I was watching an episode of ABC TV’s Landline the other day (I’m a big fan). They had a great segment on using good old-fashioned steam distillation to extract eucalyptus oil from the crude Mallee leaf on a small industrial scale. The program went on to talk about the use of plant biology in the search for high oil-yielding eucalyptus species, and is worth a look. Video here.
The major component of eucalyptus oil is cineole, also known as eucalyptol. But it was the process of steam distillation itself that brought back the memories. This took me back to my time as a tutor in the undergraduate laboratories, where twice a term, a fresh crop of second years had to do two steam distillations as part of the practical component of second year Organic Chemistry. The first was part of the workup of a classic tin and hydrochloric acid reduction of nitrobenzene to aniline.
For those who’ve never seen one, here’s a picture of the basic setup from Wikipedia. Although we always had our round-bottom flasks vertical with a Claisen head, not just a bent bit of tube with a couple of rubber bungs. That picture is also a bit vague as to the heat source. We always used to use a pair of bunsen burners. Anyway the basic idea is explained pretty well in the Wikipedia article, but in summary, bunsen #1 heats the water in a big brass kettle, and the steam is pushed through the round-bottom flask containing the crude reaction product. Extra heating is applied via bunsen #2 to the RBF and the volatile oils, in our case aniline, passes through the condenser together with the steam and is collected, and ultimately separated from the water, and redistilled.
What that particular picture can’t convey however was the smell. Remember this was the “Good Old Days” where all the bench chemistry was done, well, at the bench. In the open air. And up to 80 undergraduate students at a time doing steam distillation in the open lab made for an awful stink. The tutors like me then collected all the product for grading etc, and redistilled it for the following week, where the undergraduates then converted the aniline to iodobenzene.
This was done using the classic diazotisation of aniline with hydrochloric acid and sodium nitrite, followed by potassium iodide. And the crude product was once again purified by steam distillation. Except this time, the smell was worse. And also the cleanup of the apparatus was awful as the reaction produced huge glutinous black blobs which was the point of the steam distillation.
Ah fun times. I wish back then we’d been doing eucalyptus oil extraction instead. These days, if steam distillation is used at all in undergraduate programs, it’s commonly used to extract eugenol from cloves, often on a micro scale to save space and money, and always in a fumehood!
Besides the extraction of eucalyptus oil, steam distillation is still widely used in the perfumes and essential oils industries. It’s just that the large scale steam distillation in the open air of undergraduate laboratories has gone.