Organic and Inorganic Chemistry Lesson of the Day – DO NOT USE THE PREFIXES (d-) and (l-) TO CLASSIFY ENANTIOMERS

In a recent Chemistry Lesson of the Day, I introduced the concept of optical rotation, and I mentioned the use of (+) and (-) to denote dextrorotary and levorotary compounds, respectively.

Some people use d- and l- instead of (+) and (-), respectively.  I strongly discourage this, because there is an old system of classifying stereogenic centres that uses the prefixes D- and L-, and the obvious similarity between the prefixes of the 2 systems causes much confusion.

This old system classifies stereogenic centres based on the similarities of their configurations to the 2 enantiomers of glyceraldehyde.  It is confusing, non-intuitive, and outdated, so I will not discuss its rationale or details on my blog.  (If you are interested, here is a good explanation from the University of Maine’s chemistry department.)

Also, note that D- and L- classify stereogenic centres, whereas d- and l- classify enantiomers – this just adds more confusion.

In short,

  • DO NOT use d- and l- to classify enantiomers; use (+) and (-) instead.
  • DO NOT use D- and L- to classify stereogenic centres; use the Cahn-Ingold-Prelog priority rules (R/S) instead.