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.

 

[LudusRex]

The reduction of benzophenone to benzhydrol involves the addition of sodium borohydride (NaBH4) as a reducing agent. This reaction must be carried out slowly and in the presence of iced water to ensure the safety and success of the reaction.

Sodium borohydride is a highly reactive compound and can react violently when added to certain substances. Therefore, it is important to add it slowly to the reaction mixture to avoid any unexpected reactions or hazards. By adding it slowly, we can control the rate of the reaction and prevent any potential dangers.

Placing the reaction in iced water also serves a crucial purpose in this reduction reaction. The addition of sodium borohydride to benzophenone is an exothermic reaction, meaning it releases heat. By placing the reaction in iced water, we can regulate the temperature and prevent the reaction from becoming too exothermic. This can help prevent the formation of unwanted by-products or even explosions.

Furthermore, keeping the reaction in iced water also helps to maintain the stability of the reaction intermediates and products. Benzophenone is a carbonyl compound, which is highly reactive and can easily undergo side reactions. By cooling the reaction mixture, we can slow down any unwanted side reactions and ensure the desired product, benzhydrol, is formed.

In summary, adding sodium borohydride slowly and placing the reaction in iced water are important safety measures that help control the rate and temperature of the reaction, prevent unwanted side reactions, and ensure the successful reduction of benzophenone to benzhydrol.