Allylbenzene to propenylbenzene

[FriendlyFinger]

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 120-160°C 12-18hrs, then fractionally distilling. Sounds good to me, but Allylbenzene boils at around 158°C.

So how about dumping in 2%KOH, taking the temp to 60°C 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.

Thanks,
FF

[Rhodium]

THe reason people use the vacuum reflux is because they don't want to boil safrole with KOH at up to 250°C... 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.

[FriendlyFinger]

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

Regards,
FF.

[Osmium]

No!
Skip all that washing! More trouble than its worth! Leave the KOH in and simply distill. Less work and higher yields.

[Rhodium]

It depends on if some extra % yield propenylbenxene or the RB flask is worth most to him. That KOH + crud gets stuck!

[FriendlyFinger]

Actually, I love my glass as much as the product! 1g KOH was added to 53g Allylbenzene and heated to 120°C 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 170°C, vapor temp was 158°C 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 150°C when the azeotrope came over at 93°C, but it was very small. One drop of water, that's it. Is that enough to prevent isomerisation?

Regards,
FF.

[Rhodium]

What is the bp difference between allylbenzene and propenylbenzene? Maybe it isn't big enough to measure like that?

[Aurelius]

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

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

almost 20°C difference