Reducing Sugars

[au5233]

Hey guys,

so I just want to make sure I'm clear on carbohydrates. So basically if a carbohydrate has a free hemiacetal grp, it is a reducing sugar, can mutarotate, and yield both + Tollens and Benedicts. As well, ketose sugars will yield the same results because they can isomerize to aldoses.

But doesn't every monosaccharide have a free hemiacetal grp (ketal grp), so they will all be reducing sugars?
And since disaccharides don't have a free hemiacetal grp, they are all considered non-reducing?

Will you ever get a + Tollens and - Benedicts, or vice versa?

Thanks guys!

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[IsRaEl21]

U should know that sucrose is not a reducing sugar. It contains an acetal group. (Sucrose is derived from Fructose+Glucose) fructose has the ketone group on it not an aldehyde like in reducing sugars. U really need to read the Destroyer Dr. Romano has the best explanations to it, he also draw the molecules out so u will see if it's acetal or hemiacetal.

 

[hrndawg]

In general, reducing sugar capability is only concerned with the anomeric carbon. Meaning, its not the presence of a hemiacetal group in any part of the ring that decides whether a sugar is a reducing sugar or not, but if a hemiacetal is present by the anomeric carbon.
In regards to a disaccharide, although a bond might be formed btw two sugars, if the bond is not btw the two anomeric carbons, then at least one of the sugars can still have a hemi group at its anomeric carbon, thereby making the whole disaccharide a reducing sugar. In a sugar like sucrose, however, the bond is btw the anomeric carbons of a glucose and fructose. Neither of them has a hemi group available, and that's why sucrose is a non-reducing sugar.

 

[IsRaEl21]

u are right nice explanation, the hemiacetal must be in the anomeric carbon.

 

[hrndawg]

Also, in regards to the question about monosaccharides, not all of them are reducing sugars. For example, if glucose is put in a methanol solution, with some acid catalyst, the hemiacetal will change into an acetal (OMe replacing the OH). This group of acetal compounds are known as glucosides (a term used only when the sugar is glucose. The products of this reaction when any other sugar is used, are generally known as glycosides). The glucosides lack a hemiacetal group and therefore are non-reducing sugars. So even a monosaccharide can be a non-reducing sugar.

I just looked this up online, so if you want more details either google it, or try this website http://chemistry2.csudh.edu/rpendarvis/dipolysacch.html

 

[au5233]

Wow, you guys are awesome. Thanks. That totally helped me out.

I haven't come across the destroyer questions yet dealing with carbohydrates...prob over the next few days.