Question

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?

Answer 1

The balanced equation for the reduction of 9-fluorenone to 9-hydroxyfluorene by sodium borohydride is provided, and the concept of limiting reagent is explained.

Step-by-step explanation:

The balanced equation for the reduction of 9-fluorenone to 9-hydroxyfluorene by sodium borohydride is as follows:

9-fluorenone + NaBH4 + H2O → 9-hydroxyfluorene + NaBO2 + H2

In this reaction, the limiting reagent is the reactant that is completely consumed and determines the amount of product that can be formed. To determine the limiting reagent, we can compare the moles of each reactant to the stoichiometric ratio in the balanced equation. The reactant that has the smallest ratio is the limiting reagent. In this case, we would need to know the amounts of the reactants to determine the limiting reagent.

Answer 2

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-