🧪 Chemical Properties

This lesson explains the key chemical reactions of carbohydrates, especially monosaccharides, and how these reactions are used for identification and analysis in biochemistry.

CHEMICAL PROPERTIES OF CARBOHYDRATES

Monosaccharides

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

Dilute alkali

• Sugars in weak alkaline solutions undergo isomerization to form 1,2-enediol followed by the formation of a mixture of sugars.

Strong alkali

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 w hich then react with -naphthol to produce the pink color.

Summary Cheat Sheet

Topic	Key takeaway
Main focus	Chemical Properties.
Section context	Revise this lesson with the rest of Carbohydrates for stronger conceptual continuity.