Question

How do you explain the following ? (a) Presence of a carbonyl group in glucose (b) Presence of five --OH groups in glucose

Hint:

Oxime and cyanohydrin formation indicate the presence of a carbonyl group, while formation of glucose pentaacetate confirms the presence of five hydroxyl groups in glucose.

Answer 1

Concept: Glucose is an aldohexose having molecular formula: \[ C_6H_{12}O_6 \] Many structural features of glucose were established experimentally through characteristic chemical reactions. The presence of a carbonyl group and five hydroxyl groups in glucose was deduced from separate experimental observations.

(a) Presence of a carbonyl group in glucose

Step 1: Oxime formation. Glucose reacts with hydroxylamine to form glucose oxime. \[ \text{Glucose} + NH_2OH \rightarrow \text{Glucose oxime} \] Only compounds containing aldehyde or ketone groups undergo this reaction. This indicates the presence of a carbonyl group.

Step 2: Cyanohydrin formation. Glucose reacts with hydrogen cyanide to form cyanohydrin. \[ \text{Glucose} + HCN \rightarrow \text{Cyanohydrin} \] Formation of cyanohydrin is a characteristic reaction of carbonyl compounds.

Step 3: Reduction reaction. Glucose on reduction with HI and red phosphorus yields n-hexane. This indicates that glucose possesses an unbranched six-carbon chain and contains a carbonyl functional group.

Step 4: Conclusion. The formation of oxime, cyanohydrin and other carbonyl derivatives confirms the presence of a carbonyl group in glucose. \[ \boxed{\text{Glucose contains a carbonyl group}} \]

(b) Presence of five --OH groups in glucose

Step 1: Acetylation reaction. Glucose reacts with excess acetic anhydride. \[ \text{Glucose} + 5(CH_3CO)_2O \rightarrow \text{Glucose pentaacetate} \] The product formed is glucose pentaacetate.

Step 2: Interpretation of pentaacetate formation. Each hydroxyl group reacts with one molecule of acetic anhydride to form one acetate group. Since five acetate groups are introduced, glucose must contain five hydroxyl groups. \[ \text{Five acetate groups} \Longrightarrow \text{Five hydroxyl groups} \]

Step 3: Additional support. Glucose shows reactions characteristic of alcohols such as ester formation, further supporting the presence of multiple hydroxyl groups.

Step 4: Conclusion. Formation of glucose pentaacetate proves the presence of five hydroxyl groups in glucose. \[ \boxed{\text{Glucose contains five hydroxyl groups}} \]