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Topic: how to separate cis, trans 2-hydroxybenzalaniline  (Read 8028 times)

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briteyellowness

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how to separate cis, trans 2-hydroxybenzalaniline
« on: October 16, 2005, 01:13:02 AM »
i'm not sure how to separate cis and trans 2-hydroxybenzalaniline.  I would think maybe by enantiomeric resolution?  But this is a double bond with cis and trans, and I only know that the chemical resolution works in the R, S way.   So it can separate them by forming diasteromers like RR and SR and then separating them by their different physical properties?  Wait.. so geometric isomers already have different physical properties, so I can separate them already by things like melting point?

Offline madscientist

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #1 on: October 16, 2005, 01:24:23 AM »
I love how you asked and answered your own question, very efficient lol
The only stupid question is a question not asked.

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #2 on: October 16, 2005, 01:48:44 PM »
Yes, in principle you should be able to separate cis/trans alkene mixtures.  It's not always easy though.

briteyellowness

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #3 on: October 17, 2005, 08:30:54 AM »
i can't find a website that tells of its different physical properties. ahh!!  is there any site that can tell me this?  i'm supposed to make 2-hydroxybenzalaniline from salicylaldehyde and aniline.

should it be mostly trans and not cis due to steric hindrance?

and then once i make the product i should separate it. now that i know a "general background" of the fact it's separable on physical properties, i don't have a website of its physical properties now. ahhh.  anyways, it's solid at room temp, so how can i separate that?

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #4 on: October 17, 2005, 01:08:50 PM »
If it's a solid then recrystallization is a good bet.  Aldimines like that will favor the trans form by a lot.  Do you have an idea of what the ratio of isomers is?

briteyellowness

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #5 on: October 17, 2005, 08:06:29 PM »
nope  :-\ i'm supposed to find out the ratio by separating the different isomers.  why would recrystallization be a good bet?  would the cis isomer just be an "impurity" so recrystallization would probably just be the trans isomers?  But they would both be.. insoluble at room temperature and soluble at high temperature... so i thought in recrystallization, the impurity needed to be soluble in both temperatures....

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #6 on: October 17, 2005, 08:44:16 PM »
Both can be somewhat insoluble at room temp.  Of course, it's better if one is insoluble because then you can carry out a hot filtration and remove all the insoluble impurities.

The real key to crystallization is that many molecules prefer to crystallize as a lattice of like molecules, so impurities are excluded from the crystals.  Even relatively similar compounds can be separated by iterative (or "fractional") crystallization.  It can be a very painstaking process though.

You could find out the mixture ratio much more rapidly (and accurately) with an NMR spectrum though.  Fractional crystallization is kind of a shady way to get ratios because you don't always get 100% recovery, plus you can't really be fully sure that your product is totally pure.

briteyellowness

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #7 on: October 17, 2005, 09:21:50 PM »
i thought NMR is just structure related.  How does it even tell you the ratios of two different isomers?  Wouldn't they be the same... proton- or carbon-wise?

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Re:how to separate cis, trans 2-hydroxybenzalaniline
« Reply #8 on: October 18, 2005, 06:24:07 PM »
Different molecules have different NMR resonances, right?  The cis and trans imine isomers will have protons in magnetically different environments.  So if you can assign two resonances to two different isomers, then you can compare the relative integration of those resonances and then figure out what the product ratio is.  In your example then imine C-H will have a distinct resonance and the two isomers might be different enough that you could compare the two integrations.

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