Question

In 1891, Emil Fischer determined the structures of glucose and seven other D-aldohexoses using only simple chemical reactions and clever reasoning about stereochemistry and symmetry. He received the Nobel Prize for this work in 1902. Fischer has determined that D-glucose is an aldohexose, and he used Ruff degradation to degrade it to (+)-glyceraldehyde. Therefore, the eight D-aldohexose structures shown in Figure 23-3 are the possible structures for glucose.

Pretend that no names are shown in Figure 23-3 except for glyceraldehyde, and sue the following results to prove which of these structures represent glucose, mannose, arabinose, and erythrose.

(a)Upon Ruff degradation, glucose and mannose gives the same aldopentose: arabinose.Nitric acid oxidation of arabinose gives an optically active aldaric acid. What are the two possible structures of arabinose?

(b) Upon Ruff degradation, arabinose gives the aldotetrose erythrose. Nitric acid oxidation of erythrose gives an optically inactive aldaric acid, meso-tartaric acid. What is the structure of erythrose?

(c) Which of the two possible structures of arabinose is correct? What are the possible structures of glucose and mannose?

(d) Fischer’s genius was needed to distinguish between glucose and mannose. He developed a series of reactions to convert the aldehyde group of an aldose to an alcohol while converting the terminal alcohol to an aldehyde. In effect, he swapped the functional groups on the ends. When he interchanged the functional groups on D-mannose, he was astonished to find that the product was still D-mannose. Show how this information completes the proof of the mannose structure, and show how it implies the correct glucose structure.

(e) When Fischer interchanged the functional groups on D-glucose, the product was an unnatural L sugar. Show which unnatural sugar he must have formed, and show how it completes the proof of the glucose structure.

Short answer

a)

b)

c)

d)

e)

Step-by-step solution

D- and L- series of sugars

For D- series of sugars, the OH group of the bottom asymmetric carbon atom is on the right side in the Fischer projection. Again, for L- series of sugars, the OH group of the bottom asymmetric carbon atom is on the left side in the Fischer projection. D- and L- refers to the absolute configuration around the asymmetric carbon atom while (+) and (-) refers to the direction of rotation of the plane polarised light.

 Step 2: 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:

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

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

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 ( -CO OH ) groups. The resulting dicarboxylic acid formed is known as aldaric acid.

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.

 Step 5: Possible structures and reactions

a)

b)

c)

correct structure for arabinose

d)

e)