(a). Glucose and fructose form the same osazone, what does it prove?
(b). Why more than `3mol "of"PhNHNH_(2)` do not react with glucose and fructose and how does `PhNHNH_(2),` which is a powerful reducting agent, act as an oxidising agent in this case?

Explanation

(a)
Glucose and furctose form the some osazoe which proves that the configuration of `OH` group or stereochemistry at `C-3,C-4,C-5,` and `C-6` in glucose and fructose is same.
(b). Mechanism: (Amadori rearrangement)

In the above mechanism, we find that `PhNHNH_(2)` does not act as an oxidising agent.
The reaction takes place only up to two `C` atoms because the intermediate osazone formed can be stabilised by `H-`bonding between `N` and `H` by forming a six-membered ring as shown below.