Organic Chemistry

Module 7. Carbohydrates

Lesson 20
DEFINITION, CLASSIFICATION AND ISOMERISM

20.1 Introduction

• Major class of organic compounds occurring in nature- important to maintain life
• Include- glucose, sucrose, starch, cellulose etc

20.2 Definition

• The French term ‘hydrate de carbone’- meaning – hydrates of carbon – therefore defined as hydrates of carbon having an impirical formula C_nH_2nO_n or (CH₂O)_n
• Some non-carbohydrate compounds also fit into the impirical formula—e.g. formaldehyde, acetic acid, lactic acid etc
• Some carbohydrates do not have H to O ratio of 2:1 – e.g. deoxy ribose- C₅H₁₀O₄
• Carbohydrates containing N and S, in addition to C, H and O do not fit into the empirical formula so the word carbohydrate has only historical importance.
• Carbohydrates now defined as –“ polyhydroxy aldehyde or ketone compounds or compounds which yield these on hydrolysis and contain atleast one asymmetric/ chiral carbon atom ”

20.3 Nomenclature

Two types – nature of source and trival names

1. According to nature of source

Beet sugar, cane sugar, grape sugar, malt sugar, milk sugar

2. Trival names

Prefix- related to source, number of carbon atom content, aldehyde or ketone group content of rotation of plane polarized light

Suffix- ‘ose’ – indicates compound to be carbohydrate (sugar)

20.3.1 Source

Fruit – fructose
Lactis – Lactose (Lactis- Milk)
Malt- Maltose
Xylum - Xylose (Xylum- wood)
Cell – Cellulose

20.3.2 Number of carbon atom content

3- Tri- Triose
4- Tetra- Tetrose
5-Penta- Pentose
6-Hexa- Hexose
7- Hepta- Heptose

20.3.3 Type of functional group content

Aldehyde- Aldose
Ketone- Ketose

20.3.4 Rotation of plane polarized light

Dextro rotatory- Dextrose
Laevo rotator- Levulose

20.3.5 Classification )

Three major class: sugars, non-sugars and derived carbohydrate

1. Sugars- sweet, crystalline and water-soluble
2. Non-sugars- Tastelss, Amorphous and Water-insoluble
3. Derived carbohydrates- Derivatives of carbohydrates

• Oxidation products- ascorbic acid, saccharic acid etc
• Reduction products- glycerol, inositol, sorbitol etc
• Deoxy products- 2-deoxy ribose, methyl pentose etc
• Amino products- glucose amine, galactosamine