DIBALL
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Schiffs base / imine with an amine salt?
Right now I´m trying to make a Schiffs base from a ketone with methylamine. And I wanted to know, if I can use methylamine HCl for the reaction with
the ketone or if it could cause problems with the salt form?
I dissolved the ketone with toluene and anhyd. ethanol together with the methylamine HCl, put some magnesium sulfate in it and let it stand for a
couple of days. Then I boiled the ethanol off and it looks like everything reacted, since no methylamine HCl precipitate out of the solution.
And now I want to do a Grignard reaction.
Does the amine salt group interfere with the reaction? Or is the Schiffs base even possible to make with an amine salt?
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zed
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Free base is traditional.
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Reboot
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The reaction of an amine with a ketone is driven by the attraction between the electron poor carbon (with the oxygen double-bonded to it) in the
ketone and the electron rich nitrogen atom of the amine. If the amine is protonated (as it is in acid salt form) the nitrogen is no longer electron
rich. I wouldn't expect a reaction at all with an amine salt.
Grignards do not play nice with acidic hydrogens floating around, so if there's residual amine salts in the mix, that could completely screw up the
reaction (as those acidic hydrogens react with the electron rich carbon ion formed during the Grignard.)
You'll have to free that amine from the salt I expect. On the small scale it could be as simple as a bit of hydroxide, molecular sieves to remove
the water, and patience.
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AvBaeyer
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The best practical way to efficiently form a Schiff base from ketones is to react the carbonyl compound with a weak acid salt of the amine such as an
acetate. This can be accomplished by dissolving the amine hydrochloride and the ketone in ethanol or methanol and adding an equimolar amount of
anhydrous sodium acetate. Because the formation of Schiff bases from ketones is relatively slow, an excess of either the ketone or the amine salt is
typically used. Usually the amine salt is used in excess. Aldehydes are usually so reactive that equimolar amounts of aldehyde and amine free base can
be mixed with immediate product formation.
AvB
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zed
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Yup. Amine salts of strong acids, are usually neutralized before attempting to react with ketones or aldehydes.
Same goes for oximes (and phenylhydazones?). These may drop out of solution, upon formation. And, some analytical procedures, utilize this property.
Been a while, since I ruminated on this. I should upgrade my mindware.
Hydroxylamine HCl, is neutralized via Sodium Acetate, to form NaCl plus Hydroxyamine Acetate. This acetate reacts readily. And, shazaam..... A
crystalline derivative.
[Edited on 7-1-2018 by zed]
[Edited on 7-1-2018 by zed]
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xxX_Qwerty_Xxx
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Hey, i still dont get it - as Reboot said, the nitrogen in the salt is no longer electron rich. So why does it work with amine salts?
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Sigmatropic
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There's theory, and then there's practice. By the same logics diazotizations in dil. HCl or H2SO4 cannot work.
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xxX_Qwerty_Xxx
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Oh man, i dont even know anything about diazotizations, lol. I just want to know - should i use amine salt or its freebase form to make imines?
[Edited on 14-1-2019 by xxX_Qwerty_Xxx]
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Dan Vizine
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As the above answers clearly indicate, use the free base or its salt with a weak acid (so that there's an equilibrium to furnish some concentration of
free base).
Just a heads up, anybody can read this column and you aren't as anonymous as you may think. I was born with a suspicious mind and I spent decades
making pharmaceuticals and so naturally I'm curious about seeing an under-informed new user who is inquiring about how to potentially make a meth
precursor (if the ketone he has is methyl benzyl ketone). Even your user name "DIBALL" suggests familiarity with the convenient reducing agent which
converts imines to amines. So, yeah, I know that you'll assure me that this isn't your goal and I'm not going to call you a liar. But just be aware
that to some, your post says "I'm worth investigating". Just owning methylamine is a huge red flag for at least the last 20 years in the US. Probably
in many other countries, too.
I'd be curious about the aim of your work, if you'd care to share.
[Edited on 1/14/2019 by Dan Vizine]
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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Elrik
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I think your confused.
If he was accurate in his outline of his synthesis, ketone to N-methyl imine to grignard reaction, he is not making meth.
To make meth by a route like that phenylacetone would not even be involved, that would start by making acetaldehyde methylimine. No ketones would
enter the reaction.
People are so paranoid about meth on here.
Not that I blame you 
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zed
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Indeed, he is not making Meth. U.S. Department of Justice communications, suggest reacting a Grignard reagent with Acetaldehyde-Methylimine, is a
poor reaction path. They would prefer that apprehended criminals utilize some approach more likely to produce usable evidence.
Quite difficult to completely remove formed H2O, from an etheric Acetaldehyde/Methylamine solution.
Phenylacetaldehyde/Methylamine+Grignard..... Purportedly works better, but aside from illegality.... It is still fairly nasty chemistry, for such a
trivial result.
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Dan Vizine
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I was suggesting that perhaps no Grignard was actually planned. Benzyl methyl ketone can be condensed with methylamine to give a Schiff base which can
be reduced to the methylamino compound.
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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