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 which then react with -naphthol to produce the pink color.
Download this lecture as PDF here