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In glucose solution glucose is present in ring form but 1% can be in open chain form. As carbon carbon bond (c-c) roation is possible. Then why does Glucose can't be converted in galactose or mannose in solution?

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    $\begingroup$ Optical isomers are quite distinct from rotamers. $\endgroup$
    – Mithoron
    Aug 6, 2017 at 18:53

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Yes, carbon-carbon bond rotation can happen in the open chain form of these carbohydrates. But you are confusing rotation with stereochemical configuration. This can only be altered by breaking bonds. Consider the following Fischer projection of D-glucose.

D-glucose

Compare this to the Fischer projection of D-galactose.

D-galactose

These two molecules differ only in the orientation of the hydroxyl, OH, and hydrogen, H groups. If you want to turn one into the other, you will have to break bonds and reattach them. This does not spontaneously happen in water and is frankly difficult even in the laboratory. You cannot merely turn one into the other through rotations around a bond.

It is a common mistake that chemistry students make thinking that they can simply rotate around one of the carbons to change the stereochemistry. If you build a model, you will tangibly see why this cannot happen. Models are great true to life representations of molecules (the better ones at least). You will find out that you cannot turn glucose into galactose without breaking several bonds.

And remember, rotations do not change stereochemistry. Only bond breaking and reforming does.

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