Question

why ribose gave a positive test in lasker test even it is an aldose???

Answer 1

I can provide perhaps the start to an answer.

Right - so I was not familiar with the Lasker Test, but an internet source provided this: "The Lasker and Enkelwitz test utilizes Benedict's solution, although the reaction is carried out at a much lower temperature." http://generalchemistrylab.blogspot.com/2011/12/lasker-and-enkelwitz-test-for-ketoses.html

Benedict's I'm a bit more knowledgeable about. Benedict's tests for reducing sugars - these are compounds that have aldehyde groups, which essentially means that the C=O group is located at the end of the carbon chain. WHY this reaction occurs and why you get a color change is beyond me; haven't taken Orgo or anything like that. However, because ribose is an aldose...Benedict's reagent gives off a positive.