Answer:In 1875 Emil Fischer prepared phenylhydrazine (PhNHNH2) by the reduction of a phenyldiazonium salt. Years later (1891) this compound proved invaluable during his studies on the stereochemistry of glucose. The two aldohexoses, D-glucose 1 and D-mannose 3, and the D-ketohexose, D-fructose 2, are interconvertible with one another in the presence of calcium hydroxide without altering the stereochemistry at C3, C4 and C5 (atoms within the blue circles). The intermediate for this transformation is the enediol 4, which is formed via enolization. The process is known as the Lobry de Bruyn - Alberda van Eckstein rearrangement. Under mild conditions both D-glucose and D-mannose form their respective phenylhydrazones 5 and 7 (as does D-fructose which is not shown). Upon treating the phenylhydrazones with two additional equivalents of phenylhydrazine, all three hexoses are converted to the same osazone 6. Notice that one of the equivalents of phenylhydrazine is converted into aniline (PhNH2) and ammonia (NH3). This is a reduction of phenylhydrazine. The osazone is an oxidation product. Effectively, the C2 hydroxyl group of the phenylhydrazones has been oxidized to a ketone level removing the C2 stereochemistry. In addition the C1 hydroxyl of the fructose phenylhydrazone is oxidized to the level of an aldehyde. The question remains, how does this process occur? While the mechanism of the phenylhydrazone formation is a straightforward addition of phenylhydrazine to a carbonyl group with concommitent elimination of water, the osazone formation has been formulated by several mechanisms.
Fischer Mechanism:
Emil Fischer's proposed a mechanism invoking initial formation of a phenylhydrazone 8 (Ar = Ph, ignore the red nitrogen) from an α-hydroxy aldehyde or α-hydroxy ketone. In an unspecified manner the second and third equivalents of phenylhydrazine effected oxidation of the secondary alcohol (structure 9) --- note the formation of the reduction products aniline and ammonia --- and derivatization to form the osazone 10. While this mechanism may be correct, the lack of evidence for the transformation 8 ---> 9 leaves Fischer's proposal as part speculation.
Weygand Mechanism A:
Weygand proposed two possible mechanisms based upon hydrogen isotope studies, both of which were claimed to be operable. Mechanism A has the phenylhydrazone 8 (Ar = Ph, ignore the red nitrogen) break the N-N bond to form aniline (reduction) and oxidize the Both of the Weygand mechanisms predict that the labeled nitrogen will appear in ammonia. The difference is that mechanism A predicts all of the label is in ammonia and none in the osazone 12. On the other hand, mechanism B predicts half of the label in ammonia (21/22) and half in the osazone 19/20. The p-nitrophenylhydrazones of D-fructose (24), benzoin (25) and α-hydroxycyclohexanone (26) were prepared (40% labeled) and subjected to osazone formation with unlabeled p-nitrophenylhydrazine. In all three cases the ammonia collected was greater than 50% of the 40% label in 24-26. In addition, unreacted hydrazones 24-26 had the same percentage of label as they did before the reaction (no hydraz
Answers & Comments
Answer:In 1875 Emil Fischer prepared phenylhydrazine (PhNHNH2) by the reduction of a phenyldiazonium salt. Years later (1891) this compound proved invaluable during his studies on the stereochemistry of glucose. The two aldohexoses, D-glucose 1 and D-mannose 3, and the D-ketohexose, D-fructose 2, are interconvertible with one another in the presence of calcium hydroxide without altering the stereochemistry at C3, C4 and C5 (atoms within the blue circles). The intermediate for this transformation is the enediol 4, which is formed via enolization. The process is known as the Lobry de Bruyn - Alberda van Eckstein rearrangement. Under mild conditions both D-glucose and D-mannose form their respective phenylhydrazones 5 and 7 (as does D-fructose which is not shown). Upon treating the phenylhydrazones with two additional equivalents of phenylhydrazine, all three hexoses are converted to the same osazone 6. Notice that one of the equivalents of phenylhydrazine is converted into aniline (PhNH2) and ammonia (NH3). This is a reduction of phenylhydrazine. The osazone is an oxidation product. Effectively, the C2 hydroxyl group of the phenylhydrazones has been oxidized to a ketone level removing the C2 stereochemistry. In addition the C1 hydroxyl of the fructose phenylhydrazone is oxidized to the level of an aldehyde. The question remains, how does this process occur? While the mechanism of the phenylhydrazone formation is a straightforward addition of phenylhydrazine to a carbonyl group with concommitent elimination of water, the osazone formation has been formulated by several mechanisms.
Fischer Mechanism:
Emil Fischer's proposed a mechanism invoking initial formation of a phenylhydrazone 8 (Ar = Ph, ignore the red nitrogen) from an α-hydroxy aldehyde or α-hydroxy ketone. In an unspecified manner the second and third equivalents of phenylhydrazine effected oxidation of the secondary alcohol (structure 9) --- note the formation of the reduction products aniline and ammonia --- and derivatization to form the osazone 10. While this mechanism may be correct, the lack of evidence for the transformation 8 ---> 9 leaves Fischer's proposal as part speculation.
Weygand Mechanism A:
Weygand proposed two possible mechanisms based upon hydrogen isotope studies, both of which were claimed to be operable. Mechanism A has the phenylhydrazone 8 (Ar = Ph, ignore the red nitrogen) break the N-N bond to form aniline (reduction) and oxidize the Both of the Weygand mechanisms predict that the labeled nitrogen will appear in ammonia. The difference is that mechanism A predicts all of the label is in ammonia and none in the osazone 12. On the other hand, mechanism B predicts half of the label in ammonia (21/22) and half in the osazone 19/20. The p-nitrophenylhydrazones of D-fructose (24), benzoin (25) and α-hydroxycyclohexanone (26) were prepared (40% labeled) and subjected to osazone formation with unlabeled p-nitrophenylhydrazine. In all three cases the ammonia collected was greater than 50% of the 40% label in 24-26. In addition, unreacted hydrazones 24-26 had the same percentage of label as they did before the reaction (no hydraz
Explanation: Pa brainliest THANKS