REDUCING sugar
- Using new samples of the solutions, add 2cm of dilute hydrochloric acid
- Boil the test tubes over a Bunsen burner passing it through a blue flame so it doesn’t reach boiling point to rapidly
- Once the solutions are bubbling remove from Bunsen burner
- Neutralise the solutions by adding small amounts of NaHCO until it stops fizzing
- Test the pH using litmus paper to make sure that the solutions are neutral
- Using the same solutions carry out the test for NON-REDUCING sugars
Results:
REDUCING sugars test:
I therefore did the NON-REDUCING sugars test with solutions A and C and then proceeded to the REDUCING sugars test the results were as following:
From my results I can tell:
Conclusion:
Reducing or non-reducing refers to whether a carbohydrate gives a positive or negative test towards the weakly oxidizing Cu in benedict’s solution. All reducing sugars contain free or potentially free aldehde. Only sugars that can’t open to the carbonyl form are non-reducing.
The Benedict’s test allows us to detect the presence of reducing sugars (sugars with a free aldehyde or ketone group). All monosaccharides (carbohydrates that can’t be hydrolysed to simpler compounds) are reducing sugars; they all have a free reactive carbonyl group. Some disaccharides have exposed carbonyl groups and are also reducing sugars. Other disaccharides such as sucrose are non-reducing sugars and will not react with benedict’s solution. The copper sulphate present in benedict’s solution reacts with electrons from the aldehyde or ketone group of the reducing sugar to form cuprous oxide, a red-brown precipitate. The final colour of the solution depends on how much of this precipitate was formed and therefore the colour gives an indication of how much reducing sugar was present.
GREEN - ORANGE – RED – BROWN
From this I know that solution A was sucrose as it remained its original blue colour after the original benedict’s test which meant it was non-reducing, we know that sucrose is a disaccharide and therefore is non-reducing. We know that sucrose is made up of one glucose molecule and one fructose molecule glycosidicaly bonded. After hydrolysing to break the glycosidic bond to its constituent monosaccharides it then gave a positive test for reducing sugars (benedict’s test).
Solution B was glucose as it formed a red precipitate, which showed it was a monosaccharide solution. Solution C was water it also remained its original blue colour however after hydrolysing it still gave a negative test for reducing sugars (benedict’s test). Finally, solution D was maltose as it left a green precipitate. Maltose is also a disaccharide, as previously explained not all disaccharides are non-reducing; however they are not as reducing as monosaccharides.
My final conclusion is that the reducing sugars include all monosaccharides, glucose and fructose and some disaccharides, maltose.
