Organic and Inorganic Chemistry Lesson of the Day – Optical Rotation (a.k.a. Optical Activity)
September 4, 2014 Leave a comment
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.
- 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.
It is important to note that optical rotation is usually referred to as a bulk property.