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How does maltose form its osazone crystal? If osazone is to be formed, then there must be aldehyde form for phenylhydrazine to react, but maltose is formed after hemiacetal formation. How do we address this?enter image description here

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    $\begingroup$ What happens if you tautomerize the hydroxyl proton on the far right to the nearby ring oxygen? $\endgroup$ – Oscar Lanzi Oct 27 '19 at 9:05
  • $\begingroup$ Then you have to break the ring. $\endgroup$ – Nandi A Oct 27 '19 at 9:35
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    $\begingroup$ And you then get what? Sugars easily do this tautomerization. $\endgroup$ – Oscar Lanzi Oct 27 '19 at 9:38
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The hemiacetal is in equilibrium with the aldehyde unless it reacts further to form a glycosidic bond (like the glucose unit on the left). When analyzing carbohydrates, you can distinguish between reducing sugars (aldehyde group is available, will react with Benedict's reagent like glucose unit on the right) and non-reducting sugars (aldehyde group is not available, like glucose unit on the left).

"Reducing" sugars will react to form osazone derivatives while "non-reducing" will not.

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