Classification of Carbohydrates & Glucose - Preparation and

Structure

 Carbohydrates are called saccharides.

 Classification

Classification of Monosaccharides
 Monosaccharides are classified based on the number of carbon atoms and the functional group
present in them.
 Different types of monosaccharides arelisted in the given table.

Carbon atoms General term Aldehyde Ketone

3 Triose Aldotriose Ketotriose

4 Tetrose Aldotetrose Ketotetrose

5 Pentose Aldopentose Ketopentose

6 Hexose Aldohexose Ketohexose

7 Heptose Aldoheptose Ketoheptose

Glucose
 Preparation of glucose
o By boiling sucrose with dilute HCl or H 2SO4 in alcoholic solution
 o By boiling starch with dilute H2SO4, at 393 K, under pressure

 Structure

o Glucose has been assigned the above structure based on the following evidences.
(i) Molecular formula − C6H12O6
(ii) Suggestion of straight chain

(iii) Confirmation of carbonyl (> C = O) group

(iv) Confirmation of the presence of carbonyl group as aldehydic group

(v) Confirmation of the presence of five −OH groups

 (vi) Indication of the presence of a primary alcohol

o The correct configuration of glucose is given by

o Glucose is correctly named as D (+) − Glucose

Cyclic Structure of Glucose
 The following reactions of glucose cannot be explained by its open-chain structure.
o Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO 4 to form the
hydrogen sulphite addition product. However, glucose does not undergo these
reactions.
o The penta-acetate of glucose does not react with hydroxylamine. This indicates that a
free −CHO group is absent from glucose.
o Glucose exists in two crystalline forms, α and β.
The α-form (m.p = 419 K) crystallises from a concentrated solution of glucose at 303
K and the β-form (m.p = 423 K) crystallises from a hot and saturated aqueous
solution at 371 K. This behaviour cannot be explained by the open-chain structure of
glucose.
 Glucose exists in two cyclic forms, which exist in equilibrium with the open- chain structure.

 Representation of the cyclic structure of glucose by Haworth structure:

Structure of Fructose, Disaccharides & Polysaccharides
Structure of Fructose
 Open-chain structure:

 Cyclic structure:

 Representation of the structure of fructose by Haworth structures

Disaccharides
Glycosidic linkage − Linkage between two monosaccharide units through oxygen atom
 Sucrose
o Hydrolysis of sucrose:

o Structure:
 o The product formed on the hydrolysis of sucrose is called invert sugar as the sign of
rotation changes from dextro (+) of sucrose to laevo (−) of the product.
o Non-reducing sugar
 Maltose
o Structure:

o Reducing sugar
 Lactose
o Commonly known as milk sugar
o Structure:

o Reducing sugar
Polysaccharides
They mainly act as food storage or structural materials.
 Starch
o Main storage-polysaccharide of plants
o Polymer of α-glucose; consists of two components − amylase and amylopectin
  Cellulose
o Predominant constituent of the cell wall of plant cells.
o Straight-chain polysaccharide, composed of only β-D-Glucose

 Glycogen
o Storage-polysaccharide in animal body
o Also known as animal structure because its structure is similar to amylopectin.

Proteins
 Proteins are polymers of α − amino acids.
Amino Acids

 Some amino acids with their symbols are listed in the given table.

Name Side chain, R Three-letter symbol One-letter code

1. Glycine H Gly G

2. Alanine − CH3 Ala A

3. Valine (H3C)2CH− Val V

4. Leucine (H3C)2CH− CH2− Leu L

5. Isolecucine Ile I

6. Lysine H2N− (CH2)4 − Lys K

7. Glutamic acid HOOC − CH2 − CH2− Glu E

8. Aspartic acid HOOC − CH2 − Asp D

9. Cysteine HS − CH2 − Cys C

10. Methionine H3C− CH2 − CH2− Met M

11. Phenylalanine C6H5−CH2 − Phe F

12. Tryptophan Trp W

Classification of Amino Acids

 Based on the relative number of amino and carboxyl groups, they are classified as acidic, basic
and neutral.
 Non-essential amino acids:
o Amino acids that can be synthesised in the body
o Example − Glycine, alanine, glutamic acid
  Essential amino acids:
o Amino acids that cannot be synthesised in the body, and must be obtained through
diet
o Example − Valine, leucine, isolecuine
Properties of Amino Acids
 Colourless and crystalline solids
 Exist as dipolar ions, known as zwitter ions, in aqueous solution

 In zwitter form, amino acids show amphoteric behaviour.

 All naturally occurring α-amino acids are optically active.
Structure of Proteins
 Proteins are polymers of α-amino acids, joined to each other by peptide linkage or peptide
bond.
 Peptide linkage: Amide formed between −COOH group and −NH 2 group of two amino acid
molecules.

 Depeptide − Contains two amino acid molecules

Tripeptide − Contains three amino acid molecules
Polypeptide − Contains more than ten amino acid molecules
 Based on the molecular shape, proteins are classified into two types −
o Fibrous proteins
o Globular proteins
 Fibrous Proteins
o In fibrous proteins, polypeptide chains run parallel and are held together by hydrogen
and disulphide bonds.
 Globular Proteins
o Polypeptide chains coil around, giving a spherical shape. Structures and shapes of
proteins are studied at four different levels: primary, secondary, tertiary and
quaternary.
o Primary structure of proteins: Contains one or more polypeptide chains, and each
chain has amino acids linked with each other in a specific sequence. This sequence of
amino acids represents the primary structure of proteins.
o Secondary structure of proteins: Shape in which a long polypeptide chain can exist;
two types of secondary structures: α-helix, β-pleated sheet
o α-helix structure of protein is as follows:

o β-pleated sheet structure of proteins is as follows:

o Tertiary structure of proteins: Overall folding of the polypeptide chains; results in

fibrous and globular proteins; secondary and tertiary structures of proteins are
stabilised by hydrogen bonds, disulphide linkages, van der Waals forces and
electrostatic forces.
o Quaternary structure of proteins: Spatial arrangement of subunits, each containing
two or more polypeptide chains
o The diagrammatic representations of the four structures of proteins are given below.
Denaturation of Proteins
 Loss of biological activity of proteins due to the unfolding of globules and uncoiling of helix.
 Example − Coagulation of egg white on boiling, curdling of milk
Enzymes, Vitamins & Nucleic Acids
Enzymes
 Enzymes are biocatalysts.
o Specific for a particular reaction and for a particular substrate
o For example, maltase catalyses hydrolysis of maltose

 The name of an enzyme ends with ‘−ase’.

 Reduce the magnitude of activation energy
Vitamins
 Organic compounds required in the diet in small amounts to maintain normal health, growth
and nutrition
 Classified into groups −
o Water-soluble vitamins: Vitamin C, B-group vitamins (B 1, B2, B6, B12)
o Fat-soluble vitamins: Vitamins A, D, E and K
 Some vitamins with their sources and the diseases caused by their deficiency are given in the
following table.

Name of Sources Deficiency diseases

vitamins

Vitamin A Fish liver oil, carrots, X e r o p h t h a l m i a,

butter and milk night blindness

Vitamin B1 Yeast, milk, green vegetables Beri beri

and cereals
 Vitamin B2 Milk, egg-white, liver, Cheilosis, digestive disorders and burning
kidney sensation of the skin

Vitamin B6 Yeast, milk, egg yolk, Convulsions

cereals and grams

Vitamin B12 Meat, fish, egg and Pernicious anaemia

curd

Vitamin C Citrus fruits, amla and Scurvy

green leafy vegetables

Vitamin D Exposure to sunlight, Rickets and osteomalacia

fish and egg yolk

Vitamin E Vegetable oils like wheat germ Increased fragility of

oil, sunflower oil RBCs and muscular
weakness

Vitamin K Green leafy vegetables Delay of blood clotting

Nucleic Acids
 Two types:
o Deoxyribonucleic acid (DNA)
o Ribonucleic acid (RNA)
 Chemical composition of nucleic acids:
o Nucleic acid contains a pentose sugar, phosphoric acid and a base (heterocyclic
compound containing nitrogen).
o In DNA, sugar is β-D-2-deoxyribose; in RNA, sugar is β-D-ribose

o Bases in DNA: Adenine (A), guanine (G), cytosine (C) and thymine (T)
 o Bases in RNA: Adenine (A), guanine (G), cytosine (C) and uracil (U)

 Structure of nucleic acids

o Structure of a nucleoside:

o Structure of a nucleotide:

o Formation of a di-nucleotide:

o In secondary structure, the helices of DNA are double-stranded while those of RNA are
single-stranded.
o The two strands of DNA are complementary to each other.
Reason: H-bonds are formed between specific pairs of bases.
 o Double-strand helix structure of DNA:

 Types of RNA:
o Messenger RNA (m-RNA)
o Ribosomal RNA (r-RNA)
o Transfer RNA (t-RNA)
 Functional differences between RNA and DNA:

- RNA DNA

DNA is not responsible for

1. DNA is the chemical basis of heredity.
heredity.

DNA molecules do not synthesise proteins, but transfer

Proteins are synthesised by
2. coded messages for the synthesis of proteins in the
RNA molecules in the cells.
cells.
MAZHAR SIR
(M.Sc. ORGANIC CHEMISTRY)
CONTACT NO. 7878114456