What is a disaccharide
Carbohydrates that yield two monosaccharide units on hydrolysis are disaccharides — a sub-class of the oligosaccharides. The two units released may be identical or different. One molecule of sucrose hydrolyses to one molecule of glucose and one of fructose; one molecule of maltose hydrolyses to two molecules of glucose. The general reaction adds a single water molecule across the bridging oxygen:
$\ce{Disaccharide + H2O ->[H+/enzyme] Monosaccharide(I) + Monosaccharide(II)}$
All three NCERT disaccharides share the molecular formula $\ce{C12H22O11}$ — two hexose units ($\ce{C6H12O6}$ each) minus one water. They differ only in which hexoses are involved and which carbons the linkage joins, and those two facts determine every property NEET asks about.
The glycosidic linkage
When two monosaccharides condense, a hydroxyl group on each unit is lost as water, leaving the two rings tied together through a single oxygen atom. This C–O–C bridge is the glycosidic linkage (NCERT calls it an oxide linkage). At least one of the participating hydroxyls is the one on an anomeric carbon — the carbon that, in the open-chain form, was the carbonyl. The fate of those anomeric carbons is the whole story of reducing behaviour.
Schematic of a glycosidic linkage between two hexose rings
One hydroxyl from each ring is eliminated as water; the surviving oxygen bridges the two units. The linkage is named by the carbon numbers it joins, e.g. C1–C4 or C1–C2.
Glycosidic linkages are named by the two carbon positions they connect and by the anomeric configuration (α or β). The same notation underlies polysaccharides: amylose is a chain of α-D-glucose held by C1–C4 linkages, while cellulose strings β-D-glucose units through C1–C4 — a small change with enormous biological consequences, covered in the polysaccharides note.
Reducing vs non-reducing sugars
Carbohydrates that reduce Fehling's solution and Tollens' reagent are reducing sugars; all monosaccharides, aldose or ketose, are reducing. For a disaccharide, the rule from NCERT is precise:
If the reducing groups of the monosaccharides — the aldehydic or ketonic groups — are bonded in the glycosidic linkage, the sugar is non-reducing (e.g. sucrose). If those functional groups remain free, the sugar is reducing (e.g. maltose, lactose).
Mechanistically, a sugar reduces Tollens'/Fehling's only if it can open into a free aldehyde (or an α-hydroxy ketone) in solution. That requires a free anomeric carbon — one whose hemiacetal hydroxyl is not committed to the linkage. Maltose and lactose each retain one such free anomeric centre; sucrose locks up both.
Sucrose — cane sugar
Sucrose is the common table sugar obtained from cane and beet. On hydrolysis it gives an equimolar mixture of D-(+)-glucose and D-(−)-fructose. The two units are held by a glycosidic linkage between C1 of α-D-glucose and C2 of β-D-fructose.
$\ce{C12H22O11 (sucrose) + H2O ->[H+] \underset{(+)}{C6H12O6} (glucose) + \underset{(-)}{C6H12O6} (fructose)}$
The C1–C2 linkage is the defining feature. C1 is the anomeric (reducing) carbon of glucose; C2 is the anomeric (reducing) carbon of fructose, a ketohexose. Because both reducing groups are tied up in the bond, neither ring can open into a free carbonyl — so sucrose is a non-reducing sugar. It does not respond to Fehling's solution or Tollens' reagent.
Sucrose: the C1(glucose)–C2(fructose) linkage locks both reducing carbons
Glucose contributes a six-membered (pyranose) ring, fructose a five-membered (furanose) ring. With C1 and C2 both committed, no free aldehyde or ketone can form.
Sucrose is the odd one out
Among glucose, maltose, lactose and sucrose, examiners ask "which is non-reducing?" almost verbatim. Only sucrose qualifies, because its linkage uses both anomeric carbons (C1 of glucose and C2 of fructose). Glucose, maltose and lactose all reduce Tollens'/Fehling's.
Memory hook: "Sucrose Seals both ends." The bond name C1–C2 itself flags that two anomeric carbons are tied up.
Invert sugar & inversion of cane sugar
Sucrose is dextrorotatory. On hydrolysis it produces dextrorotatory glucose and laevorotatory fructose. The specific rotations matter:
| Species | Sign & specific rotation | Effect |
|---|---|---|
| Sucrose (before hydrolysis) | dextrorotatory, (+) | solution rotates plane to the right |
| D-(+)-glucose (product) | +52.5° | dextrorotation |
| D-(−)-fructose (product) | −92.4° | stronger laevorotation |
| Product mixture (invert sugar) | net laevorotatory, (−) | sign of rotation flips |
Because the laevorotation of fructose (−92.4°) exceeds the dextrorotation of glucose (+52.5°) in magnitude, the equimolar mixture is net laevorotatory. The sign of optical rotation thus changes from dextro (+) to laevo (−) during hydrolysis. This sign reversal is the inversion of cane sugar, and the resulting glucose–fructose mixture is called invert sugar.
The rotation signs trace back to the parent monosaccharides. Revise glucose & fructose structure to see why fructose is laevorotatory and glucose dextrorotatory.
Maltose — malt sugar
Maltose is composed of two α-D-glucose units. The linkage joins C1 of the first glucose to C4 of the second (a C1–C4 linkage). Crucially, the anomeric carbon (C1) of the second glucose unit is free. In solution that C1 can open to give a free aldehyde group, so maltose shows reducing properties — it is a reducing sugar.
$\ce{Maltose + H2O ->[maltase] 2\, \alpha\text{-D-glucose}}$
The enzyme maltase specifically catalyses this hydrolysis of maltose into glucose. Note the contrast with sucrose: maltose's linkage uses only one anomeric carbon (C1 of unit I), so the other (C1 of unit II) is left free to do reducing chemistry.
Lactose — milk sugar
Lactose is the sugar present in milk. It is composed of β-D-galactose and β-D-glucose, joined by a glycosidic linkage between C1 of galactose and C4 of glucose (C1–C4). A free aldehyde group can be produced at C1 of the glucose unit, so lactose is also a reducing sugar.
Lactose contains galactose, not two glucoses
Students routinely confuse maltose and lactose because both use a C1–C4 linkage and both are reducing. The composition differs: maltose = glucose + glucose, while lactose = galactose + glucose. The hydrolysis products are the cleanest way to tell them apart.
Lactose → β-D-galactose + β-D-glucose. Maltose → two α-D-glucose. Sucrose → glucose + fructose.
The three disaccharides compared
Every NEET property follows from the two columns "constituent units" and "linkage carbons". The table consolidates the NCERT data point by point.
| Disaccharide | Constituent units | Linkage | Reducing? | Note |
|---|---|---|---|---|
| Sucrose (cane sugar) | α-D-glucose + β-D-fructose | C1–C2 | No (non-reducing) | both anomeric C bonded; dextrorotatory; inverts on hydrolysis |
| Maltose (malt sugar) | two α-D-glucose | C1–C4 | Yes (reducing) | free anomeric C at C1 of unit II; hydrolysed by maltase |
| Lactose (milk sugar) | β-D-galactose + β-D-glucose | C1–C4 | Yes (reducing) | free aldehyde at C1 of glucose unit; found in milk |
Hydrolysis products at a glance
Hydrolysis — by dilute acid or by a specific enzyme — cleaves the glycosidic linkage and restores the monosaccharide units. NCERT lists the products explicitly:
- Sucrose: $\ce{C12H22O11 + H2O -> }$ D-(+)-glucose + D-(−)-fructose (equimolar; invert sugar).
- Maltose: two molecules of α-D-glucose.
- Lactose: β-D-galactose + β-D-glucose.
Industrially, glucose itself is prepared by boiling sucrose with dilute $\ce{HCl}$ or $\ce{H2SO4}$ in alcoholic solution, giving glucose and fructose in equal amounts — the same reaction that defines invert sugar.
Disaccharides in one screen
- Disaccharides hydrolyse to two monosaccharides joined by a glycosidic (oxide) linkage formed with loss of water.
- Sucrose = α-glucose + β-fructose, C1–C2, non-reducing (both reducing groups bonded).
- Maltose = two α-glucose, C1–C4, reducing (free C1 on second glucose); hydrolysed by maltase.
- Lactose = β-galactose + β-glucose, C1–C4, reducing (free aldehyde at glucose C1).
- Invert sugar: sucrose (dextro, +) → glucose (+52.5°) + fructose (−92.4°) gives a net laevorotatory mixture; sign flips (+) → (−).