|MadSci Network: Chemistry|
Benedictís test is an analytical chemical test that reveals the presence of aldehydes. An aldehyde is an organic compound having the chemical functional group CHO, which consists of a carbon atom doubly bonded to an oxygen atom and singly bonded to a hydrogen atom. The graphic that accompanies this answer shows you what an aldehyde group looks like.
Some sugars exist in solution as an equilibrium mixture of their ring form and an open chain form. The open chain form contains an aldehyde group, which is formed when the ring opens. When the ring closes, the aldehyde group is no longer present. Thus Benedictís test will only give a positive test for a sugar that is present at least partly as an open chain, or aldehyde, form. Maltose is, but sucrose is not.
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