* The Al/Hg reducing system is active at whatever pH you want, but the closer to neutral, the slower it will go.
* If the pH is too acidic, all of the methylamine will be in the form of methylammonium ion, which is not nucleophilic enough to add to the ketone and form the carbinolamine.
* If the pH is too basic, the dehydration of the carbinolamine to form the imine will be too slow.
What pH are we aiming for here? We have three reactions that needs to take place at the same time, and each of them wants a different pH environment - we cannot make the solution too acidic or too basic, but on the other hand we cannot make it too neutral either...