For mutarotation, at least one hemiacetal group must be present in the sugar. Therefore, non-reducing sugars do not show mutarotation.


Consider the following reaction:

$$\ce{\beta-D{-glucose} + CH3OH + HCl -> \alpha-D{-glucopyranoside} + \beta-{D-glucopyranoside}}$$

Even though the pyranoside has an acetal group, the reaction is still reversible? Therefore, shouldn't pyranoside show mutarotation? Isn't the mechanism for the reversible reaction for acetal same as the one for hemiacetal?


In principle, the acetalisation reaction is indeed reversible. Typical conditions to reverse it would be to use $\ce{TsOH}$ or $\ce{PPTS}$ and water.

However, both full acetalisation and liberation of the aldehyde require acidic conditions. Hemiacetalisation — the prime mechanism of mutarotation — proceeds without acidic catalysis, i.e. the inherent (low) acidity of water is sufficient. And to observe mutarotation, you typically dissolve the pure sample in a pure solvent (with no acid added).

If you dissolve e.g. methyl α-ᴅ-glucopyranoside in water and add catalytic amounts of acetic acid, you will observe mutarotation but not if you simply dissolve the O,O-acetal in water.


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