In imine formation, why are the reaction conducted at #pH=5-6#?

1 Answer
Jan 31, 2017

Because such a #pH# is optimal for bond-making, and for the elimination of water.

Explanation:

The general equation for the formation of an imine is:

#R_1C(=O)R_2 + H_2NR_3rarrR_1C(-O-H)(NHR_3)R_2#

With arrow pushing, the lone pair on the nitrogen is conceived to make a #C-N# bond, and one of the originally nitrogen bound hydrogens transfer to the oxygen centre. If this oxygen centre becomes protonated to give:

#R_1C(-""^+OH_2)(NHR_3)R_2#

We now have a potential leaving group, that can be lost to give a #C=N# bond:

#R_1C{=N(R_3)}R_2 + H_2O#

The given #pH# optimizes bond formation, the formation of the #N-C# bond, and is acidic enough to allow the elimination of water.

Most of this will be discussed in greater detail, and with much greater sofistication in your organic text. I urge you to consult it.