THE REDUCTION OF A CARBONYL WITH SODIUM BOROHYDRIDE
pp 495-498
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  4 [(C6H5)2CHO-]-BNa
Benzophenone
(2)
4 [(C6H5)2CHO-]BNa + HCl + 3H2O  4 [(C6H5)2CHOH] + H3BO3 + NaCl
diphenylmethanol
Table of Physical Constants
Compound
C6H5COC6H5
NaBH4
C6H5CH(OH)C6H5
M.W.
183.0
37.83
184.0
density
-------------
mp
47
--66
bp
305
--297
Tabled Amounts of Reactants Used and Amounts of Products Theoretically Possible
MW
C6H5COC6H5
NaBH4
HCl
H2 O
C6H5CH(OH)C6H5
H3BO3
NaCl
182.2
37.83
36.5
18
184.2
61.8
58.5
2.2
0.5
Moles
Grams
ml
Procedure
1.
Dissolve 2.2 g of benzophenone in 15 ml of methanol in a 150-ml beaker.
2.
In a smaller beaker prepare a solution of 0.5 g of sodium borohydride in 7.5 ml of cold water. Add the
aqueous sodium borohydride solution in small portions and with stirring to the benzophenone solution at
such a rate that the temperature does not exceed 45. The reaction is exothermic and the rate of addition,
therefore, should not be too rapid.
3.
After all the sodium borohydride solution has been added, continue to stir the reaction mixture for
approximately 15 minutes. A white gum results which is the product of eq. 1 above. IF
your product oils or is not gummy, leave the flask in your desk until the next lab period before
proceding to step 4.
4.
Decompose the excess sodium borohydride and hydrolyze the white gum by adding the crystalline slurry
slowly and with stirring to a mixture of 50 g of crushed ice and water and 10 ml of concentrated hydrochloric
acid in a 400-ml beaker. Note: The decomposition of sodium borohydride is a vigorous reaction and
hydrogen is liberated rapidly. Unless a large beaker is used, the froth which results from the decomposition
cannot be confined.
5.
Collect the diphenylmethanol on the 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 2.2 g
(95%). Include in your notebook and in your Lab Report, also calculate the % yield.
8.
Use this product in the alcohol classification assignment.
Explanation of the Procedure
STEP 1 Sodium borohydride reacts with water to form H 2 gas so, instead of weighing, which exposes the reducing
agent to the water in the air for a significant length of time, you quickly dump a rounded teaspoonful
into the methanol. The reaction with water is
H2O + Na+BH4- ---> Na+(BH3OH)- + H2 (gas)
STEPS 2 & 3
The reaction between sodium borohydride and benzophenone is kept below 50 oC to minimize the
side reaction of sodium borohydride with the solvent methanol. The side reaction is
CH3OH + Na+BH4- ----> (CH3OBH3)-Na+ + H2
and can go to (CH3O)4B-Na+ + 4H2 if all of the NaBH4 reacts with the solvent.
The mechanism of the main reaction involves hydride, H-, leaving NaBH4 + going to the
benzophenone, see pg 496 of lab text.
The mechanism is
C6H5
H
C=O + H-B-H
H
C6H5
C6H5
Na+ -----> H-C-OBH3 Na+
C6H5
The reaction continues until all of the H on BH4 have been replaced by
H
C6H5-C-C6H5
O
C6H5
C6H5 H
3 C=O reacting with BH3- ---> HC-O-B-O-C-C6H5
C6H5
O
C6H5 O C6H5
H5C6-C-C6H5
C6H5-CH
H
C6H5
which is the white gum produced in equation (1) shown at the beginning of the experiment.
STEP 4 The borohydride salt of diphenylmethanol is hydrolyzed in two steps by the aqueous HCl. Reaction (a) is
H
C6H5CC6H5
C6H5 O H
OH
OH
HC-O- B-O-C-CH6H5 + 4 H2O ---> HO-B-OH Na+ + 4C6H5-C-C6H5
C6H5 O C6H5
OH
H
C6H5-C-C6H5
H
OH
HO-B-OH
OH
OH
Na+ + HCl ----> B + H2O + NaCl
HO OH
Lab Report. Follow outline on pg 5 and the instructions at step 7 of this experiment.
HOMEWORK
1.
Determine the molar ratio of NaBH4 to benzophenone that you ACTUALLY used in the experiment. why do
you think that it was necessary to use a greater molar ratio than the equations at the beginning indicate?
2.
After the reaction between the sodium borohydride and the carbonyl compound is complete, the reaction
mixture is treated with water to produce the desired secondary alcohol. Explain this reaction by indicating
the source of the hydrogen that ends up on oxygen.
3.
Although sodium borohydride is fairly unreactive toward methanol, the addition of a mineral acid to this
solution results in the rapid destruction of NaBH4. Explain. (Think in terms of the reactivity of Zno with
acetic acid compared to that with HCl.)
4.
Give the structure of the white precipitate formed in the reaction of benzophenone with sodium borohydride,
and write an equation for its reaction with water.
5.
What reagents will be used for the conversion of benzophenone to diphenylmethanol. Determine the limiting
reagent and the theoretical yield of product. Show your work.
6.
What types of carbonyl compounds could be used to prepare a primary or secondary alcohol? Could a
tertiary alcohol be prepared by the reduction of a carbonyl compound ? Explain.