Lesson 6 - Reducing and non-reducing sugar tests Flashcards
Name 3 common monosaccharides
Glucose, galactose and fructose
Monosaccharides are the monomers from which larger ____________ are made
Carbohydrates
What bond does a condensation reaction between 2 monosaccharides form?
A glycosidic bond
Which monosaccharides are these disaccarides formed from?
Maltose
Sucrose
Lactose
Maltose - formed from the condensation of 2 glucose molecules
Sucrose - formed from the condensation of a glucose and fructose molecule
Lactose - formed from the condensation of a glucose and galactose molecule
Which glucose isomers are these polysaccharides formed from the condensation of?
Glycogen & starch
Cellulose
Glycogen & starch - alpha glucose
Cellulose - beta glucose
Why are reducing sugars sometimes called reducing agents?
Reducing sugars can donate electrons to reduce another chemical (e.g. in this case Benedict’s reagent)
Which sugars are/aren’t reducing sugars?
● ALL monosaccharides are reducing sugars - able to reduce (give electrons to) other molecules
● Some disaccharides are reducing sugars, including maltose & lactose
● Sucrose is not a reducing sugar, we call it a non-reducing sugar
How can we check for the presence of reducing sugars?
● Benedict’s reagent is an alkaline 2+ solution of copper II sulphate (Cu2+ ions)
● When a reducing sugar is heated with
● When using Benedict’s reagent to carry out the reducing sugar test: If there is a colour change from blue to red(orange/yellow/green) there is a reducing sugar present.
What’s oxidised/reduced in Benedict’s test for reducing sugars?
The soluble, blue Cu2+ ions are reduced (accept electrons donated by the reducing sugar) and form insoluble, red Cu+ ions.
Ionic equation for Benedict’s test for reducing sugars
Cu2+ + e- -> Cu+ (insoluble precipitate)
Blue Brick red
Why can the results for Benedict’s test for reducing sugars be semi-quantitative?
You may get some other colours (green, yellow, orange) appear other than brick red, this is due to the concentration of reducing sugar found in the unknown
Test for non-reducing sugars
● add Benedict’s reagent
● heat in water bath
● observe the result - negative (stays blue)
● in a new test tube, add the unknown sugar (that got the -ve result) and boil with HCl in a water bath
● add an alkali (sodium hydrogen carbonate) to neutralise
● add Benedict’s reagent & heat in water bath
● if present, there should be a colour change to brick red (or green, orange, brown) if a non-reducing sugar was present
Results for glucose in reducing sugars test
Glucose showed a colour change from blue to brick red, producing a positive result. It turned red because Glucose is a reducing sugar (as all monosaccharides are reducing sugars), which can act as a reducing agent by donating an electron to the Cu2+ ions in the Benedict’s solution, converting them to Cu+ ions. These Cu+ ions then combine with Oxygen in the solution to form Copper Oxide (CuO), which is a red precipitate.
Results for sucrose in reducing sugars test
Sucrose did not show a colour change, it remained blue, showing a negative result. This is because Sucrose is not a reducing sugar, it is a non-reducing sugar.
Results for sucrose in non-reducing sugars test
Sucrose, produced a positive result in the non-reducing sugars test (hopefully!), as it turned from blue to brick red. This was because, in the non-reducing sugars test, we heat the sucrose with HCl, which resulted in the hydrolysis of the glycosidic bonds in sucrose, breaking sucrose down into its monomers - glucose and fructose. These are both reducing sugars, so when then heated with Benedict’s solution, produces a positive result (a colour change from blue to brick red). This shows the presence of a non-reducing sugar (sucrose) as before heating with HCl, there was no colour change (in the reducing sugars test).
