Mutarotation: Measurement of Mutarotation: Example: Reaction Mechanism (For CBSE, ICSE, IAS, NET, NRA 2022)

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Title: Mutarotation

Mutarotation is the change in the optical rotation because of the change in the equilibrium between two anomers, when the corresponding stereo centers interconvert.
Mutarotation is defined as the change in the optical rotation due to the in equilibrium between the two anomers.
Cyclic sugars show mutarotation as alpha and beta anomeric forms of interconverting
Cyclic sugars show mutarotation as α and β anomeric forms interconvert.
The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution
Mutarotation was discovered by French chemist Dubrunfaut in 1844, when he noticed that the specific rotation of aqueous sugar solution changes with time.

The α and β anomers are diastereomers of each other and usually have different specific rotations.
A solution or liquid sample of a pure α anomer will rotate plane polarized light by a different amount and/or in the opposite direction than the pure β anomer of that compound.
Optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution

For example, if a solution of is dissolved in water, its specific optical rotation will be
Some of the will undergo mutarotation to become , which has an optical rotation of .
Each of these two forms can be synthesized and isolated as pure compounds.
The anomer of D-glucose has a specific rotation of + 112 degrees in water.
The beta of D-glucose has a specific rotation of + 19 degrees. (18.7 actually, but rounding up to 19) .
The rotation of the solution will increase from to an equilibrium value of as some of the β form is converted to the α form.
Equilibrium mixture is about 64 % of and about 36 % of , though there are also traces of the other forms including furanoses and open chained form.