Aldehydes and ketones
Read our general notes on Risk Assessment
What's the mechanism when Benedict's Test acts with monosaccharides and disaccharides?
250509
Ulex replies
I hope you don’t really mean the mechanism i.e. the individual steps by which the reaction takes place. If you do, then I must disappoint you because I do not know and have never seen such a full account. I can, however, try to explain what happens and, to some extent, why.
Benedict’s solution is a mixture containing copper(II) ions, a complexing agent (a salt of tartaric acid) and an alkaline substance (sodium carbonate). Normally if you make a solution of copper(II) ions alkaline, a precipitate of copper(II) hydroxide (and in this case carbonate) would result. The complexing agent is there to prevent this happening. When a suitable carbohydrate is boiled with Benedict’s solution, the copper(II) ions are reduced to copper(I) ions which are then precipitated as copper(I) oxide, a red solid.
Turning to the carbohydrate and what makes it ‘suitable’. You may already know that the monosaccharide glucose is an equilibrium mixture of two molecular forms, a ring form and a straight chain form. The straight chain form includes the group –CHO, an aldehyde group. This is easily oxidised to an acidic group, which means that glucose is a reducing agent which will reduce Benedict’s solution.
One little surprise, here, is that the straight chain form of fructose is not an aldehyde but a ketone which, you might think, would not be a reducing agent; yet fructose does, in fact reduce Benedict’s solution, presumably because it isomerises to an aldehyde.
Disaccharides, in general, do not reduce Benedict’s solution because the relevant carbon atoms are being used to join two monosaccharide units together. On boiling with the alkaline Benedict’s solution, hydrolysis to monosaccharides may occur so that some positive reaction may be seen.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 2 ratings so far
, rated at 
updated: 26 May 2009
