mass of lactose?
Show answer
12(12.011) + 22(1.008) + 11(15.999) = 342.30 g/mol.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Disaccharide
- Two monosaccharides joined by one glycosidic bond
- Glycosidic bond
- Acetal C–O–C link between two sugars, formed with loss of water
- Anomeric carbon
- The carbonyl carbon of the cyclic sugar; the one that forms the glycosidic bond
- Reducing sugar
- Sugar with a free anomeric carbon that can reduce Tollens'/Benedict's reagent
- Hydrolysis
- Cleavage of a glycosidic bond by addition of water
- Invert sugar
- Equimolar glucose + fructose mixture from sucrose hydrolysis
Sources & references
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