Consider the following two reactions:
(i) Sucrose is boiled with dilute HCl in alcoholic solution.
(ii) Starch is boiled with dilute H2SO4 at 393 K under a pressure of 2–3 atm.
Which of the following gives the carbohydrates formed in reaction (i) and in reaction (ii)?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (i) Glucose and fructose in equal amounts; (ii) Glucose
Both methods are acid hydrolyses: the glycosidic linkages are broken by water in the presence of H+, and each method gives back the monosaccharide units the carbohydrate is built from.
(i) From sucrose: one molecule of sucrose is made of one glucose and one fructose unit, so hydrolysis gives glucose and fructose in equal amounts:
\( \displaystyle \ce{C12H22O11 + H2O} \xrightarrow{\ce{H+}} \ce{C6H12O6 + C6H12O6} \)
Sucrose → Glucose + Fructose
(ii) From starch (the commercial method): starch is a polymer of α-glucose, so every unit is set free as glucose:
\( \displaystyle \ce{(C6H10O5)_{n} + nH2O} \xrightarrow[\text{393 K; 2-3 atm}]{\ce{H+}} \ce{nC6H12O6} \)
Starch → Glucose
Why the other options are incorrect
Option (1) (i) Glucose and galactose in equal amounts; (ii) Glucose — Incorrect. Galactose and glucose are the units of lactose (milk sugar), not of sucrose.
Option (2) (i) Glucose and fructose in equal amounts; (ii) Glucose and fructose in equal amounts — Incorrect. Starch contains no fructose units; it gives glucose alone.
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.”
“2. From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure.”
10.1.4 Polysaccharides – “…It is a polymer of α-glucose and consists of two components— Amylose and Amylopectin.”