Author Topic: Oxime success, Reduction Failure  (Read 2601 times)

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Argon

  • Guest
Oxime success, Reduction Failure
« on: April 08, 2003, 11:32:00 AM »
An oxime was prepared by combining .5 mole of ketone with 1 mole of NH2OH, and bringing the contents to reflux in 800ML MeOH. The oxime was a white oil, that partialy separated upon cooling. It was isolated by stripping of the solvent in a rotorvap, but not allowed to dry up to a solid.

This oil was placed in ethanol, which was dryed with silica gel in the freezer for a couple of hours, along with 20g Raney Ni, into a hydrogenation apparatus. The pressure was brought up to 2.5 bar, and the stirrer speed was 1500 rpm.
(The Ni was washed down with a bit of water, dew to time constaints.)

After only 1.5 hours the absorbtion has completely stopped, and after 2 hours the reaction was stopped. The catalyst was fitered off, and a crystal clear oil dropped out of the solution. It was separated off, and the remainder of the alcohol was evaporated off in a rotorvap, leaving more crystal clear residue. The clear oil was combined, and represented a 92% yeild!!! It was not the target molecule :(

Where am I going wrong??

P.S. The ketone was tested with the bisulfate test.

becomezen

  • Guest
H20 may be the problem
« Reply #1 on: April 08, 2003, 12:08:00 PM »
Just a thought , but swim would follow Sonson's words to the T. In other words , dry the oxime as much as possible. If he used dry, powdered oxime , then swim would too. Good luck and keep the bees updated .


terbium

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How much H2 was absorbed?
« Reply #2 on: April 08, 2003, 04:45:00 PM »
After only 1.5 hours the absorbtion has completely stopped, and after 2 hours the reaction was stopped.

And is the product basic?


Chromic

  • Guest
Yields
« Reply #3 on: April 08, 2003, 07:27:00 PM »
If I'm using powdered oxime, I get 80% yields with Al/Hg reduction, if I'm using the gooey oil oxime I get 20% yields.

I think you should more carefully distill your ketone and really watch the reaction conditions for the oxime formation.

Barium

  • Guest
Hydrogen uptake
« Reply #4 on: April 09, 2003, 02:01:00 AM »
If you are performing a hydrogenation with which you are unfamiliar you would do yourself a great favor by measuring the hydrogen uptake. This will make it possible to better understand what has happened inside the reactor. Was the product neutral or alkaline? Have you distilled it? Done any A/B wash? You have left out a lot of information.


Argon

  • Guest
How much H2 was absorbed?
« Reply #5 on: April 09, 2003, 10:47:00 AM »
How much H2 was absorbed?    --> 1 mole, or very close to it

And is the product basic?    --> The crystal clear residue was neutral, the reaction contents were PH 6.5

Have you distilled it?Done any A/B wash?       --> No, distilled off the alcohol, and attempted an A/B, a little product went into the acidic water, the clear oil left on bottom was separated off, and the acidic water washed once with toluene. Then basified, and extracted with a preweighed amount of toluene. Toluene was then separated and weighed again. It only was 1 gram heavier. And that is with sloppy lab technique. So I assume there was no product in there.

synthesys

  • Guest
Initial Ketone/NH2OH mixture PH
« Reply #6 on: April 09, 2003, 10:53:00 AM »
I believe that the ideal PH for oxime formation is 5.5
Someone correct me if I am wrong.

What was your PH at that stage?

You should definately distill the oxime to dryness in your rotorvap.
Can someone explain why that is? The properties of the oxime state that it could be an oil, or a crystalline solid, and basic or acidic. Why does it matter for further reduction purposes to specificly turn it into a solid?
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Antibody2

  • Guest
Why does it matter for further reduction ...
« Reply #7 on: April 11, 2003, 12:32:00 PM »
Why does it matter for further reduction purposes to specificly turn it into a solid? so you know it isn't ketone or some other kind of crap. oxime that won't crystalize isn't usually very pure either.