Identify the incorrect statement about glucose
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Glucose exists as a 5-membered ring
In its cyclic form, the –OH group at C5 of glucose adds to the –CHO group (C1) and forms a cyclic hemiacetal. The ring holds five carbon atoms and one oxygen atom, so it is a six-membered (pyranose) ring.
Option (1) Glucose exists as a 5-membered ring — An incorrect statement about glucose. The ring of glucose is six-membered (five C and one O), like pyran. A five-membered (furanose) ring is found in fructose.
Option (2) It forms n-hexane on prolonged heating with HI — A correct statement about glucose. HI reduces glucose completely to n-hexane, which shows that its six carbon atoms are joined in a straight chain.
\( \ce{CHO-(CHOH)4-CH2OH} \xrightarrow[\Delta]{\ce{HI}} \ce{CH3-CH2-CH2-CH2-CH2-CH3} \)
Option (3) Acetylation of glucose takes place with acetic anhydride — A correct statement about glucose. Acetic anhydride acetylates the five –OH groups and gives glucose pentaacetate, which shows that glucose has five –OH groups.
\( \ce{CHO-(CHOH)4-CH2OH} \xrightarrow{\ce{(CH3CO)2O}} \ce{CHO-(CHOCOCH3)4-CH2OCOCH3} \)
Option (4) It does not give 2,4-DNP test. — A correct statement about glucose. In the cyclic structure the –CHO group is not free, so glucose does not react like a normal aldehyde: it gives no Schiff’s test and no addition product with \( \ce{NaHSO3} \), and in the same way no 2,4-DNP test.
NCERT (2026-27 edition):
Structure of Glucose – “… 2. On prolonged heating with HI, it forms n-hexane, suggesting that all the six carbon atoms are linked in a straight chain. … 5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –OH groups.”
Cyclic Structure of Glucose – “… 1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3. … The six membered cyclic structure of glucose is called pyranose structure (α– or β–), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring.”