Rotational Direction of Plane-Polarized Light: Dextrorotation & Levorotation

Instructor: Laura Foist

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

Chiral molecules will rotate plane-polarized light. The direction the light is rotated is described as dextrorotation or levorotation. In this lesson we will learn about the direction the light is rotated.

Chirality and Specific Rotation

When you look in a mirror, everything looks identical to you and your surroundings. However, it is backwards (opposite) to the way it actually is. Mirror images of molecules are also identical except for being opposite or backwards.

Sometimes when you look at the mirror image of an object (such as something simple like a piece of paper with no writing) the mirror image really is the exact same as the original. You simply need to rotate it around and you've got the exact same thing.

For many other objects, no matter how you rotate the mirror image, it will never be the same as the original. A common example of this is your left and right hand. They are mirror images of each other, but no matter how you rotate your hands they will never perfectly line up.

In chemistry, molecules that are mirror images of each other but never perfectly line up (non-superimposable), are called chiral. Chiral molecules have a special function in that they can rotate plane-polarized light, meaning that when we shine light through the sample, it will rotate the light a specific direction and a specific amount.

Direction of Light

Light can be rotated either clockwise or counter-clockwise. If light is rotated clockwise (to the right) it's called dextrorotation. If light is rotated counter-clockwise (to the left) then it's called levorotation. You can remember this because the word 'levorotation' starts with 'l' just like the word 'left' starts with 'l'.

In order to tell which direction the light is rotated, we can use a fairly simply apparatus. This apparatus first ensures that all of the light starts out going in the same direction by using slits:

The light is first rotated so that it is all in the same plane
Light all in one plane

When we shine the light through a non-chiral compound, the direction of the light doesn't change. So if we have slits (in the same direction as the original slits) on the other end of the apparatus, then we will see the light shining through:

If the light shines though a compound that is non chiral, then there will be no change in the rotation. We will see the light shining through
No rotation on light

If, the compound is chiral, the light will be rotated. This rotated light won't be able to fit through the slits, thus we will see no light:

If the light shines through a chiral compound, then it will rotate. We will not see the light shining through
Light rotate

If we put in slits that are turned towards the left, and we see light, then the compound is levorotatory. If we put in slits that are turned towards the right, and we still see no light, then the compound must be dextrorotatory.

Every chiral compound also has an enantiomer that is also chiral. If the original chiral compound is levorotatory, then its enantiomer must be dextrorotatory.

Other Chiral Descriptors

Chiral compounds and enantiomers have a lot of descriptors. You may see +, -, d, l, D, L, R, or S as descriptors. Of these,

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