[vikram]

Hi all,

I am synthesizing a schiff base using 2 hydrazino pyridine and an aldehyde. Little of the product precipitated which I filtered and washed with minimum amount of ethanol.Now the solid product has 2 hydrazino pyridine. What may be the best procedure to remove it?

Thank you:)

[billnotgatez]

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[AWK]

Usually Schiff bases are formed nearly quantitatively. Consider some modifications in reaction conditions.

[rolnor]

Perhaps you can use a little excess of the aldehyde?

[vikram]

I poured the mixture into water.But I was not able to recover the aldehyde in reasonable amount.

[AWK]

Your isolation procedure is also aldehyde dependent. We do not know it.
Since water is formed during reaction I suggest removing it through azeotropic distillation even without Dean-Stark apparatus. Azeotropes with n-propanol or n-butanol contain ~30 % of H₂O and boil about 80 or 90 C, respectively. Proceed synthesis under nitrogen or argon. You can check a progress of reaction through TLC (I guess 1-2 hrs will be sufficient). The only precipitate after cooling your reaction mixture should be a needed Schiff base.

[wildfyr]

Schiff base formation is also pH dependent, being most effective at pH ~4 and 8. Its a balance between protonating the carbonyl to make it electrophilic, vs not protonated the amine too much with acidic conditions. With the basic conditions, its about allowing the amine to be a good nucleophile by not having it be protonated at all. Its a complex interplay of factors that varies from substrate to substrate, but pH certainly has a strong effect on this equilibrium dependent reaction.

[vikram]

Ok. But now I have a good amount of schiff base which also has 2 hydrazino pyridine in it.I want to remove 2 amino pyridine now.

[AWK]

Usually solids are purified by crystallization.

[rolnor]

Chromatography?

[wildfyr]

Try recrystallizing in ether or an alcohol, those are early good bets.

[phth]

Usually Schiff bases are formed nearly quantitatively. Consider some modifications in reaction conditions.

ortho/para pyridines function as α-diketo functionalities or electron withdrawing group stabilizing the tetrahedral hemiaminal intermedita often causing these rxns not to go to completion para-pyridins being worse than ortho.  I would try heat and molecular sieves in tol.  Possible dean stark, and/0r just run a column.

[rolnor]

Usually Schiff bases are formed nearly quantitatively. Consider some modifications in reaction conditions.

ortho/para pyridines function as α-diketo functionalities or electron withdrawing group stabilizing the tetrahedral hemiaminal intermedita often causing these rxns not to go to completion para-pyridins being worse than ortho.  I would try heat and molecular sieves in tol.  Possible dean stark, and/0r just run a column.

But this is hydrazinopyridine, not aminopyridine, is not that an important difference?

[AWK]

Every Schiff base contain linear -CH=N- bonds. Aldimines and hydrazones are the special subgroups within Schiff bases.

[rolnor]

Yes, but you have one more N-atom between the pyridine ring and the dubbelbond in the imine, thats a different system alltogether?