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Chapter 23: Q25P (page 1228)

(a) Show the product that results when fructose is treated with an excess of methyl iodide and silver oxide.

(b) Show what happens when the product of part (a) is hydrolyzed using dilute acid.

(c) What do the results of parts (a) and (b) imply about the hemiacetal structure of fructose?

Short Answer

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  3. The results show that the acetals are easily hydrolyzed by dilute acid whereas ethers are stable under these conditions.

Step by step solution

01

Fructose

Fructose is a ketohexose which contain a ketone group. It is used as a sweetening agent in many confectionaries and also as a substitute of cane sugar.

02

Formation of methyl ether

Hydroxy groups of sugar can be converted to methyl ethers by treating with methyl iodide (CH3I) and silver oxide (Ag2O). CH3-I group can be polarised by silver oxide(Ag2O), which makes the methyl carbon more strongly electrophilic. Attack by the -OH group of carbohydrate is then followed by deprotonation which gives the ether.

03

Furanose

Furanose is a five-membered cyclic hemiacetal whose name is derived from the five-membered cyclic ether furan. Five membered ring of fructose is known as fructofuranose.

04

Explanation


  1. Fructose on treatment with excess methyl iodide and silver oxide gives methyl 2,3,4,6, -tetra-O-methyl-伪-D-fructofuranoside as the product as the product

  2. Treatment with dilute acid only hydrolyzes the acetal methyl group.
  3. The results show that the acetals are easily hydrolyzed by dilute acid whereas ethers are stable under these conditions.

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Most popular questions from this chapter

Which of the sugars mentioned in Problem 23-53 and 23-54 are reducing sugars? Which ones undergo mutarotation?

Trehalose is a nonreducing disaccharide (C12H22O11) isolated from the poisonous mushroom Amanita muscaria. Treatment with an 伪 -glucosidase converts trehalose to two molecules of glucose, but no reaction occurs when trehalose is treated with a 尾 -glucosidase. When trehalose is methylated by dimethyl sulfate in mild base and then hydrolyzed, the only product is 2,3,4,6-tetra-O-methylglucose. Propose a complete structure and systematic name for trehalose.

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

  1. Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimer results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.
  2. Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has twoprotons and removing one or the other gives two different enolate ions.

The Wohl degradation, an alternative to the Ruff degradation, is nearly the reverse of the Kiliani-Fischer synthesis. The aldose carbonyl group is converted to the oxime, which is dehydrated by acetic anhydride to the nitrile (a cyanohydrin). Cyanohydrin formation is reversible, and a basic hydrolysis allows the cyanohydrin to lose HCN. Using the following sequence of reagents, give equations for the individual reaction in the Wohl degradation of D-arabinose to D-erythrose. Mechanisms are not required.

(1) hydroxylamine hydrochloride

(2) acetic anhydride

(3) OH-,H2O

Question. Cellulose is converted to cellulose acetateby treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibres. Show the structure of cellulose acetate.
See all solutions

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