Organic and Inorganic Chemistry Lesson of the Day – Optical Rotation is a Bulk Property

It is important to note that optical rotation is usually discussed as a bulk property, because it’s usually measured as a bulk property by a polarimeter.  Any individual enantiomeric molecule can almost certainly rotate linearly polarized light.  However, in a bulk sample of a chiral substance, there is usually another molecule that can rotate light in the opposite direction.  This is due to the uniform distribution of the stereochemistry of a random sample of the molecules of one compound.  (In other words, the substance consists of different stereoisomers of one compound, and the proportions of the different stereoisomers are roughly equal.)  Because one molecule’s rotation of the light can be cancelled by another molecule’s optical rotation in the opposite direction, such a random sample of the compound would have no net optical rotation.  This type of cancellation will definitely occur in a racemic mixture.  However, if a substance is enantiomerically pure, then all of the molecules in that substance will rotate linearly polarized light in the same direction – this substance is optically active.

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Organic and Inorganic Chemistry Lesson of the Day – Optical Rotation (a.k.a. Optical Activity)

A substance consisting of a chiral compound can rotate linearly polarized light – this property is known as optical rotation (more commonly called optical activity).  The direction in which the light is rotated is one way to distinguish between a pair of enantiomers, as they rotate linearly polarized light in opposite directions.

Imagine if you are an enantiomer, and linearly polarized light approaches you.

  • If the light is rotated clockwise from your perspective, then you are a dextrorotary enantiomer.
  • Otherwise, if the light is rotated counterclockwise from your perspective, then you are a levorotary enantiomer.

In a previous Chemistry Lesson of the Day, I introduced the concept of diastereomers, and I used threose as an example.  Let’s use threose to illustrate some notation about optical activity.

D-threose.svg 2

(-)-Threose

  • Levorotary compounds are denoted by the prefix (-), followed by a hyphen, then followed by the name of the compound.  The above molecule is (-)-threose.
  • Dextrorotary compounds are denoted by the prefix (+), followed by a hyphen, then followed by the name of the compound.  The enantiomer of (-)-threose is (+)-threose.

A compound’s optical rotation is determined by a polarimeter.

I strongly discourage the use of the prefixes (d)- and (l-) to distinguish between enantiomers.  Use (+) and (-) instead.

Beware of the difference between designating enantiomers as (+) or (-) and designating stereogenic centres as either (R) or (S).

It is important to note that optical rotation is usually referred to as a bulk property.