Chapter 23: Q57P (page 1256)

After a series of Kiliani–Fischer syntheses on (+)-glyceraldehyde, an unknown sugar is isolated from the reaction mixture. The following experimental information is obtained:
(1) Molecular formula C₆H₁₂O6
(2) Undergoes mutarotation.
(3) Reacts with bromine water to give an aldonic acid.
(4) Reacts with HNO₃ to give an optically active aldaric acid.
(5) Ruff degradation followed by HNO₃ oxidation gives an optically inactive aldaric acid. (6) Two Ruff degradations followed by HNO₃ oxidation give meso-tartaric acid.
(7) When the original sugar is treated with CH₃I and Ag₂O, a pentamethyl derivative is formed. Hydrolysis gives a tetramethyl derivative with a free hydroxy group on C5.
(a) Draw a Fischer projection for the open-chain form of this unknown sugar. Use Figure 23-3 to name the sugar.
(b) Draw the most stable conform

Short Answer

Expert verified

_(a)

D-altrose

(b)

(2S,3R,4R) - 2,3,4,5,6 - pentahydroxyhexanal

Step by step solution

Kiliani- Fischer synthesis (lengthening of the carbon chain)

Adding one carbon atom to the aldehyde end of the aldose lengthens an aldose chain. The sugar chain is lengthened as a result of this process with a new carbon atom at C1 position and the aldehyde group (C1 position earlier) is now moved to C2 position. For determining the structure of existing sugars and also for synthesizing new sugars, Kiliani-Fischer synthesis is used.

Mutarotation

A spontaneous change in the specific rotation of a solution of an optically active compound is known as mutarotation. This implies that the two samples are different but in solution they form an equilibrium mixture. Sugars which are reducing can undergo mutarotation while non-reducing sugars cannot undergo mutarotation.

Bromine water oxidation of monosaccharides

Bromine water oxidises aldehyde group (-CHO) of an aldose into carboxylic acid (-COOH). Moreover, bromine water does not oxidize alcohols and ketoses. The product formed from bromine water oxidation is known as an aldonic acid.

Nitric acid oxidation of monosaccharides

Nitric acid (HNO₃) is considered to be stronger oxidizing agent than bromine water since it can oxidize both aldehyde group and the terminal -CH₂OH group of an aldose to carboxylic acid (-COOH)groups. The resulting dicarboxylic acid formed is known as aldaric acid.

Ruff degradation (shortening of the carbon chain)

Synthesis of new sugars and its structure can be known by a process known as Ruff degradation. The sugar chains are shortened in Ruff degradation. It is a two- step process which is given as below:

oxidation of aldose to aldonoic acid by bromine water (Br₂/H₂O) .

aldonic acid formed is treated with hydrogen peroxide (H₂O₂) and ferric sulfate Fe₂(SO₄)₃, that oxidises the carboxyl group to carbon dioxide (CO₂) and the resulting aldose is formed with one fewer carbon atom.

Meso compounds

Meso compounds are defined as those which contain two or more asymmetric centres that are optically inactive. They consist of internal plane of symmetry such that one half of the molecule forms the mirror image of the other half. Since the optical activity due to one half is counterbalanced by the optical activity of the other half, so meso compounds are optically inactive.

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.

Drawing the structure and writing its systematic name

(a)

D-altrose aldonic acid

D-altrose aldaric acid

The name of the unknown sugar is D-altrose

(b) The most stable conformation of the most stable cyclic hemiacetal form of this sugar ( D-altrose ) is (2S,3R,4R) - 2,3,4,5,6 - pentahydroxyhexanal.