MadSci Network: Chemistry

Re: Why does the disaccharide sucrose give a negative test with benedict's soln

Date: Mon Feb 28 11:31:02 2000
Posted By: Alvan Hengge, Faculty, Chemistry and Biochemistry, Utah State University
Area of science: Chemistry
ID: 951162388.Ch

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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