Whenever we add $\ce{H+}$/ Ethanol to glucose in its hemiacetal form, why doesn't pinacone/pinacolone rearrangement take place in place of nucleophillic substitution. The product would lead to formation of a carbonyl bond and so, should be the favoured product.
The actual pathway is:
But, why can't the reaction occur like this with ethanol as a polar solvent?:
First of all, there's no pinacole in the first place, so pinacole/pinacolone rearrangement isn't possible. Your rearrangement reaction/hydride shift wouldn't be called pinacole/pinacolone rearrangement.
Second, we have to think about why this would happen. Such rearrangements happen because we produce a more stable carbocation. In case of pinacole/pinacolone, if we do the rearrangement we end up with:
Which is well stabilized and after deprotonation a stable compound.
In case of sugars it's a little bit different, since it's not an alcohol in 1-position, it's a hemiacetale. This is extremly important since it has quite different reactivities.
We can cleave of water under much weaker acidic and mild conditions than in case of alcohols. The produced carbocation is quite stable since we got an oxygen stabilizing the cation, therefore no rearrangement is happening.
