Why Must Sodium Borohydride Be Added Slowly in Benzophenone Reduction?

[ephemeral1]

Why do you have to add sodium borohydride slowly to reduce benzophenone to benzhydrol? And why do we have to place the reaction in iced water?

 

[Char. Limit]

Because you need to control the amount of heat released in the reaction. You don't want the whole thing to explode... or maybe you do. That might be a fun thing to try.

 

[ephemeral1]

After we got the solid from the reaction of benzophenone with sodium borohydride, then we dissolved the solid in a minimum amount of hot methanol. Why do we add the same amount of water to get the resulting solution to a cloudy point?

 

[Char. Limit]

The methanol, I assume, would isolate the (former) benzophenone, and water is needed to complete the reaction. (any acidic substance with a pKa than can protonate a diphenylmethoxide ion, really) Since diphenylmethanol has a solubility of just 0.5 g/L at room temperature, the cloudy stuff is probably that.