question_answer 3)

How do you explain the absence of aldehyde group in thepenta acetate of D-glucose? Free aldehyde group (—CHO) gives reaction with Tollen's reagent, Fehling reagent etc.  

Answer:

D-glucose, an aldohexose gives the characteristic reactionsof aldehydic group (e.g., with HCN Tollen's reagent, Fehling reagentetc.), but penta acetate of D-glucose does not give these reactions. Itmeans that either ?CHO group is absent or it is not available for thechemical reactions in penta acetyl glucose. Infact, the aldehydic groupis a part of hemiacetal structure which penta acetyl glucose has. So, it isnot available for reaction.