FriendlyFinger (Hive Bee)
05-06-03 00:23
No 431689
      Allylbenzene to propenylbenzene  Bookmark   

Hi there, I'm stuck on this isomerising deal. If I wanted to isomerise allylbenzene the way everyone seems to be for safrole. What temp would I aim for?

From what I've read people are dumping in around 2% KOH, heating with slight vacuum to remove water, keeping the temp at 120-160C 12-18hrs, then fractionally distilling. Sounds good to me, but Allylbenzene boils at around 158C.

So how about dumping in 2%KOH, taking the temp to 60C with vacuum then once water goes heat to reflux. Does the Allyl decompose at reflux? I only have a small amount so don't want to screw it.


Be joyful, though you've concided all the facts!
(Chief Bee)
05-06-03 17:55
No 431806
      Allylbenzene isomerization  Bookmark   

THe reason people use the vacuum reflux is because they don't want to boil safrole with KOH at up to 250C... Allylbenzene is a much tougher molecule as it hasn't got the relatively fragile methylenedioxy ring. It will absolutely not decompose at reflux.

I suggest that you just add those 2 weight% of KOH, heat to drive off the water without a condenser, then attach the condenser and reflux the mixture without vacuum for 3-6 h (test for completion by TLC or by checking the temperature of the vapors given off from the boiling solution - when the temp doesn't increase anymore, it should have reached the bp of propenylbenzene, and you are done. Wash the product with water, then with brine, and dry over CaCl2 or other strong drying agent. After fractional distillation you should have pure propenylbenzene. You can distill without the wash/drying too, but your flask will be much harder to clean afterwards.
(Hive Bee)
05-06-03 19:18
No 431827
      Beautifull. Straight clean answers.  Bookmark   

Beautifull. Straight clean answers. Rhodium, your better and faster than any anti-anxiety pill I've ever tried.

(Stoni's sexual toy)
05-08-03 06:50
No 432136
      No! Skip all that washing!  Bookmark   

Skip all that washing! More trouble than its worth! Leave the KOH in and simply distill. Less work and higher yields.

I'm not fat just horizontally disproportionate.
(Chief Bee)
05-08-03 10:32
No 432176
      Yield or flask?  Bookmark   

It depends on if some extra % yield propenylbenxene or the RB flask is worth most to him. That KOH + crud gets stuck!
(Hive Bee)
05-08-03 19:39
No 432236
      Actually, I love my glass as much as the ...  Bookmark   

Actually, I love my glass as much as the product! 1g KOH was added to 53g Allylbenzene and heated to 120C with stiring and water left to evaporate for about 10 minutes. The KOH turned brown and I couldn't see any water coming off, but the allyl evaporates quickly.

Added condenser, thermometer, took oil bath to 170C, vapor temp was 158C and refluxed for 23hrs. No temp rise. Why? This is simple isn't it! I've made safrole via grignard, Why can't I do this?

Anyway, 25ml Butanol was added and everything set up for fractional distillation to see how much water was still in the system. Oil was bath at 150C when the azeotrope came over at 93C, but it was very small. One drop of water, that's it. Is that enough to prevent isomerisation?

(Chief Bee)
05-09-03 11:41
No 432357
      bp difference?  Bookmark   

What is the bp difference between allylbenzene and propenylbenzene? Maybe it isn't big enough to measure like that?
(Hive Addict)
05-09-03 16:27
No 432392
      Tisk, tisk...  Bookmark   

Rhodium, have you forgotten how to use the search engine?winktongue

allylbenzene has a bp of 156-157 degrees
propenylbenzene has a bp of 172-172 degrees

almost 20C difference