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From Wikipedia:

It involves two reactions. Firstly glucose with phenylhydrazine gives glucosephenylhydrazone by elimination of a water molecule from the functional group. The next step involves reaction of one mole of glucosephenylhydrazone with two moles of phenylhydrazine (excess). First phenylhydrazine is involved in oxidizing the alpha carbon to a carbonyl group, and the second phenylhydrazine involves in removal of one water molecule with the formyl group of that oxidized carbon and forming the similar carbon nitrogen bond.

Why doesn't it first react with the formyl group and stop with just one phenylhydrazine? That is, why doesn't the nitrogen of phenylhydrazine attack the $\mathrm{sp^2}$ carbonyl carbon (a nucleophilic addition reaction)?

Why does it oxidize glucose first and then proceed?

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As according to modern mechanism it has first reacted with formyl then goes for tautomers and to form an intermediate..which further form iminoketone ,as phenylhydrazine is in excess then it again react with the c=o formed on C2 and then with carbon first and give osazone . And as further reaction stops by H- bonding between H and Nitrogen of each phenyl hydrazine which forms chelation and makes it stable.

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    $\begingroup$ This is very difficult to understand at the moment. I can't work out whether it is actually answering the question or not. Please consider rewriting it. $\endgroup$ – bon Sep 4 '16 at 8:32
  • $\begingroup$ Three equivalents of phenylhydrazine are required to form an osazone. One acts as an oxidant. Look here: chemistry.stackexchange.com/questions/42702/… $\endgroup$ – user55119 Dec 14 '18 at 23:06

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