Identify Chemical Structure of Cellulose Triacetate

To determine the correct chemical structure of cellulose triacetate, we analyze the structure of cellulose and the effect of complete acetylation:

1. Structure of Cellulose:
   Cellulose is a linear polymer composed of $\text{D}$-glucose monomeric units joined by $\beta(1 \to 4)$-glycosidic linkages.
   Due to the geometry of the $\beta(1 \to 4)$ linkage, adjacent glucose units are flipped by $180^\circ$ relative to one another along the polymer chain to maintain a linear, extended ribbon-like conformation.

2. Acetylation Reaction:
   Each $\text{D}$-glucose unit in cellulose contains three free hydroxyl ($\text{-OH}$) groups:

   • Secondary $\text{-OH}$ groups at $\text{C-2}$ and $\text{C-3}$
   • Primary $\text{-OH}$ group at $\text{C-6}$ (in the $\text{-CH}_2\text{OH}$ group attached to $\text{C-5}$)

   When cellulose is treated with excess acetic anhydride in the presence of a catalytic amount of $\text{H}_2\text{SO}_4$, all three $\text{-OH}$ groups per monomer unit undergo acetylation to form acetoxy ($\text{-OAc}$) groups:
   $\text{Cellulose-OH} + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4} \text{Cellulose-OAc} + \text{CH}_3\text{COOH}$

3. Comparison of the Options:

   • Structure (A) correctly depicts the alternating inverted pyranose rings characteristic of the $\beta(1 \to 4)$-glycosidic linkages of cellulose, with complete acetylation at positions $\text{C-2}$, $\text{C-3}$, and $\text{C-6}$ ($\text{-CH}_2\text{OAc}$).
   • Structure (B) contains unreacted $\text{-OH}$ groups at $\text{C-2}$ and $\text{C-3}$, representing incomplete acetylation.
   • Structures (C) and (D) show non-inverted pyranose rings aligned in the same direction, which corresponds to $\alpha(1 \to 4)$-glycosidic linkages found in amylose (starch) rather than cellulose.

Thus, structure (A) correctly represents cellulose triacetate.

Correct Answer: A