Question

What are 2 half reactions for the reaction of glucose with Benedict’s reagent?

Answer 1

Benedict's reagent (often called Benedict's qualitative solution or Benedict's solution) is a chemical reagent named after American chemist Stanley Rossiter Benedict.[1]

Benedict's test

Positive Benedict's test

ClassificationColorimetric methodAnalytesReducing sugars

It is a complex mixture of sodium carbonate, sodium citrate and copper(II) sulphate pentahydrate.[2] It is often used in place of Fehling's solution to detect the presence of reducing sugars. The presence of other reducing substances also gives a positive reaction.[3] Such tests that use this reagent are called the Benedict's tests. A positive test with Benedict's reagent is shown by a color change from clear blue to a brick-red precipitate.

Generally, Benedict's test detects the presence of aldehydes and alpha-hydroxy-ketones, also by hemiacetal, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone, and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent.

The principle of Benedict's test is that when reducing sugars are heated in the presence of an alkali they are converted to powerful reducing species known as enediols. Enediols reduce the cupric compounds (Cu2+) present in the Benedict's reagent to cuprous compounds (Cu+) which are precipitated as insoluble red copper(I) oxide(Cu2O).

The color of the obtained precipitate gives an idea about the quantity of sugar present in the solution, hence the test is semi-quantitative. A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% and red indicates 2 g% or higher concentration.