Organic Chemistry

Module 7. Carbohydrates

Lesson 23B
REACTIONS OF MONOSACCHARIDES

23. B.1 Introduction

The reactions of monosaccharides are mentioned below

1. Osazone formation

• With calculated amount of phenylhydrazine reducing sugars form phenylhydrazones. (Fig. 23.5 Hydrazone formation)
  
• With excess of phenyldrazine – a more complex reaction takes place- forming osazone (Fig. 23.6)
• Time required for osazone formation- vary with sugar and typical for a given sugar.
• Osazones- yellow solid- each sugar has its own characteristic crystalline osazone- therefore useful in identification of sugars-which are difficult to crystallize.

2. Oxidation

• Product- depends on type of oxidizing agent (mild or strong)
• Mild oxidizing agent- only aldehydic group is oxidized to –COOH group.
• Strong agent- terminal hydroxyl group oxidized- dicarboxylic acid (terminal).
• Very strong agent- both aldehyde and terminal hydroxyl group oxidized- saccharic acid (mucic acid - very old name) (Fig. 23.7)

3. Reduction

• Carbonyl group (-CHO or >C=O) of sugar- reduced to an alcohol group- product known as alditol
• By variety of agent = H₂, Pt,Sodium amalgam (Fig. 23.8)
• Sorbitol- obtained by reduction of glucose- has commercial importance- used in manufacture of emulsifying agents and ascorbic acid.

4. Action of alkali (Alkaline interconversion)

• Lobry de Bruyn and Alberda Van Ekensiein rearrangement – reversible isomerization of monosaccharides- bring about the dilute alkalies (Fig. 23.9)
• Ketoenol transformation- tautomerization of carbonyl compounds- therefore any one of these sugar is treated with alkali- leads to equilibrium mixture of the three- via enediol intermediate= from which arrangement to any of the sugar can occur- because asymmetry at carbon 1 & 2 is destroyed in the enediol intermediate
• Because of this isomerism ketose (fructose) gives positive test with Fehling, Benedict and Tollen’s reagent
• Simple ketones do not respond to these reagents
• This interconversion also occurs in biological systems- therefore animals have ability to utilize mannose and fructose for energy – converted first into glucose and then utilized.

5. Dehydrocyclization (Action of acid and heat)

• Monosaccharides- relatively stable in dilute acid
• Treatment with hot strong acid- complex reaction – dehydrocyclization- loss of water and formation of furfural or furfural derivative
• Aldo pentose yields furfural

Fig. 23.10 Dehydrocyclization

• Aldohexose yields hydroxymethyl furfural (HMF)

Fig. 23.11 HMF formation

• Furfurals- very reactive- give characteristic color with polyhydric phenols (resorcinol, orcinol etc) = used in qualitative and quantitative analysis of sugars
• Appearance of color – identify the sugar- pentose or hexose, aldose or ketose etc
• Intensity of color- used to determine concentration
• HMF- an treatment of hot acid= transferred into levulinic acid and large amount of dark, insoluble condensation products (humins)- leads to browning – caramalization
• HMF determination- serves as an index of caramalization