Why Must Sodium Borohydride Be Added Slowly in Benzophenone Reduction?

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SUMMARY

The reduction of benzophenone to benzhydrol using sodium borohydride must be conducted slowly to control the exothermic reaction and prevent hazardous outcomes. The reaction is performed in iced water to manage heat release effectively. After the reaction, the solid product is dissolved in hot methanol, followed by the addition of water to reach a cloudy point, indicating the formation of diphenylmethanol. This process highlights the importance of temperature control and solvent choice in organic synthesis.

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  • Understanding of organic chemistry reaction mechanisms
  • Familiarity with sodium borohydride as a reducing agent
  • Knowledge of solubility principles, particularly with diphenylmethanol
  • Experience with temperature control in chemical reactions
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  • Learn about temperature control techniques in organic synthesis
  • Explore the solubility characteristics of diphenylmethanol
  • Investigate the use of iced water baths in exothermic reactions
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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?
 
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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.
 
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?
 
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.
 

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