Mutarotation in Chemistry: Definition, Mechanism & Examples

Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Mutarotation is a type of change in the specific rotation of a solution. In this lesson we learn how it occurs and look at a couple examples, including D-glucose and D-fructose.

Isomers & Light Rotation

Have you ever hung out with someone enough that you started picking up some of their habits and they started picking up some of yours? Eventually, you both mutate into a combination of each other. Over time you both change, which will also change the nature of the relationship.

Molecules also slowly undergo changes. One of these common changes is when a molecule changes from one isomer into another isomer.

For example, the sugar molecule D-glucose has two common isomers in the cyclic form: alpha and beta. In water the molecule can open up into the straight chain, and when it reforms the cyclic form it can form either the alpha or the beta form, thus allowing D-glucose to interchange between alpha and beta forms.


Alpha and beta glucose are isomers of each other but have different specific rotations
Alpha and beta glucose


Now, recall that each molecule (in fact each isomer of a molecule) will rotate plane polarized light to a specific degree, and this is called the specific rotation. This means that alpha-D-Glucose will rotate plane polarized light at a different rotation than beta-D-Glucose will rotate it. The specific rotation for pure alpha-D-Glucose is 112 ° while the specific rotation for the pure beta-D-Glucose is 18.7 ° .

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  • 0:04 Isomers & Light Rotation
  • 1:13 Mutarotation
  • 2:55 Other Mutarotations
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Mutarotation

Okay, now that we've reviewed a bit of background knowledge, let's learn about mutarotation. Mutarotation refers to the change in specific rotation over time due to a change between isomers. 'Muta' means 'change', so it literally means a change in rotation.

The specific rotation of a molecule never changes, but the specific rotation of the entire solution can change. This is because the molecule can change between isomers in some cases (such as with glucose).

Let's look at what happens when we put pure alpha-D-Glucose into water and measure the specific rotation over time. At first, it starts out at 112° just as we would expect, but it slowly starts to change until it reaches 52.5° .

Now, let's look at what happens when we put pure beta-D-Glucose into water and measure the specific rotation over time. Once again, it starts out where we would expect it at 18.7°, but it slowly changes until it also reaches 52.5°.


Alpha and beta glucose can convert between each other because an open, straight chain form is present than can convert to either alpha or beta glucose
Alpha to beta reaction


This change in rotation within the entire sample is mutarotation. So, what exactly is happening here?

Well, we know that in water, glucose will form the open chain and then reform into the cyclic, either as alpha or as beta. The specific rotation will change based on the concentrations of alpha and beta forms in the sample.

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