THE REDUCTION OF A CARBONYL WITH SODIUM BOROHYDRIDE

 

 

Sodium (or potassium) borohydride is a highly selective reagent that reduces aldehydes or ketones to the corresponding alcohols but normally does not reduce nitro, nitrile, olefinic, amide, carboxylic acid, or ester functional groups. The reagent is less reactive (i.e., more selective) than lithium aluminum hydride. Moreover, it may be used in aqueous or alcoholic solutions.  Lithium aluminum hydride, on the other hand, reacts violently with such hydroxylic compounds and must be used in inert solvents such as ether, tetrahydrofuran, etc. In this experiment a simple example of the use of sodium borohydride is illustrated by the reduction of benzophenone (diphenyl ketone) to diphenylmethanol (diphenylcarbinol).

 

Equation:

 

(1)           4 C6H5-(C=O)-C6H5 + NaBH4 6 4 [(C6H5)2CHO-]-BNa

                    Benzophenone

 

(2)           4 [(C6H5)2CHO-]BNa + 4 CH3OH  6 4 [(C6H5)2CHOH] +(CH3O)4B- Na +

                                                                                diphenylmethanol

 

Table of Physical Constants

 

   Compound                         M.W.                         density        mp                           bp

   C6H5COC6H5                     183.0                        -----             47                            305

   NaBH4                                 37.83                        -----             ---                             ---

   C6H5CH(OH)C6H5            184.0                        -----             66                            297

 

 

Tabled Amounts of Reactants Used and Amounts of Products Theoretically Possible

 

 

C6H5COC6H5

NaBH4

H2O

C6H5CH(OH)C6H5

CH3OH

 

MW

182.2

37.83

18

184.2

28

 

Moles

 

 

 

 

 

 

Grams

1

0.25

 

 

 

 

ml

 

 

 

 

 

 

 

 

Procedure

 

 1.            Dissolve 1 g  of benzophenone in 7.5 ml of methanol in a 50‑ml Erlenmeyer flask.

 

2.                  In a 5 or 10 mL beaker add 0.25 g of sodium borohydride.

 

3.                   Add the sodium borohydride in small portions and with swirling to the benzophenone solution at such a rate that the temperature does not exceed 45E.  The reaction is exothermic and the rate of addition, therefore, should not be too rapid.

 

 4.            After all the sodium borohydride has been added, heat the reaction to boiling for 2 minutes.  Add the hot solution to 15 gm of ice in a 50 mL beaker.

 

 5.            Allow the ice to melt and collect the diphenylmethanol on a small  Buchner funnel.

 

 6.            Wash the crystal cake twice with 25‑ml portions of water and allow to dry overnight.

 

 7.            At the beginning of the next lab period, weigh the product, and determine its melting point. Yield about 1 g (95%).  Include in your notebook and in your Lab Report, also calculate the % yield.

 

 7.            Use this product in the alcohol classification assignment.