JEE Chemistry Biomolecules Notes Set 03

What are Carbohydrates

Carbohydrates are organic compounds of carbon, hydrogen and oxygen, classically written as \( C_x(H_2O)_y \). In modern terms they are polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.
The formula is only a rough guide. Rhamnose, \( C_6H_{12}O_5 \), is a carbohydrate that does not fit it, while acetic acid, \( CH_3COOH \), fits it but is not a carbohydrate.

Classification of Carbohydrates

By hydrolysis:
1. Monosaccharides cannot be hydrolysed further: glucose, fructose, ribose.
2. Oligosaccharides give a few monosaccharide units: sucrose, maltose (\( C_{12}H_{22}O_{11} \)).
3. Polysaccharides give many units: starch, cellulose, glycogen, gums.
By reducing nature: reducing sugars (glucose, fructose, maltose, lactose) reduce Tollens' and Fehling's reagents; non-reducing sugars include sucrose and all polysaccharides.

Monosaccharides with an aldehyde group are aldoses (glucose, ribose, glyceraldehyde) and those with a keto group are ketoses (fructose, dihydroxyacetone). They are named by chain length: triose (3 C), tetrose (4 C), pentose (5 C), hexose (6 C) and heptose (7 C), with the prefix aldo- or keto-.

Preparation of Glucose

1. From sucrose: boiling with dilute acid in alcohol gives an equimolar mixture of glucose and fructose.
\( C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 + C_6H_{12}O_6 \)
Sucrose rotates +66.5°, glucose +52.7° and fructose -92.4°, so the mixture rotates about -19.95°. The sign changes from dextro to laevo, which is why the product is called invert sugar.
2. From starch or cellulose: hydrolysis with dilute acid at 393 K and 2-3 atm.
3. From maltose: the enzyme maltase hydrolyses it into two glucose molecules.

Properties of Glucose and What They Prove

1. Prolonged heating with HI and red phosphorus gives n-hexane: the six carbons are in a straight chain.
2. Mild oxidants (bromine water) give gluconic acid, a monocarboxylic acid with six carbons. Nitric acid gives saccharic acid, a dicarboxylic acid, which shows a primary alcohol group at the end of the chain.
3. Tollens' reagent, Fehling's and Benedict's solutions are reduced: a free aldehyde group is present. Tollens' gives a silver mirror and Fehling's gives red \( Cu_2O \).
4. Acetylation with acetic anhydride gives a pentaacetate: there are five -OH groups.
5. Hydroxylamine (oxime) and HCN (cyanohydrin) additions confirm one carbonyl group.
6. Phenylhydrazine gives an osazone. Glucose and fructose give the same osazone, because they differ only at C-1 and C-2.
7. In dilute alkali glucose, mannose and fructose interconvert through an enediol (Lobry de Bruyn-van Ekenstein rearrangement). This is why glucose and fructose behave alike in basic medium.
8. Mutarotation: pure α-D-glucose (+112°) or pure β-D-glucose (+19.2°) dissolved in water drifts to the equilibrium value of +52.7°.

Some reactions (such as the pentaacetate not reacting with hydroxylamine, and glucose not giving Schiff's test) cannot be explained by the open-chain form. They need the cyclic hemiacetal structure, in which C-1 becomes a new stereocentre. The two ring forms that differ at C-1 are called anomers (α and β).

Disaccharides

A disaccharide gives two monosaccharide units on hydrolysis.
1. Maltose: two α-D-glucose units joined from C-1 of one to C-4 of the other. A free hemiacetal group remains, so it is reducing.
2. Lactose: β-D-galactose joined through C-1 to C-4 of glucose. It is reducing.
3. Sucrose: α-D-glucose and β-D-fructose joined through C-1 of glucose and C-2 of fructose. Both anomeric carbons are used up in the bond, so sucrose is non-reducing and shows no mutarotation.

Polysaccharides

Polysaccharides are long chains of monosaccharides with the general formula \( (C_6H_{10}O_5)_n \).
1. Starch is the storage food of plants, built from α-glucose. Amylose is the water-soluble, unbranched part (100-3000 units, C1-C4 links). Amylopectin is the water-insoluble, branched part.
2. Cellulose is the structural material of plant cell walls and the most abundant organic compound in nature. It is a chain of β-glucose units. It is used for rayon, gun cotton, cellulose acetate, paper and textiles.
3. Glycogen is the reserve carbohydrate of animals (liver and muscles) and is a highly branched glucose polymer.

Amino Acids

Proteins are polymers of α-amino acids, which carry an -NH2 and a -COOH group on the same carbon. Every α-amino acid except glycine is optically active, because it has a chiral carbon.
Essential amino acids such as valine, leucine, isoleucine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine cannot be made in the body and must come from food.
Properties: colourless, crystalline, water soluble, high melting solids that behave like salts. In water they exist as zwitterions, \( R-CH(NH_3^+)-COO^- \), which are electrically neutral overall.
The isoelectric point is the pH at which the amino acid does not move to either electrode.

Proteins

Amino acids join by peptide bonds (-CO-NH-), formed with loss of water. A polypeptide has a free amino (N-terminal) end and a free carboxyl (C-terminal) end. A chain with molecular mass above 10000 is conventionally called a protein.
By shape: fibrous proteins (keratin, myosin) are insoluble in water with chains held side by side by hydrogen and disulphide bonds. Globular proteins (insulin, albumin) are coiled into spheres and are water soluble.
By structure:
1. Primary: the sequence of amino acids in the chain.
2. Secondary: regular folding held by hydrogen bonds between C=O and N-H groups, as the α-helix (right-handed screw) or the β-pleated sheet.
3. Tertiary: the overall folding of the chain, held by hydrogen bonds, disulphide links, van der Waals forces and electrostatic attraction.
4. Quaternary: the arrangement of two or more polypeptide sub-units.
Denaturation changes the shape and properties of a protein (through heat, strong acid or base) without breaking the primary structure. Boiling an egg coagulates albumin and is irreversible.