Q.What is mutarotation? Briefly explain why pure $\alpha$-D-glucose and pure $\beta$-D-glucose, dissolved separately in water, both end up showing the same equilibrium optical rotation.

Mutarotation is the phenomenon in which the specific optical rotation of a freshly prepared aqueous solution of a pure sugar anomer gradually changes over time until it settles at a fixed, constant value.

It happens because the cyclic ring form of glucose is in a dynamic equilibrium with the open-chain form: the ring can reopen (via the reverse of the hemiacetal-forming reaction) back to the open-chain aldehyde, and then reclose again, potentially forming either the $\alpha$- or the $\beta$-anomer at random, regardless of which anomer it started as. When pure $\alpha$-D-glucose is dissolved in water, some of it opens and recloses as $\beta$-D-glucose, and vice versa for pure $\beta$-D-glucose dissolved separately; both starting solutions keep interconverting until they reach the very same equilibrium mixture of roughly 36% $\alpha$-anomer and 64% $\beta$-anomer (with only a trace of open-chain form present at any moment). Since that final equilibriu …

Recognise that the cyclic and open-chain forms of glucose interconvert in solution, so any pure anomer dissolved on its own will drift to …

Thinking mutarotation means the sugar itself chemically changes into a different compound -- it remains glucose throughout; only the ring-open …