Question

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

Answer 1

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.