Experiment 5: Reducing a
ketone using NaBH4
Risk: Class B
This experiment needs close supervision by the teacher because there are significant risks due
to the nature of the chemicals or equipment used. Wear goggles and lab coat. Mop up spills
using water. Wash hands thoroughly afterwards. Read the hazard warning labels carefully
and treat accordingly. Do not carry out the experiment until you have the permission and
attention of your teacher.

NaBH4 is properly known as sodium tetrahydroborate(III); it is commonly called sodium
borohydride.
 The method involves dissolving the carbonyl compound in warm ethanol (contains
about 10 % water). The NaBH4 is added, the mixture stirred and left for 10 to 15
minutes – heat is usually evolved. Water is added and the mixture boiled - this destroys
the excess NaBH4.
 These reactions happen in 2 steps:
a) Step 1: initiated by the nucleophilic attack of an H- to the C of the carbonyl group,
b) Step 2: followed by the attack of a H+ ion from water to the anion.
NaBH4
Overall: R1COR2 + 2 H ( H- + H+ )
R1CH(OH)R2
H2O / heat
The mechanism is:
1) Add 0.7g of 1,2-diphenylethanedione (Benzil)
to 7cm3 of ethanol in a small conical flask
2) Warm the mixture in a hot water bath at about 50oC until the solid dissolves.
3) Cool.
When the solid reappears as fine suspension add 0.15g sodium
tetahydroborate (III). Allow to stand for 10 minutes.
4) Add 15cm3 of distilled water and heat until the water boils. Allow the mixture to
cool.
5) Collect the crystals by suction (Buchner) filtration. Suck dry and wash with 100cm3
of distilled water. Transfer the solid to a clean filter paper and press dry.
6) Design some chemical experiments to show that the starting material and the
product have different functional groups.
7) Write down a balanced equation using displayed formulae and 2H as the reductant.