Naming Limonene Correctly

With regard to the article on limonene supplies (C&EN, April 4, page 21): To the few who might really care, this is already well-known and crystal clear. To the rest, it seems to be confusing. But to a middle-aged-plus biochemist, it deserves repeating.

The stereochemical indicators “D” and “L” describe the absolute stereochemistry by analogy to D- and L-glyceraldehyde, with groups on the stereocenters of the subject compound and glyceraldehyde related pairwise. This system is meaningful only for monosaccharides, amino acids, and, less straightforwardly, inositols and glycerides. There is just no way it can be applied to limonene. (You can say the H on the stereocenter of limonene is related to the H of glyceraldehyde, but then what?)

Optical isomers that are identified by their sign of rotation—which, of course, is unrelated to their absolute configuration—are called d- or l- (for dextro- and levo-rotatory). Natural limonene from citrus fruits is dextrorotatory. The subject of the referenced article was d-limonene. There is no such thing as D-limonene.

Eric C. Ressner
St. Louis