1. A sugar's chain –OH attacks its own carbonyl to close a hemiacetal ring.
The open-chain aldehyde and its cyclic hemiacetal form are the same molecule in equilibrium.
2. Ring closure turns the flat carbonyl carbon into a new stereocenter, the anomeric carbon.
Because the incoming –OH can add to either face, that carbon becomes chiral where it was not before.
3. The two configurations at the anomeric carbon are the α and β anomers.
α has the anomeric –OH pointing down, opposite the CH₂OH; β points up, on the same side (Haworth view).
4. Dissolving pure α or β lets the ring open and re-close the other way, interconverting them.
The path runs through the achiral open chain, so α and β equilibrate to a fixed ratio (glucose ≈ 36% α, 64% β).
5. Because the anomers rotate light differently, the observed rotation drifts to a constant — that is mutarotation.
The change is acid/base catalyzed since it must pass through the open-chain carbonyl.
Summary
Ring closure makes a new anomeric stereocenter · gives α and β anomers · open/re-close interconverts them · through the open chain · optical rotation drifts to equilibrium = mutarotation · acid/base catalyzed.
Quiz yourself
Tap a question to reveal the answer — free, no login.
The former carbonyl carbon, now called the anomeric carbon.
The configuration of the anomeric –OH: α points down (opposite CH₂OH), β points up (same side) in the Haworth drawing.
The open-chain form, where the ring has opened back to the free carbonyl.
Because α and β have different specific rotations, so as the ratio equilibrates the net rotation drifts to a constant value.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.