I’ve significantly improved the original batch method I posted last year, what follows is my latest method.
This approach uses the same setup as the last steam distillation variation graphically depicted in in this thread.
Basically, the items used were:
hot plate, pressure cooker steam generator & associated tubing, etc
3L 3-neck RBF & mantle
vigreux column, right angle bend & still head
condenser
Vacuum pump & glassware for purfying the final product & recycling unreacted cinnamaldehyde.
chemicals: 2 litres tap water, lecithin, cinnamon oil, NaOH
Essentially, a large reaction vessel and a means of steam & vacuum distilling are all the equipment really required.
Begin by setting up all equipment, add 2 L water & ~1.5g lecithin.
The first variation is to use OTC sodium hydroxide instead of sodium carbonate, simply add the alkali to adjust the pH within the range of 12-13, only
small amounts are required.
The second variation is to slowly drip in the cinnamon oil during the course of the reaction, the benzaldehyde being steam distilled as it forms.
Begin the addition once the pH adjusted solution has heated to boling temperature and the steam is passing though the solution.
The third variation is to stop the cinnamon oil addition after 5 min, then check the distillate. If benzaldehyde is clearly distilling over, then
continue with the addition.
A crude test is to simply taste the steam distillate as it drips into the receiving vessel, if it burns your tongue off, then you're doing well. If it
tastes sweet, then significant quantities of cinnamon are being carried over. Should this not change after a short while, then double check the pH !
Provided that the pH is correct and the cinnamaldehyde content is high (70%+) then benzaldehyde laden distillate should start collecting almost
immediately after the addition has begun.
However, if it is not, wait for the benzadehyde to start collecting, THEN continue with the addition. This prevents unnecessary amounts of
cinnamaldehyde being steam distilled at the start of the process, which is a serious problem with the old batch mode reaction.
Note that by slowly dripping the cinnamon oil instead of adding it all at once to the reaction mix, single run conversion is increased by 20-30%.
Ensure the rate of cinnamon oil drip rate is approximately equivalent to the quantity of oil being steam distilled, thus maintaining high dilution in
the reaction mixture.
After 300 to 500 ml of cinnamon oil had been added, the drip rate is stopped and the steam distillation continued until all the aldehyde has been
steam distilled from the reaction mix. If using less than 2L of water, then adjust proportionally to maintain the same dilution. At this point, dump
the mix and replace with clean pH adusted water with at least ~1.5g lecithin.
This step removes tar and other junk that slows down the reaction and reduces benzaldehyde production. Significantly reduced tar formation will also
result from periodically carrying out this step (very imporant if high yields are desired).
Also note that with this method, the pH tends to slowly drop over time during the course of the steam distillation, so check the pH every couple
hours, and top up with NaOH if and when required. Its preferable to simply add small amounts concentrated NaOH solution when topping up, using a
dropper makes the alkali addition much simpler since NaOH pellets will rapidly collect moisture and stick to the sides of the reaction vessel.
Efficiency is further improved if the steam distillate (after oil separation) is also extracted with DCM, though this isn't absolutely necessary.
Once the the product oil is pooled, vacuum distillation will remove the benzaldehyde - no need to vacuum distill the recycle cinnamon aldehyde, just
drip it into the steam distillation vessel, just as with the cinnamon oil feed. Expect the quantity of unreacted cinnamon aldehyde to be circa 25%,
but if the reaction mix was kept clean during the process then it may be considerably less. The benzaldehyde distillate is clear, & much purer
than the original solvent extraction method that doesn't employ the steam distillation. Give the batch solvent extraction a rest folks, use the steam
distillation variation if producing benzaldehyde from cinnamon oil!
Far, far less work.
Using the above approach, overall molar conversion to benzdehyde is at least 80%.
I’ve managed higher conversion efficiencies (~90%), but the reaction efficiency is subject to variation according to varying reaction conditions,
i.e. feed rate, reaction mix volume (i.e. dilution), and the quantity of cinnamon oil fed to the reaction mix before changing the solution. >80%
seems to be a realistic overall figure.
Also...
Remember to keep the reaction mix free of tar & cinnamon oil impurities as much as possible to give consistent high yields.
If dripping the aldehyde into the mixture isn't practical for any reason, you can periodically add the cinnamon oil in small batches, say 20-50 ml at
a time to give good conversion, but expect a little more cinnamonaldehyde to distill over. |
(benzaldehyde oxime----->benzaldehyde)