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

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.

Reaction Mechanism

Reaction Mechanism