[HP]

Hi
Now i am working on lab exs with amines...
I know the formation of azomethyne compounds(Shiff's bases) between primary amines and carbnyl compounds is an equilibrium reaction. The amine i use have pKb=6.5 and the aldehyde i tried first was the aromatic isovanilie in the presense of acetic acid/H2O/ethanolic media. The pH of the reaction media was 4 and at this pH my IR analyze show little Shiff base formation. When i adjust the pH of the media to 5 and above to 6-7 with ading NaOH solution the reaction seems proceed very well with formation the Shiff's base(also visible with the typical colour formation).
How can i explain this fact ???Is this Shif's base unstable at pH=4 or theres any other explanation i wonder.Also what could be the role of ethanol in this reaction mixture except for disolving the isovaniline which is little soluble in water..
Thanks and greetings alles!

[movies]

Control of the pH is important because the amine is basic.  When the amine is protonated it isn't very nucleophilic so it can't start the condensation reaction.

[HP]

I agree the pH of the media is important especially because the reaction proceed in water containing media and in the imine formation reaction water is product in equilibria. I red that the aliphatic aldehydes form less stable imines than the aromatic because aromatic pi- conjugation but when i used formaldehyde(37%) it seems the reaction proceed well also in acidic media does it mean formaldehyde may form methylene imines with the protonated NH3+ form of the amine?

[movies]

Formaldehyde is kind of a special case because it is a completely unstabilized aldehyde; therefore formaldehyde is a lot more reactive in general.  In any case though you need a free amine to attack the carbonyl carbon.  A protonated amine doesn't have any available electrons to attack with.