A disaccharide is two monosaccharides united into one, as one might expect from the di- prefix, which means “two.” A disaccharide is formed when two monosaccharides combine in a chemical process known as dehydration synthesis, which results in the loss of a water molecule. The condensation reaction is another term for this phenomenon. The disaccharide sucrose is created when the monosaccharides fructose and glucose are combined.

What are Disaccharides?

Disaccharides are the carbohydrates that on hydrolysis carry two identical or disparate monosaccharides. Their general formula is C₁₂H₂₂O₁₁.  The consequential building blocks belonging to disaccharides are sucrose, maltose, and lactose. On hydrolysis with dilute acids or enzymes, these carry the following two molecules of monosaccharides.  

C₁₂H₂₂O₁₁ (sucrose) + H₂O ⭢ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)

C₁₂H₂₂O₁₁ (Maltose) + H₂O ⭢ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (glucose)

C₁₂H₂₂O₁₁ (Lactose) + H₂O ⭢ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (Galactose)

The disaccharides are fashioned up of two molecules of monosaccharides associated with each other by the condensation reaction. The associating is crystallized exactly as hemiacetals react with alcohols to solidify acetal with the elimination of a water molecule.  

In a similar way, the hydroxyl groups of hemiacetals of two monosaccharides nits condense to form a disaccharide. The two monosaccharide units are linked to each other through an oxygen atom by a bond called glycosidic linkage.

Examples of Disaccharides

Sucrose

It’s the most familiar disaccharide and is extensively apportioned in plants specifically sugar cane and sugar beet. It’s fashioned either from cane sugar or sugar beet. The sugar acquired from sugar beet is called beet sugar. It’s a tintless, crystalline, and sweet substance. It’s veritably soluble in water and its aqueous solution is dextrorotatory having (a) p = 66. °. On hydrolysis with dilute acids or enzyme invertase, cane sugar gives an equimolar admixture of)-glucose and D- (-)-fructose. 

C₁₂H₂₂O₁₁ (Sucrose) +H₂O ⭢ C₆H₁₂O₆ D-(+)-Glucose + C₆H₁₂O₆ D-(-)-Fructose

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. D-(-)-fructose has a higher concrete gyration than D-(+)-glucose. Thus, the attendant result upon hydrolysis is laevorotatory nature with distinct gyration of (-39.9°). Since there’s a modification in the sign gyration from Dextro before hydrolysis to leave after hydrolysis, the reaction is called inversion reaction and the admixture (glucose and fructose) is called invert sugar.  

Sucrose solution is fermented by yeast when the enzyme invertase hydrolysis the sucrose to glucose and fructose and enzyme zymase proselytizes these monosaccharides to ethanol.  

C₁₁H₂₂O₁₁ (sucrose) + H₂O ⭢ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)

C₆H₁₂O₆ (glucose or fructose) + H₂O ⭢ 2C₂H₆OH (Ethanol) + 2CO₂

Maltose

It is known as malt sugar. It is the principal disaccharide obtained by the partial hydrolysis of starch by an enzyme present in malt (sprouted barley seeds).

2(C₆H₁₀O₅)_n (Starch) + nH₂O ⭢  nC₁₂H₂₂O₁₁ (Maltose)

On Hydrolysis, one mole of maltose gives two moles of α-D glucose. It is a reducing sugar. It is composed of α-D-glucose units which are condensed together through C₁ of one unit and C₄ of the other unit. Both glucose relatives are in pyranose form.

Lactose

Lactose occurs in milk and, thus, it’s also called milk sugar.  Lactose on hydrolysis with dilute acid gives an equimolar mixture of β-D glucose and B-D-galactose. It’s reducing sugar. Thus, it’s formulated of β-D-glucose and β-D-galactose units. These units are held together by the glycosidic connection between the C₁ of galactose and the C₄ of the glucose unit.  

Lactose gets hydrolyzed by emulsion, an enzyme that especially hydrolyses glycosidic linkages.  

Sugar and Sweetness

The monosaccharides and disaccharides possess a sweet flavour and, thus, they’re called sugars. The degree of sweetness, still, varies as shown below-

As is see-through from the table, fructose is the sweetest of all the natural sugars.  Nevertheless, it may be noted that in addition to carbohydrates, some other chemical compounds are also veritably sweet. For exemplification,  

• Saccharin (applied as a man-made sweetener) is about 500 times as sweet as sucrose.
• The protein monoallelic is around 2000 times sweeter than sucrose.
• The peptide aspartame is around 160 times sweeter than sucrose.

FAQs on Disaccharides

1. Is a diet consisting mainly of rice an adequate diet? Why or why not? 

Ans. Adequate and good quality food is necessary for physical and mental development. This food enter with energy for our daily work. Nutritious food should contain carbohydrates, fats, proteins, and vitamins. Rice alone cannot supply these nutrients. Therefore, a diet mainly of rice is not an adequate diet.

2. What is meant by the inversion of sugar?

The alteration of the especial rotation of sugar from dextrorotatory to laevorotatory is called the inversion of sugar. 

3. Amylose and cellulose are both straight chains polysaccharides containing only D-glucose units. What is the structural difference between the two?

Amylose is a straight-chain polysaccharide having D-glucose units joined together by C-glycoside linkage involving C, of one glucose unit, and C, of the next glucose unit. On the other hand, cellulose is a straight t polysaccharide having D-glucose units joined by B-glycosidic linkages between C, of one glucose unit and C next glucose unit.

4. What is reducing and non-reducing sugars? What is the structural feature characterizing reduce sugars?

Sugars that can degrade Tollen’s reagent or Fehling’s solution are called reducing sugars. The sugars which de reduce Tollen’s reagent or Fehling solution are called non-reducing sugars.  

The distinctive structural affection of reducing sugars in the sight of either-hydroxy aldehyde g (-CHOH-CHO) similar as glucose, mannose, galactose, etc. α-hydroxy keto group (-CO-CH ₂ OH) as present in the fruit.

5. What are the products of Hydrolysis of sucrose?

Glucose and fructose are the products of the Hydrolysis of sucrose.

6. What’s the nomenclature given away to the linkage which holds simultaneously monosaccharide units in polysaccharides?

Glycosidic linkage is the linkage that holds simultaneously monosaccharide units in polysaccharides.