IIT-JEE 2008 · Paper 2 · Chemistry Q46

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ChemistrySingle correct+3 / -1Question 46 of 66

Cellulose upon acetylation with excess acetic anhydride/ $\ce{H2SO4}$ (catalytic) gives cellulose triacetate whose structure is

Three β-1,4-linked glucopyranose units with every free OH (C2, C3 and C6) acetylated (OAc and AcO groups): cellulose triacetate with β-glycosidic links.

Three β-1,4-linked glucopyranose units with only C6 acetylated (AcO) and the C2/C3 OH groups free.

Three α-1,4-linked glucopyranose units with C2, C3 and C6 acetylated (an α-linked, amylose-type triacetate).

Three α-1,4-linked glucopyranose units with acetates drawn on C2 and C3 both pointing down and C6 acetylated (wrong stereochemistry at C3).

Answer and worked solution

Official answer: A

Cellulose is a polymer of D-glucose joined by β-1,4 links, each unit having free OH groups at C2, C3 and C6. Excess acetic anhydride acetylates all three, giving cellulose triacetate with the β-links unchanged.

(B) is only a monoacetate; (C) and (D) have α-links (amylose-type) or wrong stereochemistry.

(A)

Topics:Carbohydrates

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IIT-JEE 2008 · Paper 2, Chemistry question 46