Organic Chemistry

Module 7. Carbohydrates

Lesson 23A
REACTIONS OF MONOSACCHARIDES

23. A.1 Introduction

Carbohydrates which cannot be hydrolyzed into simpler compounds- therefore also known as simple sugars
Contain 3 to 10 carbon atoms- triose, tetrose, pentose, hexose etc
Contain –CHO or >C=O group- aldose or ketose
Hexoses- the most important class of monosaccharides – widely distributed in nature
The most important hexoses- glucose, galactose, mannose and fructose
Some aldopentoses- biologically important- Ribose and Deoxy ribose – component of DNA, RNA , ATP etc.

Table 23.A.1 Example of monosaccharides

23.A.2 Reactions of Monosaccharides

• Differences in structures- frequently affect rate of reaction and sometimes ability to react at all – the differences – useful in detection and determination of sugar.
• Reactive groups

1. Hydroxyl groups

-Anomeric hydroxyl group- more reactive
- primary alcohol group

2. Carbonyl group- -CHO and >C=O

1. Glycoside formation

• Acetal formed from carbohydrate- glycoside
• Glycoside- general name- specific name given – based on particular sugar involved – e.g. glucoside, galactoside, fructoside etc
• The hydroxyl group on anomeric carbon- involved in glycoside formation. (Fig. 23.1)

• Disaccharides are glycoside in which ‘alcohol’ is another monosaccharide (i.e. its primary hydroxyl group)
• Glycoside formation fixes the ring structure of monosaccharide – no shift from α to β or β to α form – therefore glycosides do not mutarotate and prevents many reactions of –CHO or >C=O

2. Ester formation

• Esterification of hydroxyl group- useful reaction – helps in identification of structure
• Esterified by treatment with acid anhydride or acid halide – to acetate, stearate etc. (Fig. 23.2)

• Formation of esters with inorganic acids- e.g. phosphoric acid= phosphoesters (phosphates)- great biological significance= phosphates of ribose and deoxyribose- component of vital natural compounds – Nucleic acids (Fig.23.3)

3. Copper reduction(Reducing properties)

• Possessing free of potentially free aldehyde or ketone group – therefore have ability to reduce certain metal ions – copper
• Cupric hydroxide (blue) in alkaline medium (Fehling solution) –heated in presence of reducing agent (sugar)- reduced to insoluble cuprous oxide (brick red)- used as qualitative and quantitative tests for the reducing sugars.