Question about sucrose

[sfoksn]

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Why does sucrose give negative tollen's test?

I understand the reason why it gives negative Benedicts test, but not sure about negative tollen's.

Can someone please explain?

THank you.

 

[JBarr29]

There is no aldehyde or alpha hydroxy ketone present in sucrose for it to react with the tollens reagent.

 

[sfoksn]

isn't sucrose made of glucose and fructose?

I thought glucose was aldohexose and fructose was aldopentose..

Doesn't that mean that it has aldehyde groups?

Sorry if this question is too obvious...

 

[JBarr29]

in the closed ring form of sucrose, there is no free aldehyde. Take a look at the structure. no double bonded oxygens present at all i believe. i could be wrong though. and nothing is too obvious haha

 

[sfoksn]

Oh i see..

So only the sugars that mutarotate back into linear fashion can be tollen +, right?

 

[redchesus]

isn't it the same reason as the Benedict's test?

sucrose is a non reducing sugar so there's no aldehyde/hemiacetal or ketone/hemiketal interconversion

 

[sfoksn]

yeah i understand why benedict's test was negative. it's in my post.

thanks for your input, though

 

[jmiami6]

Doesn't both the glucose and fructose have a free aldehyde?

Shouldn't it give a + Tollens test?

 

[Dotoday]

Both Glucose and Fructose have free aldehyde. However, when they are united with a glycosidic linkage, they loose their Hemiacetal ( open chain aldehyde groups) , therefore cannot give a positive Tollens and/or Benedicts test. Hope this helps.

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