Read the case carefully and answer the questions that follow. Carbo...

Question

Read the case carefully and answer the questions that follow. Carbohydrates are essential for life in both plants and animals. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Chemically they are polyhydroxy aldehydes or ketones. On the basis of their behaviour on hydrolysis, carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides. All monosaccharides are reducing sugars, i.e., they are oxidized by Tollens' reagent and Fehling's solution. A monosaccharide like glucose is aldohexose and its molecular formula was found to be $C_6H_{12}O_6$. After reacting with different reagents like HI, $H_2N-OH$, Bromine water, $(CH_3CO)_2O$, etc. its structure was found to contain one aldehyde group, one primary alcoholic group $(-CH_2OH)$ and four secondary alcoholic groups $(>CHOH)$. Despite having the aldehyde group, glucose does not give some of the reactions of aldehyde group like Schiff's test, $NaHSO_3$ addition. This explains the existence of glucose in two cyclic hemiacetal forms which differ only in the configuration of the hydroxyl group at C-1. Answer the following questions : (a) What are reducing sugars ? [1] (b) Classify the following into monosaccharide and disaccharide : Fructose, Sucrose, Lactose, Galactose [1] (c) Name the polysaccharide which is known as 'animal starch'. Why is it called 'animal starch' ? [2] OR (c) (i) Name the isomers of glucose which in the cyclic form differ only in the configuration of the $-OH$ group at C-1. (ii) Presence of which functional group was detected when glucose reacted with $Br_2$ water ? [2]

Accepted Answer

(a) Reducing sugars Carbohydrates that contain a free aldehyde or free ketone group and are therefore able to reduce Tollens' reagent and Fehling's solution are called reducing sugars. The sugar itself gets oxidised while it reduces $Ag^+$ to metallic silver (silver mirror) or $Cu^{2+}$ to $Cu_2O$ (red-brown precipitate). All monosaccharides are reducing sugars. Among disaccharides, maltose and lactose are reducing, while sucrose is non-reducing because both its anomeric carbons are locked in the glycosidic linkage. (b) Classification Monosaccharides Disaccharides Fructose, Galactose Sucrose, Lactose Basis : monosaccharides cannot be hydrolysed into simpler sugars, whereas each disaccharide gives two monosaccharide units on hydrolysis (sucrose $ightarrow$ glucose + fructose; lactose $ightarrow$ glucose + galactose). (c) The 'animal starch' Name : Glycogen Why it is called animal starch : Glycogen is the polysaccharide in which animals store their carbohydrate, held mainly in the liver and in muscle. Its role in animals is exactly the role starch plays in plants — a reserve of glucose that can be broken down by enzymes whenever the body needs energy. Structurally too it resembles starch, being a polymer of glucose units, but glycogen is more highly branched than amylopectin. For these reasons it is called animal starch. OR (c) (i) Isomers differing at C-1 They are called anomers, specifically $\alpha$-D-glucopyranose and $\beta$-D-glucopyranose. When the open-chain form cyclises, C-1 becomes a new chiral centre (the anomeric carbon). The $-OH$ at C-1 can point down ($\alpha$-form) or up ($\beta$-form), giving these two cyclic hemiacetal forms. (c) (ii) Functional group detected with bromine water The aldehyde group $(-CHO)$. Bromine water is a mild oxidising agent that oxidises only an aldehyde group, converting glucose into gluconic acid : $$\underset{	ext{glucose}}{CHO-(CHOH)_4-CH_2OH} \xrightarrow{Br_2\ 	ext{water}} \underset{	ext{gluconic acid}}{COOH-(CHOH)_4-CH_2OH}$$ Since only the $-CHO$ group is affected and the $-CH_2OH$ group survives, this reaction proves that glucose contains an aldehyde group.

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