D-glucose and D-fructose have different structures. Why do they form the same product on reacting with excess of phenyl hydrazine ?

Explanation

On reacting with excess of phenyl hydrazine `(C_(6)H_(5)NHNH_(2))` both D-glucose and D-fructose form an osazone. Although both these monosaccharides have different structures, but the osazone is the same for them. Actually in D-glucose and D-fructose, onyl the configurations of the first two carbon atoms are different while the remaining four carbon atoms have the same configuration. In the osazone, only the first carbon atoms in both of them are involved and have the same atoms or groups attached to them. Therefore, both of them form the same osazone.