25.8 Mutarotation

1. 25.8Mutarotation

2. Mutarotation • Mutarotation is a term given to the change in the observed optical rotation of a substance with time. • Glucose, for example, can be obtained in either its a or b-pyranose form. The two forms have different physical properties such as melting point and optical rotation. • When either form is dissolved in water, its initial rotation changes with time. Eventually both solutions have the same rotation.

3. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2°

4. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2° Final: [a]D +52.5°

5. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2° Final: [a]D +52.5°

6. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose • Explanation: After being dissolved in water, the a and b forms slowly interconvert via the open-chain form. An equilibrium state is reached that contains 64% b and 36% a. 1 1 b-D-Glucopyranose a-D-Glucopyranose