Question

How will you confirm the presence of five -- OH groups in a glucose molecule, which are attached to different carbon atoms ?

Hint:

A very important structural proof of glucose is: \[ \text{Glucose} \xrightarrow[\text{Pyridine}]{(CH_3CO)_2O} \text{Glucose Pentaacetate} \] Formation of a pentaacetate derivative confirms the presence of five alcoholic \((-OH)\) groups in glucose.

Answer 1

Concept: The molecular formula of glucose is \[ C_6H_{12}O_6 \] Experimental studies show that glucose contains oxygen atoms in the form of hydroxyl groups and a carbonyl group. One of the important objectives in determining the structure of glucose is to establish the number of hydroxyl groups present and whether these hydroxyl groups are attached to different carbon atoms. This information is obtained through the acetylation reaction of glucose.

Step 1: Understanding acetylation of alcohols. Alcohols containing hydroxyl groups react with acetic anhydride in the presence of pyridine to form acetate derivatives. The general reaction is \[ R-OH + (CH_3CO)_2O \longrightarrow R-OCOCH_3 + CH_3COOH \] Each hydroxyl group present in a molecule gets converted into an acetate group. Therefore, the number of acetate groups formed indicates the number of hydroxyl groups originally present.

Step 2: Reaction of glucose with acetic anhydride. When glucose is heated with excess acetic anhydride in the presence of pyridine, all the hydroxyl groups present in glucose undergo acetylation. The reaction can be represented as \[ \text{Glucose} \xrightarrow[\text{Pyridine}]{(CH_3CO)_2O} \text{Glucose Pentaacetate} \] The product obtained is called glucose pentaacetate.

Step 3: Significance of the formation of pentaacetate. The prefix ``penta'' means five. Since glucose forms a pentaacetate derivative, exactly five acetyl groups are introduced into the molecule. Each acetyl group replaces one hydrogen atom of a hydroxyl group. Therefore, glucose must contain five hydroxyl groups. Hence, \[ \boxed{\text{Glucose contains five } -OH \text{ groups}} \]

Step 4: Establishing that the hydroxyl groups are attached to different carbon atoms. If two hydroxyl groups were attached to the same carbon atom (geminal diol structure), such an arrangement would generally be unstable and would not account for the formation of a stable pentaacetate derivative. The formation of glucose pentaacetate demonstrates that five separate hydroxyl groups are available for acetylation. Therefore, the five hydroxyl groups must be attached to five different carbon atoms.

Step 5: Writing the conclusion. Since glucose forms glucose pentaacetate upon acetylation, it is confirmed that glucose possesses five hydroxyl groups and each hydroxyl group is attached to a different carbon atom. \[ \text{Glucose} \rightarrow \text{Glucose Pentaacetate} \] \[ \boxed{\text{Five } -OH \text{ groups present}} \]

Final Answer: Glucose reacts with excess acetic anhydride in the presence of pyridine to give

glucose pentaacetate. The formation of glucose pentaacetate proves that glucose contains \(\boxed{five}\) hydroxyl groups, and these hydroxyl groups are attached to different carbon atoms. \[ \boxed{ \text{Glucose} \xrightarrow[\text{Pyridine}]{(CH_3CO)_2O} \text{Glucose Pentaacetate} } \]