Sucrose is boiled with dilute hydrochloric acid, and the product mixture is called invert sugar. Which of the following explains this name?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The sign of rotation changes from dextro (+) to laevo (−), because the laevorotation of fructose is more than the dextrorotation of glucose.
Boiling with dilute acid hydrolyses the glycosidic linkage of sucrose. One molecule of sucrose takes up one molecule of water and gives one molecule of glucose and one of fructose:
\( \displaystyle \ce{C12H22O11} + \ce{H2O} \xrightarrow{\ce{H+}} \ce{C6H12O6} + \ce{C6H12O6} \)
Sucrose → D-(+)-glucose + D-(−)-fructose
Sucrose itself is dextrorotatory. The products are formed in equal amounts, and the laevorotation of fructose (−92.4°) is larger than the dextrorotation of glucose (+52.5°), so the mixture is laevorotatory.
The sign of rotation is thus inverted from (+) to (−), and the product is named invert sugar.
Why the other options are incorrect
Option (1) The sign of rotation changes from laevo (−) to dextro (+), because the dextrorotation of glucose is more than the laevorotation of fructose. — Incorrect. It reverses both facts: sucrose is dextrorotatory to begin with, and fructose, not glucose, has the larger rotation.
Option (2) The configuration changes from D to L, because the glycosidic linkage between glucose and fructose is broken. — Incorrect.
Breaking the glycosidic linkage does not change D into L; both products, D-(+)-glucose and D-(−)-fructose, belong to the D-series.
The name refers to the sign of rotation, not to the configuration.
Option (4) The sign of rotation changes from dextro (+) to laevo (−), because more fructose is formed than glucose. — Incorrect.
The change of sign is right, but the reason is wrong: glucose and fructose are formed in equal amounts (an equimolar mixture).
The mixture is laevorotatory because of the larger rotation of fructose.
NCERT (2026-27 edition):
10.1.2.1 Glucose – “1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.”
10.1.3 Disaccharides – “(i) Sucrose: One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of D-(+)-glucose and D-(-) fructose.”
“Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar.”