Question

When the gum of the shrub Sterculia setigera is subjected to acidic hydrolysis, one of the water-soluble components of the hydrolysate is found to be tagatose. The following information is known about tagatose:

(1) Molecular formula C₆H₁₂O₆

(2) Undergoes mutarotation.

(3) Does not react with bromine water.

(4) Reduces Tollens reagent to give d-galactonic acid and d-talonic acid.

(5) Methylation of tagatose (using excess CH₃ I and Ag₂O) followed by acidic hydrolysis gives 1,3,4,5-tetra-O-methyltagatose.

(a) Draw a Fischer projection structure for the open-chain form of tagatose.

(b) Draw the most stable conformation of the most stable cyclic hemiacetal form of tagatose.

Short answer

(a)

open chain form of tagatose

(b)

most stable cyclic hemiacetal form of tagatose

Step-by-step solution

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.

Tollens test

Tollens reagent, [Ag(NH₃)₂]OH test is a qualitative test used to distinguish between an aldehyde an ketone. This test is also known as silver mirror test in which aldehydes are readily oxidized to corresponding carboxylic acids whereas ketones are not.

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 structures

(a)

open chain form of tagatose

Tagatose reduces tollens reagent to form D-galactonic acid and D-talonic acid.

(b)

most stable cyclic hemiacetal form of tagatose