[vikram]

Can anyone help me to know how can I separate aromatic amine from aldehyde/ketone+amine.I heard in column chromatography imine breaks down into amine and aldehyde/ketone.

Can I achieve this by any precipitation method?

Thanks in advance:)

[Dan]

Aromatic imines can quite often be purified by chromatography in my experience, sometimes you have to be quick. You can check with a 2D TLC to see if the imine is stable enough to column. If 2D TLC shows it's unstable on silica, try alumina instead.

Otherwise your only option is recrystallisation of the imine or evaporation of the aldehyde and amine (if they are sufficiently volatile).

[vikram]

Thank you for your valuable inputs

Can you please suggest me a recrystallization method?Which solvents are better?

I don't have much idea.(Based on my findings in research-gate, I tried to dissolve the product in all possible solvents to find the solvent in which my product is soluble only upon heating.But didn't succeed.)

[Dan]

Can you please suggest me a recrystallization method?

No, the best solvent depends on the structure and cannot be reliably predicted. You have to experiment.

A mixture of EtOAc and hexane is usually a good starting point. Usually I suspend the material in refluxing hexane, then add EtOAc slowly until everything has dissolved, then cool to RT (and to 0 °C if necessary).

For aromatic imines, I have had good results with EtOH as a recryst solvent, but that doesn't mean it will work in your case.

[vikram]

I followed this method:

Dissolved crude product in ethanol and added hexane drop wise. Spotted the residue and filtrate on TLC along with starting materials. (S1:aromatic ketone S2:aomatic amine R:residue F:Filtrate). please refer to the attachment.
I expected the amine to precipitate out first as it is most polar one.But in residue I found its aldehyde and in filtrate amine.Totally confused

[rolnor]

You can also add a little triethylamine to the mobilphase when you pack the column to neutralize the acid in silica. Better to recrystalize after chromatography.

[Dan]

Dissolved crude product in ethanol and added hexane drop wise. Spotted the residue and filtrate on TLC along with starting materials. (S1:aromatic ketone S2:aomatic amine R:residue F:Filtrate). please refer to the attachment.
I expected the amine to precipitate out first as it is most polar one.But in residue I found its aldehyde and in filtrate amine.Totally confused

Do you mean the imine is in the filtrate? You say above that the filtrate contains the amine, implying that the imine hydrolysed to the amine during recrysallisation.

From you TLCs it looks like you are using a large excess of aldehyde vs amine, this is not necessary when condensing aromatic amines with aromatic aldehydes. It would probably make your life easier if you mix the aldehyde and amine in a 1:1 ratio to avoid a large amount of excess crystalline reagents.

[vikram]

Sorry its ketone.(I have mentioned aldehyde in several places) I have taken ketone:amine in 2:1 molar ratio as it is a diketone.

I am trying to play with polarity to achieve selective precipitation of imine. I found amine and imine in filtrate which is contrary to the expected result.I expected amine to be precipitated first as it is more polar than imine and diketone.

[pgk]

By several extractions with saturated NaHSO3 aqueous solution (prepared by dissolving sodium metabisulfite Na2S2O5 in water), you eliminate the most of aldehyde/ketone as bisulfite adduct and the most of amine as bisulfite salt. Then, extract with water until neutral to litmus, followed by recrystallization or column chromatography. But the overall procedure must be very quick, as Dan advices.
Imines are quickly hydrolyzed under acidic conditions but they are relatively stable under alkaline or neutral conditions.
On the other hand, monobasic sulfurous acid (pka2 = 6.9) is a weak acid that cannot accelerate imine hydrolysis, cannot easily neutralize imines (pka ≈ 9.0) but can form salts with amines (pka >10). Otherwise, purification by further extractions an aqueous solution of a weak organic acid (e.g acetic or propionic, pka ≈ 4.9) can also be effectuated.

[wildfyr]

pgk's NaHSO3 trick is one I've always wanted the opportunity to try, because its so neat, give it a shot.

[vikram]

Then, extract with water until neutral to litmus, ??

Can you please elaborate a little?

[pgk]

Sorry, bad English.
NaHSO3 is a weak acid that might catalyze the imine hydrolysis, if not being eliminated and stays in contact with the imine, for a long time. So, after 4-5 extractions with aqueous NaHSO3, you continue extractions with pure water and monitor the pH of the aqueous phase with a pH strip (litmus paper) until neutral. Then, you dry the organic phase with Na2SO4, as usual, etc…

[vikram]

Now it is very clear.Thank you very much. .