[saN]

In lab, we synthesized the following oxime molecules with Y = ^-F and ^-OCH₃.



Our goal now is to separate the two isomers and find their ratios. We are certain that we have these two oxime isomers because the H NMR spectrum shows a double of every set of peaks, where in a case of a single isomer it would not.

Will these two compounds have a different boiling point to where a distillation will be able to separate the compounds?
Will these compounds separate through chromatography? If so, would some ratio of ethyl acetate and hexane be a good eluent?

This is our final step in our project besides putting a poster & presentation together. Any help would be greatly appreciated!

[AWK]

Chromatography is the best method in this case.
Note, sometimes during crystallization you can convert a mixture of oximes into one isomer

[g-bones]

if you just need to find the ratio of one to the other, you can just use gas chromatography and create a ratio between the area under the peaks.  this will be proportional to the ratio of each isomer in sol'n.

[saN]

With the attached groups at the Y position, we had only para substitution. Why would no ortho position form on the ring? I don't believe its due to stearic effects because fluorine and oxygen are not very large. If we had ortho position form, we would see more of a multiplet in the aromatic region rather than a double of doublets.

[saN]

Would performing a Beckmann rearrangement be easier to separate the two compounds?

[macman104]

I think this should be helpful:  http://www.usd.edu/~gsereda/Exp5s.ppt

Specifically look towards the last part of the slides.

[sjb]

With the attached groups at the Y position, we had only para substitution. Why would no ortho position form on the ring? I don't believe its due to stearic effects because fluorine and oxygen are not very large. If we had ortho position form, we would see more of a multiplet in the aromatic region rather than a double of doublets.

You'd be surprised how large (relatively, of course) some of these substituents are, maybe the ortho-compound did form, but in the course of your workup etc you've managed to get rid of it. Did you run crude TLCs etc before actually working up?

Would performing a Beckmann rearrangement be easier to separate the two compounds?

Possibly, but you'd need to be certain that both cis and trans oximes react stereospecifically to different amides. As it happens I think this is the case here, but in my opinion it's generally a bad thing to have to rely to rearrangements to work out relative yields of isomeric compounds in this manner.

[saN]

You'd be surprised how large (relatively, of course) some of these substituents are, maybe the ortho-compound did form, but in the course of your workup etc you've managed to get rid of it. Did you run crude TLCs etc before actually working up?

No, we just performed the work-up right after forming the ketone.

Possibly, but you'd need to be certain that both cis and trans oximes react stereospecifically to different amides. As it happens I think this is the case here, but in my opinion it's generally a bad thing to have to rely to rearrangements to work out relative yields of isomeric compounds in this manner.

Well, I have the H NMR for the oxime formation. If the H NMR of the beckmann rearrangement shows the two different compounds, I should be able to calculate how much of each product was formed, and relate it back to the oxime H NMR.