Sciencemadness Discussion Board

Benzaldehyde via cassia oil unexpected hiccup

Geewizz34 - 25-10-2020 at 08:34

I finally got around to synthesizing a little benzaldehyde from cassia oil via Cyclonight's variation of the aldol reaction.

This was performed in a uni lab so I was able to follow his exact procedure to a T, the only deviation was a slow stream of argon in the reaction vessel to eliminate the foaming.

Everything seemed to go well, I turned off the hood and an amazing aroma of cherries filled the room so I'm pretty sure benzaldehyde was being produced throughout the reaction.

The issue: in the receiving flask, the oil layer was floating on top of the aqueous layer, not below it. The oil layer should be a combination of benzaldehyde and cinnamaldehyde which both have densities lower than water so why is it not sinking? It's really bugging me and all the photos/videos I have referred to show the oil layer sinking to the bottom of the aqueous layer.

The only issue I could think of was that our PH meter has been kind of acting funny lately. The literature cites only a small amount of NaOH is needed to basify the solution, and I feel like I added considerably more. Could a PH of 14 instead of 12-13 have created other side products that caused the benzaldehyde to be less dense?

any help would be much appreciated.

[Edited on 25-10-2020 by Geewizz34]

unionised - 25-10-2020 at 08:58

You ,might do better if you post a link fo "synthesizing a little benzaldehyde from cassia oil via Cyclonight's variation of the aldol reaction."

Geewizz34 - 25-10-2020 at 09:05

Sorry here is the procedure I followed.

unionised - 25-10-2020 at 12:42

That's interesting; I don't recall seeing a reverse aldol reaction before.

The density of the aldehydes isn't much higher than water.
Sometimes drops will float on the surface of the water held up by surface tension. Swirling the container might get them to sink.
The other factor is that a less dense impurity would reduce the density significantly.

Does the original oil float or sink in water?

UC235 - 25-10-2020 at 14:19

I'd say excessively basic solution might be causing a Cannizzaro reaction that is resulting in benzyl alcohol also being present in your distillate but that is also more dense than water. How thick was your oil layer? I've had thin layers of dense liquid sit atop water probably mostly through surface tension.

Also, only 10mM NaOH is required for pH of 12. I feel like that would be rapidly consumed by side-reactions but if it works, it works.

Geewizz34 - 25-10-2020 at 21:01

The oil layer was quite thick. I had 50ml and all of it floated no matter what I did.

Someone else mentioned it might be due to an impurity in the cassia oil. That's the only thing that I can think of that makes sense since all of the other references use cinnamon bark oil.

[Edited on 26-10-2020 by Geewizz34]

Geewizz34 - 25-10-2020 at 21:32

Quote: Originally posted by UC235  
I'd say excessively basic solution might be causing a Cannizzaro reaction that is resulting in benzyl alcohol also being present in your distillate but that is also more dense than water. How thick was your oil layer? I've had thin layers of dense liquid sit atop water probably mostly through surface tension.

Also, only 10mM NaOH is required for pH of 12. I feel like that would be rapidly consumed by side-reactions but if it works, it works.


I can get the original cassia oil to sink with heavy agitation. But after some time about a third of it floats back up to the surface.

I think a hint of carrier oil in the original cassia oil may be distilling over and lowering the density of the oil layer in the receiving flask.

[Edited on 26-10-2020 by Geewizz34]

[Edited on 26-10-2020 by Geewizz34]

nzlostpass - 26-10-2020 at 00:14

I tried this recently with Cassia oil also....i found that 90 % of the oil in the receiving flask sunk and about 10% floated.....I never managed to seperate any unreacted cinnamaldehyde from the benzaldehyde though as I havnt worked out my vacuum pump yet when doing vacuum distillations.....i end up getting huge bumps where it sucks the original distilling mix through the condenser.

unionised - 26-10-2020 at 05:32

This may indicate a way forward.
http://www.sciencemadness.org/talk/viewthread.php?tid=27476

Mateo_swe - 26-10-2020 at 14:25

I think its as you suspect, some unknown present in the Cassia oil came over during distillation and is present in the oil layer making it less dense.
If you clean up your benzaldehyde it probably give you correct density.
Since benzaldehyde has density 1.04 its close to water and anything unknown could easily be in the oil layer lowering the density.
I really like that marzipan smell of benzaldehyde.

Geewizz34 - 30-10-2020 at 04:52

Quick update,

I was able to get some time with the LC/MS. The culprit, coconut oil!

Thanks for all the responses everyone.