Question

Which of the following pairs form the same osazone?
A. Glucose and Fructose
B. Glucose and Galactose
C. Glucose and Arabinose
D. Lactose and Maltose

Answer 1

‌ The osazone formation is a characteristic reaction of active groups like carbonyl group as aldehyde and ketone. The osazone is formed by the action of phenylhydrazine on sugar. In excess of reagent, the sugar is converted into diphenyl hydrazine of the sugar. The reagent can convert the carbonyl group to the phenyl hydrazine residue. The general for the aldehyde is as shown below:
$R - CHO\rightarrow{{{C_6}{H_5}NHN{H_2}}}R - CH = N - NH - {C_6}{H_5}$

Complete answer:
Sugar reacts with the excess of phenyl hydrazine to form phenyl hydrazone. This is called the osazone and the reaction is called the osazone formation reaction.
The aldehyde reacts with the phenyl hydrazine to form phenyl hydrazone. However, the sugar which has aldehyde or ketone group, on warming up with the excess of phenylhydrazine reagent gives diphenyl hydrazones.
For example, let's consider that glucose reacts with the excess of phenylhydrazine to give the corresponding phenylosazone, which is glucosazone. The reaction takes place in three steps:
Step1: The first molecule of phenyl hydrazine reacts with the aldehyde $( - CHO)$ group of the glucose to give the glucose phenylhydrazine. During this step, the one equivalent of the hydrazine is utilised to generate phenyl hydrazine.
Step2: In step2, the second molecule of the phenyl hydrazine oxidizes the reactive $\alpha -$ hydroxyl group to generate the carbonyl group. Since, the aldehyde has already reacted to give hydrazine, the next equivalent of reagent attacks on the hydroxyl group on the adjacent carbon atom, oxidizes it to the carbonyl group.
Step3: Finally, the third molecule 0f the phenyl hydrazine reacts with the earlier formed carbonyl group and converts into hydrazine. Now, the glucose molecules have two phenyl hydrazone groups thus it is called the diphenyl hydrazine or glucosazone.
Hence, the correct option is (A.)

Note: The osazone formation takes place only in the excess amount of phenyl hydrazine. In the limited supply of phenyl hydrazine, the reaction proceeds with the formation of one glucose phenylhydrazine.