Write a balanced equation for the reduction of 9-fluorenone to 9-hydroxyfluorene by sodium borohydride. what is the limiting reagent in the following procedure?

 To obtain a balanced equation you need to be somewhat familiar with the mechanism of the reaction. It can be broken down into 3 steps: 

1) Nucleophilic addition of hydride (by borohydride) to the carbonyl carbon of fluorenone 

C13H8O + BH4- ==> C13H9O- + BH3 

2) Complexation of BH3 with ethanol 

BH3 + CH3CH2OH ==> CH3CH2OHBH3 

Note that in a Lewis structure the product should appear with a formal positive charge on O and a formal negative charge on B. 

3) Formation of product (proton transfer) 

C13H9O- + CH3CH2OHBH3 ==> C13H9OH + CH3CH2OBH3- 

Next, the ethoxyborohydride formed here can react with 3 additional equivalents of fluorenone, thereby picking up additional ethoxy groups in the process. 

Adding these steps together and canceling like terms, we arrive at: 

4C13H8O + BH4- + 4CH3CH2OH ==> 4C13H9OH + B(OCH2CH3)4- 

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