Question

What happens when fructose heated with dilute acetic acid

Answer 1

Explanation:

I Experiment:  Molisch’s test:  1 ml fructose + 2 or 3 drops of Molisch’s reagent.  Mix well & add 1-2 ml Conc. Sulphuric acid along the sides of the test tube without shaking.  Observation:  A reddish violet ring at the junction of two liquids.  Inference:  Fructose is a carbohydrate.

2.  Composition of Molisch’s reagent:  1% α- naphthol in 95% ethanol.  Principle:  Carbohydrates when treated with Conc. Sulphuric acid undergo dehydration to give furfural or furfural derivatives.  These compounds condense with α- naphthol to form reddish violet ring.

3.  General test for all carbohydrates  Excess α- naphthol & impurities in reagent give green colored ring.  Molisch’s test is given by carbohydrates with at least 5 carbons.  Trioses & tetroses do not answer this test.  Interaction of acid & water produces heat & can cause charring of carbohydrates (due to precipitation of carbon) resulting in the formation of black ring.  Acid should be layered very slowly.

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