Question

Hydrolysis of sucrose is called inversion. Why?

Answer 1

We know that hydrolysis is the reaction in which a compound reacts with water. Disaccharides on hydrolysis with enzyme or acids (dilute) give different or same monosaccharides.

Complete step by step answer:

Let's first understand the classification of carbohydrates considering their behavior in hydrolysis reaction.
Carbohydrates are divided into four groups namely monosaccharides, oligosaccharides and polysaccharides.

Monosaccharide is a carbohydrate which cannot be hydrolysed to produce polyhydroxy ketone or aldehyde.

Oligosaccharide is a carbohydrate which on hydrolysis produces 2 to 10 units of monosaccharide. Depending on the number of monosaccharide units produced on hydrolysis, oligosaccharide is further classified into disaccharide, trisaccharide etc.

Polysaccharide is a carbohydrate that gives a large number of monosaccharide units on hydrolysis.

Now, come to the question. We know that sucrose is a common disaccharide whose chemical formula is ${{\rm{C}}_{{\rm{12}}}}{{\rm{H}}_{{\rm{22}}}}{{\rm{O}}_{{\rm{11}}}}$.
On hydrolysis sucrose gives equimolar mixture of D-(+)-glucose and D-(-) fructose.

${{\rm{C}}_{{\rm{12}}}}{{\rm{H}}_{{\rm{22}}}}{{\rm{O}}_{{\rm{11}}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to \,{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}} + \,{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}$

$\text {Sucrose D-(+)-glucose D-(-)Fructose}$

A glycosidic bond links the two monosaccharides formed on hydrolysis.

Now, let's understand the reason for calling hydrolysis an inversion reaction. Sucrose is dextrorotatory in nature. But after the hydrolysis reaction, it gives glucose which is dextrorotatory and fructose which is laevorotatory in nature. As the laevorotatory fructose (-92.4) is more than dextrorotation of glucose (+52.5), the mixture is laevorotatory in nature. So, we can say that hydrolysis of sucrose results in the change in sign of rotation, from dextro (+) to laevo (-). So, the hydrolysis reaction of sucrose is an inversion reaction.

Note: Always remember that, an oxide linkage joins two monosaccharide units. This oxide linkage is formed by losing a water molecule. A linkage in which two monosaccharides are joined through oxygen atoms is termed as glycosidic linkage.