BIC 101 :: Lecture 05 :: CHEMICAL PROPERTIES OF CARBOHYDRATES
Reactions of monosaccharides are due to the presence of hydroxyl (-OH) and the potentially free aldehyde (-CHO) or keto ( >C=O) groups.
Reaction with alkali
- Sugars in weak alkaline solutions undergo isomerization to form 1,2-enediol followed by the formation of a mixture of sugars.
Under strong alkaline conditions sugar undergo caramelization reactions.
Reducing property of sugars
- Sugars are classified as either reducing or non-reducing depending upon the presence of potentially free aldehyde or keto groups.
- The reducing property is mainly due to the ability of these sugars to reduce metal ions such as copper or silver to form insoluble cuprous oxide, under alkaline condition.
- The aldehyde group of aldoses is oxidized to carboxylic acid.
- This reducing property is the basis for qualitative (Fehling's, Benedict's, Barfoed's and Nylander's tests) and quantitative reactions.
- All monosaccharides are reducing. In the case of oligosaccharides, if the molecule possesses a free aldehyde or ketone group it belongs to reducing sugar (maltose and lactose).
- If the reducing groups are involved in the formation of glycosodic linkage., the sugar belongs to the non- reducing group (trehalose, sucrose, raffinose and stachyose).
Reaction with phenylhydrazine
- When reducing sugars are heated with phenylhydrazine at pH 4.7 a yellow precipitate is obtained.
- The precipitated compound is called as osazone.
- One molecule of reducing sugar reacts with three molecules of phenylhydrazine.
- D-mannose and D-fructose form same type of osazone as that of D-glucose since the configuration of C-3, C-4, C-5 and C-6 is same for all the three sugars.
- This reaction serves to distinguish between aldose and ketose sugars.
Reaction with acids
- Heating a solution of hexoses in a strong non-oxidising acidic conditions, hydroxyl methyl furfural is formed.
- The hydroxymethyl furfural from hexose is usually oxidized further to other products When phenolic compounds such as resorcinol, -naphthol or anthrone are added, mixture of coloured compounds are formed
- The molisch test used for detecting carbohydrate in solution is based on this principle.
- When conc. H2SO4 is added slowly to a carbohydrate solution containing -naphthol, a pink color is produced at the juncture.
- The heat generated during the reaction hydrolyse and dehydrate it to produce furfural or hydroxymethyl furfural which then react with -naphthol to produce the pink color.
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