Organic #86 Topscore 2

[Orgodox]

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The question (#86 in organic topscore test number 2) was about sucrose. I know sucrose is a nonreducing sugar and I was pretty sure fructose is an example of a ketohexafuranoside because its a 5 membered ring not 6 since the carbnyl is not an aldehyde but a ketone. So from that alone I should have chosen E but B really didnt look right to me...

Why is sucrose a alpha 1,2 linkage it kinda looks like an alpha 1,4 to me?

 

[hdu]

The glycosidic linkage in sucrose is between the carbon 1 of an alpha-D-glucose, and the carbon 2 of beta D fructose. Observe that the anomeric carbon of fructose is the carbon 2, and that is we say that the linkage is alpha1, beta 2. (Maybe the graphic you are looking at has the fructose in an inverted position, and it confuses you).

The fact that both anomeric carbons (the carbons that had the C=O group are compromised in the linkage -carbon 1 of glucose and carbon 2 of fructose) is precisely the reason that sucrose is a non reducing sugar.

Let me know if it is enough to understand it or you need more information.

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