Question

The number of phenylhydrazine molecules required for $1$ molecule of glucose osazone formation is:
(A) $1$
(B) $2$
(C) $3$
(D) $4$

Answer 1

Osazones are a group of derivatives of carbohydrate in organic chemistry which are formed when reducing sugars are reacted with an excess of phenylhydrazine at boiling temperatures. This is a two-step process.

Complete answer:
The reaction of osazone formation was discovered by German chemist Emil Fischer who used this reaction as a test to identify monosaccharides whose stereochemistry only has the difference of one chiral carbon.
The chemical compound used in the formation of osazone is phenylhydrazine. Its chemical formula is ${C_6}{H_5}NHN{H_2}$. The reducing sugars have a free aldehyde or ketone group attached to it to react with phenylhydrazine.
$1$ phenylhydrazine molecule reacts with the carbonyl group present on aldehyde or ketone on the sugar to form a glucose phenylhydrazine by eliminating a water molecule. This is the first step.
In the next step, $1$ equivalent of glucose phenylhydrazine with $2$ equivalents of phenylhydrazine (excess). The First phenylhydrazine which is used in oxidizing the $\alpha$ carbon on the carbonyl group and the second phenylhydrazine is used for the removal of one water molecule with the new-formed carbonyl group of that oxidized carbon and forming an osazone. $2$ molecules react in the same manner whereas ${3^{rd}}$ reacts differently.
Hence, Three molecules of phenylhydrazine are used for $1$ molecules of glucose osazone formation.

Therefore, Option (C) is correct.

Note:
The crystals of osazone have a characteristic shape under the light which is used to identify the type of sugar. This reaction requires the presence of a free carbonyl group. Thus, only reducing sugars can form osazones. Non-reducing sugars like Sucrose are unable to form an osazone.