The Reduction of
a Benzophenone 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).
Equations:
(1) 4 C6H5-(C=O)-C6H5 + NaBH4 + 4 [(C6H5)2CHO-]-BNa
Benzophenone
(2) 4 [(C6H5)2CHO-]BNa
+ 4 CH3OH + 4 [(C6H5)2CHOH]
+ (CH3O)4B- Na+
diphenylmethanol
Amounts of Reactants Used:
|
|
C6H5COC6H5 |
NaBH4 |
H2O |
C6H5CH(OH)C6H5 |
CH3OH |
|
MW |
182.2 |
37.83 |
18 |
184.2 |
28 |
|
Moles |
|
|
|
|
|
|
Grams |
1 |
0.25 |
|
|
|
|
B.P.
|
305
|
297
|
|||
|
M.P.
|
47
|
66
|
|||
|
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 45oC. 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 until next Thursday.
7. Next week: weigh the product, and determine its melting point.