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
MW
C6H5COC6H5
NaBH4
H2O
C6H5CH(OH)C6H5
CH3OH
182.2
37.83
18
184.2
28
1
0.25
Moles
Grams
ml
Procedure
1.
2.
Dissolve 1 g of benzophenone in 7.5 ml of methanol in a 50-ml Erlenmeyer flask.
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.