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.
Measurement
- 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
Example
- 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.