Question

look up the structures for mannose and galactose. what would you expect to see if you performed a Benedict's or Fehling's test on each monosaccharides? what would your observations tell you about these two sugars?

Answer 1

Frist of all we have to know that what is a Benedicts test or Fehling test

Fehling’s Test:

This forms the reduction test of carbohydrates. Fehling’s solution contains blue alkaline cupric hydroxide solution, heated with reducing sugars gets reduced to yellow or red cuprous oxide and is precipitated. Hence, formation of the yellow or brownish-red colored precipitate helps in the detection of reducing sugars in the test solution.

Benedict’s Test:

As in Fehling’s test, free aldehyde or keto group in the reducing sugars reduce cupric hydroxide in alkaline medium to red colored cuprous oxide. Depending on the concentration of sugars, yellow to green color is developed . All monosaccharides are reducing sugars as they all have a free reactive carbonyl group. Some disaccharides, like maltose, have exposed carbonyl groups and are also reducing sugars, but less reactive than monosaccharides

SO These test only responds to the reducing sugar and not to non-reducing sugars

So from the above mannose is a reducing sugar which will give +ve Benedict's and Fehking's test

and Galactose will give -ve result as it is not a reducing sugar