CIE A Level Biology复习笔记2.1.3 Testing for Non-Reducing Sugars

Testing for Non-Reducing Sugars

  • Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons (a reducing sugar that is able to donate electrons is itself oxidised)
    • OILRIG in Chemistry
  • If Benedict's test has been carried out on a solution and it shows that no reducing sugars are present then a modified version of the test can be carried out to test for the presence of non-reducing sugars

To test for non-reducing sugars:

  • Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
  • Neutralise the solution with sodium hydrogencarbonate
    • Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict’s test to work
  • Then carry out Benedict’s test as normal; add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs (orange-red precipitate), a non-reducing sugar is present

Explanation

  • The addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules
  • The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Reducing & Non-reducing Sugars Table

Table_-2.1-Reducing-and-Non-reducing-sugars

 

 

 

 

 

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