Question

Predict the products obtained when D-galactose reacts with each reagent.

(a) Br₂ and H₂O

(b) NaOH,H₂O

(c)CH₃OH, H⁺

(d) Ag(NH₃)⁺₂OH⁺

(e) H₂, Ni

(f) excess Ac₂O and pyridine

(g) excess CH₃I ,Ag₂O

(h) NaBH₄

(i) Br₂ , H₂O then H₂O₂ and Fe₂(SO₄)₃

(j) (1) KCN/HCN; (2) H₂ Pd/BaSO₄; (3) H₃O⁺

(k)excess HIO₄

Short answer

a)

b)

c)

d)

e)

f)

g)

h)

i)

j)

k)

Step-by-step solution

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.

Formation of glycosides

Aldehydes and ketones can be converted to acetals when they are treated with an alcohol and a small amount of an acid catalyst. Under these conditions, aldoses and ketoses can also be converted to acetals which are known as glycosides

Ester formation

The hydroxy (-0H) groups of a sugar can be converted to acetate esters by the treatment with acetic anhydride and pyridine. The stereochemistry is usually retained at the anomeric carbon.

 Step 4: 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.

 Step 5: Reduction of monosaccharide

Aldoses and ketoses can be reduced to their corresponding polyalcohols, known as sugar alcohols or alditols by using the reagents such as sodium borohydride (NaBH₄) or by catalytic hydrogenation using a nickel (Ni) catalyst.

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) .

(2)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.

Step 7: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.

 Step 8: Prediction of the products

(a)Bromine water oxidation of D-galactose gives galactonic acid.

b)

c)

(d) An aldose in its open chain form has an aldehyde group which reacts with Tollen’s reagent to yield an aldonic acid a silver mirror.

(e)Aldoses are reduced to its corresponding polyalcohols, known as alditols by H₂Ni .

f)

g)

(h)Aldoses are reduced to its corresponding polyalcohols, known as alditols by NaBH₄.

i)

j)

k) D- galactose when treated with HI O₄ gives formic acid and formaldehyde.