vac distillation issues

joe_aldehyde · huxleys associate

i tried vac distilling a mixture of piperonal and hydrocinnamic alcohol, but even under a nice aspirator vacuum (and with a 30cm vigreux column attached), nothing happened. no vapors would evolve out of the liquid. it just bumped a couple of times. the still head and column were packed in Al foil to prevent cooling of the vapors.

any hints?

bio · Working Bee

Well, assuming you got it hot enough, measured the vacuum at the receiver and didn't have a leak upstream I'd say are you sure that's what was in the flask.

No vapors at all? Not even a little reflux in the boiler? Strange

With a column that long it's a good idea to also wrap some insulation over the foil, still head and top of flask. Takes a lot less heat and takes off steadier.

For high boilers with a 600mm vig I wrap with a 60 watt heat tape, insulate that and trim with a dimmer to a little below the distillation temperature.

joe_aldehyde · huxleys associate

yea i'm sure that it was in the flask, since the label said it and the smell of pip. is just so characteristic. i guess the main fault was not putting a stirbar in and stirring like hell. there was a reflux in the boiling flask, if you can call it a reflux since all it did was bump a few times as i had no stirbar in it.
i'm gonna try again with a heat tape (thank god i know peeps that have all the equipment Smile

2spun · The Resistor

Thats what swiy was think'n(stir bar).
swiy as well has had sucsessful distills by bubbling air thru his oil,
push'n vapors up . a bit slo'r but works.

joe_aldehyde · huxleys associate

yeah i did that at the uni back in the days, but have no measures of making a glass capillary to put down the still head and in the liquid.

2spun · The Resistor

what about chem resistent vacum tube, with a glass tip made of(glass) radeator anti frezze tester from wally world?
it's getto but should work.
much thanks

bio · Working Bee

Capillaries have to be ultra fine to work properly and there is always the oxidation possibility with certain reagents if using air. I tried making a bubbler with the finest needle from those tiny insulin syringes attached to a piece of spaghetti tubing maybe 1/32" OD with a 1/8" needle valve and it still sucked waay too much air.

Stirring is really much better and the nice thing is it's easy to control the take-off rate by adjusting the speed. Especially when it first starts boiling, quick bursts to top speed help it get going and if you overshoot the heat a little or heat too fast just slowing down the magnet get's it under control until it cools a little.

About the only time a capillary is better than stirring is when evaporating a lot of liquid with vacuum when separation isn't an issue. Also an aquarium air pump directed at the surface can speed things up in this situation when evaping at atmospheric. especially when the soup is too thick to really boil much without erupting.

joe_aldehyde · huxleys associate

i tried again today, without heating tape but lots more insulation on the vigreux and still head. the vapors climbed up to the thermometer but no more, hence no condensation. i guess i need a much stronger vaccuum and a heat tape. it took like one hour for the vapors to reach the thermometer! crazy shit...so far, bisulfite adducts worked better for piperonal separation. but i seem to lose a sizable amount of product in the water of the solvent extraction since piperonal is also water soluble.

bio · Working Bee

Yea, sometimes it's a bitch with a lousy vacuum. Maybe try just loose packing the vertical of a claisen. This will fractionate quite well if you go slowly (1-2 drops/second) and it's a help also to have the flask immersed to the neck in the bath.

This should work if you made it over 300mm already as the claisen vertical arm is about 150mm on the Kimax 24/40.

joe_aldehyde · huxleys associate

so you mean i should leave out the vigreux and put stuff in my still head? i don't really understand what you mean, bio...

2spun · The Resistor

don't meen to be stomp'n on your thread J_A,

((there is always the oxidation possibility with certain reagents if using air))

swiy's not the smartest in the bunch can Bio of the top of his head give swiy
a few names of the reagents that may oxidize via push'n air.
so swiy don't find himslef in a fix please.
Much thanks

joe_aldehyde · huxleys associate

as far as i know 2spun, benzaldehydes are quite sensitive to air as the cannizzaro disproportionation to the corresponding benzoic acids and benzyl alcohols is promoted by oxygen radicals. back in the days at the uni, i did though distill plain benzaldehyde with an air capillary, and if you use it right away, there won't be any problems. if you plan on storing the aldehyde, be sure to put some hydrochinone in it, it inhibits radical formation. always keep it in the freezer...i left 500g of benzaldehyde in my attic at around 10-20 deg.c and it has completely turned to benzoic acid in about 3 months.

bio · Working Bee

OK Joe; The still head 3way adaptor goes on the vertical arm of a claisen adaptor and the claisens vertical is loose packed with small marbles or NiCr wire coils or other suitable material.

BTW do you know of an OTC oxidation inhibitor for PhCHO? No hydroquinone or catechol in the closet. BHA (butylated hydroxyanisole) or BHT should work but in pure form is it OTC?

2spun, That's kind of a tough question in away as even a hydrocarbon like ethylbenzene will react with O2 at 140deg after a few hours to form acetophenone!
In general aldehydes and ketones at higher temps shouldn't have oxygen especially if wet which it will be unless a drying tube of adequate capacity is used.

Star-light · Blacklight bulb

2spun · The Resistor

OK swiy understands,

well swiy had made them messups before and was woundering.
as well so to push o2 thru a propenylbenzene will give swiy a side reaction that he does not want? (acedic maybe)?
and if so is it revirseable?
Much thanks
swiy may have really blown some very nice oil.