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I understood that the representation of fructose is:

enter image description here

But during the formation of sucrose:

enter image description here

How did fructose, whose representation is given in the first picture, turn out to be different in the second. I tried to visualize myself rotating the molecule but the position of oxygen changes and it doesn't match with the one in sucrose.

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    $\begingroup$ Hint: Draw a vertical line/axis on your first diagram, connecting the oxygen to the middle of the C3-C4 bond. Then rotate the molecule 180 degrees counter clockwise. Now compare this with the fructose residue in sucrose ;) $\endgroup$ – paracetamol Jul 16 '17 at 10:13
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Ah, a (fairly) common conundrum that assails us Chemistry students when we start Biochem. ;-)

At first glance, the fructose molecule in your first picture, and that in the second picture appear to have different conformations:

enter image description here enter image description here

Believe me, they're the same. "How?" you ask? Stay with me here (it requires a little imagination):

Draw a vertical line/axis on your first diagram along the plane of the furanose ring, connecting the oxygen atom at the apex, right down to the middle of the C3-C4 bond. Then rotate the molecule 180 degrees counter-clockwise.

Now compare this with the fructose residue in sucrose.

(I lack the necessary Org. Chem. sketching tools to depict this... apologies)


If you weren't able to wrap your head around "rotation" bit, then, let's assign locant numbers to the carbon atoms in the two diagrams and then compare them:

enter image description here enter image description here

I've drawn them tiny, my bad

Must've clicked by now ;)

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