We’re starting a new lab in O-Chem this week: Extraction of a Volatile Oil from Orange Peels via Steam Distillation.

In layman’s terms: we’re taking the stuff that smells good out of the orange peels by boiling it off. 

The major compound responsible for the orange smell in peels is a volatile oil called “Limonene”, and is also found in lemons. Interestingly, the enantiomer (more on that in a minute) of this compound is not palatable at all (it has an odor similar to Turpentine).

Briefly, an Enantiomer is a mirror-image of a molecule. These 3-d arrangements behave differently (the case study always used is the Thalidomide tragedy) and thus interact differently with the smell receptors in our noses.

d-limonene	l-limonene

The enantiomers of limonene, in stick structure form, are pictured to the left. 

They may not look like mirror images, but I just need you to trust me on this. (If you were to make up a MolyMod model of it, you’d see what I mean) You may think that they look identical except for that the left one has a thick black line towards the bottom and the right one has a dashed line. 

In organic stick structures, thick black lines like that indicate a bond whose subsitiuents are coming TOWARDS you, and dashed lines indicate a bond whose substituents are receding into the paper. So the left side one, imagine that the /\\ part at the bottom is coming towards you.

Side note: “d” and “l” indicate which enantiomer is being referred to, and are necessary for drawing the structure correctly. “d” is from the latin “dextrorotatory”, and “l” from the latin “laevorotatory”. [Read more]

If you’ve ever had a citrusy-flavored alcoholic beverage, you may have seen the  bartender “zest” the orange peel into the drink as a garnish. I always used to wonder why they would zest the PEEL instead of just squeezing some juice into it. 

As it turns out, the peel contains the highest concentration of d-Limonene in the orange, so zesting is the easiest / most effective way to get that volatile oil out. 

Also contained in the orange peel are the compounds “Octanol” (like “Octane” found in gasoline, except with a hydroxyl (-OH) group on the end of the chain, making it an alcohol), and “Carvone“, a compound derived from Limonene (via Oxidation).

Carvone is another interesting case like Limonene: its flavor depends on which enantiomer you consume. Fortunately, they are both non-toxic and (in my opinion) tasty. 

The “R” orientation (For the purposes of this blog post, “R” = “d” and “S” = “l” — pardon the confusion) is a major contributor to the flavor of spearmint oil, and the “S” orientation is a “principal constituent” of caraway seeds.

Through oxidation, we can produce Carvone from d-limonene. (If you compare the structures, you’ll notice that the only difference is that extra “O” doublebonded near the top of the molecule.) I suspect that leaving lemon or orange peel out in the open air will cause it to naturally oxidize (via an SN2 reaction), and we could certainly test for this by leaving the peels out and running an FTIR scan on the extract, looking for ketone peaks (that “O” double bond attached to a secondary carbon).

At this point, we are working on developing the procedure. I’m pretty sure that our lab group will be using a distillation apparatus (the big ol’ macro variety) to try and steam off some of the orange oil. The thing I found interesting was that all the sources I consulted (admittedly: all via google) said that orange oil has a boiling point of 176 C — we’re not going to get temperatures that high with our apparatus. I suspect that the heterogeneous mixture (the orange oil is non-polar, and we’ll be mixing it with water, a very polar solvent) may have a decreased boiling point, or that it will piggy back on over or something. We’ll see. 

If it works, we’ll see a thin film on the distillate beaker, if not, we’ll know to look into the short-flask. I’ve got lab again on Thursday afternoon, so we’ll see what happens!