When Life Hands You (S)-(–)-limonene . . . You Can’t Make Lemonade
Norwegian Academics Go Ragnarök on Organic Chemistry Textbooks
There’s nothing I like better than a merciless takedown of lazy conventional wisdom. In What the Nose Knows I tracked down the origin of the bogus “10,000 smells” meme and also detonated the notion that scent-triggered memory was first described by Marcel Proust.
While nothing beats doing it yourself, there is great entertainment value in taking a seat in the bleachers and watching someone else set off the dynamite. Take, for example, a new paper by three Norwegian professors that appeared in the ACS Journal of Chemical Education.
Their target is limonene, a monoterpene that occurs naturally in range of citrus fruit oils. Specifically, they look at the notion, often cited in organic chemistry textbooks, that of limonene’s two enantiomers, (R)-(+)-limonene smells like oranges while (S)-(–)-limonene smells like lemons. This characterization, they claim, is simply incorrect.
Like Viking coastal raiders, they go after the target from multiple directions. First, they point out that (S)-(–)-limonene is almost nonexistent in citrus fruits, although it can be found in the oils of citronella and lemongrass.
Next, they conduct some elementary but credible sniff tests. Their test subjects reported that orange oil smells more like orange/mandarin/clementine than lemon/lime, and vice versa for lemon oil. All good. But while the sniffers found (R)-(+)-limonene to smell orange-like, the (S)-(–)-limonene results were far less tilted toward lemon. It turns out the less pure (technical grade) version of each enantiomer smells fruitier than its more pure (analytic grade) form. This implies that trace impurities are supplying characteristic odor notes.
Finally, they examine the fragrance chemistry literature and find a murky picture of what limonene smells like. They note that citrus scents are the result of multiple components; they quote the great fragrance chemist Günther Ohloff who attributed the character of lemon oil thusly:
geranyl and neryl acetate as responsible for the fruity note; (−)-β-pinene together with (−)-terpinen-4-ol for the green, peely odor; trans-α-bergamotene for the basic peppery aroma; [along with] the combination of citral (geranial and neral) and C7−C13 alkanals
So where did the bogosity of “(S)-(–)-limonene smells like lemons” originate? The Norwegians point the finger at a 1971 paper on odor and chiral isomers by Lester Friedman and John G. Miller published in Science. And they trace the canard as it is repeated in organic chemistry textbooks from Elsevier, Wiley, and Oxford University Press.
Kudos to Lise Kvittingen, Birte Johanne Sjursnes, and Rudolf Schmid for reminding us that skepticism is a scientist’s best friend.
Lise Kvittingen, Birte Johanne Sjursnes, and Rudolf Schmid. (2021). Limonene in citrus: A string of unchecked literature citings? Journal of Chemical Education 98(11), 3600-3607.
A tip of the hat to Joseph A. DiVerdi for alerting me to this paper.