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I'm interested in developing an undergrad organic lab that delves deeper into distillation/extraction, optical rotation and enantiomers. Our lab currently does a steam distillation extracting R-(+)-Limonene from orange peels but I am wanting to explore if there is a natural source that is rich in the S enantiomer. I have checked various places on the internet to little avail. As yet, I haven't been able to find a reliable source describing any natural occurrence (Wikipedia mentioned something about it being present in some mint essential oils but left it vague and I couldn't find any other sources to back that up). I did see some is available from Sigma Aldrich for purchase but I think it would be much more illustrative for the students to obtain this via another extraction (Or divide up into teams). Could anyone provide some insight or direction for me in this? Or else confirm that this is not richly available in natural products (Which I suspect might be the case from my superficial understanding of biology's chiral preferences)?

I have also read that the extract from oranges is actually just enriched in the R-(+) enantiomer but I am not particularly equipped to try and separate enantiomeric mixtures like that. Perhaps if someone has suggestions for making this a simpler option (enantiomeric separation) for undergrads, I would love to hear that too.

Thanks in advance!

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The (S)-form of limonene is naturally found in pine needle oils and mentha oils. However, the concentration is lower (by way of comparison, the (R)-form is present in citrus peel oils at a concentration of over 90 %). In case of pine needle oil, you would have to separate it from terpineol and other similar components.

It is also present in ginger, nutmeg, pepper, mace, coriander, and other herbs and spices.

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