Distillative workup for dithionite reduction of nitroalkene
A sodium dithionite reduction was performed on unsubstituted phenylnitropropene (legal here) once at small scale in cool to warm IPA and at 4x scale
in ethanol which hit reflux for a short time.
The procedure was an attempt of replication based on an hive post by barium, which claimed 90+% yield reduction of nitrostyrenes/alkenes to oximes.
The product did not crystallize and thus alternative workup had to be started: distillation
I do have problems cutting fractions and identifying products.
After removal of the MTBE I extracted the nitroalkene & dithionite, ~40 ml of an dark yellow to orange oil was left behind and distillation at 20
mbar was commenced.
After a short distillation summaryI'll describe the lengthy distillation, wishing to hear what I did wrong and what to improve/do further:
Over the course of 2 hours, 4 fractions of fishy smelling, uniformly yellow oil were collected over a total range of 121-163°C at steady ~20 mbar.
Fraction I: forerun of ~5ml from 121 to 123°C
Fraction II: 10 ml collected from 123 to 128°C
Fraction III: 10 ml collected from 132 to 151°C
Fraction IV: 10 ml collected from 151 to 163°C ended by smoking oil
Residue: dark but entirely liquid 5ml
Important for thermometer reading might be the fact that the thermometer might be 2-3 cm too long, as its tip ends 2 cm below the "point of no
return". Important?
First drop entered receiver at 121°C reading the thermometer at 23 mbar and pretty constant 140°C oil bath temperature. 5% of total volume was
collected at 1 drop per seconds.
Fresh receiver was rotated in at 20 mbar and 123°C after which distillation speed slowed and heat was turned up, slowed over 125°C and heat was
turned up again and after 20% of total volume was contained by second receiver, third receiver was switched in at 128°C @20mbar.
During collection of additional 20% of total volume, the temperature climbed almost steadily. I spotted a comparably heavy refluxing front of oil a
few centimeters below the thermometer tip. So I decided not to cut a fraction and to up the heat several times. The front came nearer from 128 to
140°C and jumped to 150°C pretty fast - so at 151°C at 20 mbar, last receiver was rotated in, after another 20% was held by third receiver.
Distillation speed is maximum with 2 drops per second and is slowed, while temperature climbs from 151 to 163°C. Distillation slows and heat is
turned up until bath hits 200°C and starts smoking.
The drop hanging from the thermometer tip takes on a red dye and distillation flask contents reddens and finally blackens but stays liquid refluxing
hard at just a centimeter below thermometer.
Distillation is ended in spite of remaining ~10-15% of total volume.
Setup was a 29/32 joint 250 ml RBF connected via conical 29/32 to 14,5/whatever adapter to a 5 cm high 14,5 single joint air cooled distillation
bridge with slightly too long thermometer and 4x receiver spider, heated and stirred by IKA magstirrer in cocos oil.
Total height from flask bottom to "point of no return" 2 cm above thermometer tip is ~22 cm with a conical adapter juvenating the pipe from 29 to 14,5
diameter at 2/3 of height.
What does this do for separation? Column effect? Does the specific change in gas flow caused by the adaptor influence distillation?
Products?
I would expect phenylacetone oxime as the main product,and phenylacetone, phenylnitropropene and the corresponding amine as contaminations - need to
throw them out!
It's hard to even get any bp of the nitropropene and oxime and those found have to be extrapolated.
Please take your shots at doing so!
Where is the oxime and is there still some left behind?
However, I'm unsure what to do.
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