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Santhoshini V. from R S KRISHNAN HR SEC SCHOOL , asked a question
Subject: Chemistry , asked on 22/1/13

Why does fructose reduce Fehling 's solution ?

EXPERT ANSWER

Priya G N ,Meritnation Expert added an answer
Answered on 23/1/13

An aqueous solution of copper sulphate, sodium hydroxide, potassium sodium tartarate is called the Fehling’s reagent.

Fehling’s reagent is used for reducing sugars and not specific for aldehydes.

Fructose is a reducing sugar with one free ketone group. It reduces Fehling’s solution giving brick red precipitate.

Fructose under alkaline condition converted to glucose and mannose by keto-enol tautomerism giving positive to Fehling’s solution.

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Abhinav Kumar , From Kendriya Vidyalaya , added an answer
Answered on 22/1/13

  fructose gives a positive test with Fehling 's solution , because fructose is converted to glucose and mannose under alkaline conditions. The conversion can be explained by the keto-enol tautomerism. 

equation  
Fig. 3: Conversion of fructose to glucose and mannose



The reduction of Fehling solution using fructose is not only to be attributed to the fact that the ketose is isomerized into an aldose. The treatment of fructose with alkali - e.g. Fehling solution - causes even decompostion of the carbon chain. More products with reducing capability are formed. 

  

Fig. 4: Decomposition of fructose
Santhoshini V. , From R S Krishnan Hr Sec School , added an answer
Answered on 22/1/13

@ Abhinav

Thank you sooooo much

Vinod Sharma , From Lady Irwin Girls Sr Sec School , added an answer
Answered on 23/1/13

 it so because fructose has one free ketone group in its structure....... so it can reduce fehling 's solution...