Sugars! All sorts of sugars!

[fashafosho]

Hey everyone,
Thought I had my sugars down. Guess not.
So if anyone can just clarify the following:
Reducing vs. non-reducing sugars
How do you tell an alpha linkage apart from a beta (is there an easy way to remember this- for some reason I can never picture the compound in my head when I come across a 1-4 linkage problem)
Hemi-acetals, Hemi-ketals...acetals and ketals...what are the main differences here (besides the open ring orientations?)

Basically, any easy way of systematically remembering the important concepts of sugars would be much appreciated. Thanks!

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[RSAgator]

alpha/beta cross linking seems pretty out of the scope of the MCAT

 

[zanzounita]

A reducing sugar's anomeric carbon is free to become oxidized, thus reducing another molecule such as Cu2+ in a Benedict's test..
For example, lactose has a free -OH group to become oxidized, whereas sucrose does not.

As for remembering beta vs. alpha. I always remember beta being ontop..."'tis better to be ontop"...

 

[BloodySurgeon]

A reducing sugar's anomeric carbon is free to become oxidized, thus reducing another molecule such as Cu2+ in a Benedict's test..
For example, lactose has a free -OH group to become oxidized, whereas sucrose does not.

As for remembering beta vs. alpha. I always remember beta being ontop..."'tis better to be ontop"...

Love the MNEMONIC! However this is out of scope for the mcat. You do need to know the difference between hemi-acetal vs. hemi-ketal, but I doubt it will be asked in the sugar form (which for your own curiousity) will be reducing sugars.

 

[travelbug73]

More appropriate for the alpha and beta orientations would be
alpha: OH on anomeric carbon (C1 on 6-membered ring and C2 on a 5-membered ring) opposite side of CH2OH (C6 on both 5- and 6-membered rings)
beta: OH on anomeric carbon same side of CH2OH

The reason I'm clarifying this is because I got a question wrong with the beta on top mnemonic when they changed the orientation of the molecule. In this case, it was OH on the bottom and I called it alpha and realized later that it is still beta because the CH2OH group was also on the bottom. If you are good with flipping the molecule then the suggested mnemonic is great. However, if you are like I, then you might want to remember it differently.

For reducing and non-reducing sugars, for me it helps to look at the anomeric carbon.
If the anomeric carbon has an OH group on it, it is a hemiacetal or hemiketal, depending on the original sugar.
If however, the anomeric carbon only has an H or R group, then it is an acetal or a ketal.
Hemiacetal or hemiketal = reducing sugar eg: lactose (galactose + glucose). This will give you a positve Tollen's test or Benedict's test.
Acetal or ketal = non-reducing sugar eg: sucrose (glucose + fructose). Negative Tollen's and Benedict's.

 

[fashafosho]

Thank you!