WEEK 2: SODIUM BOROHYDRIDE REDUCTION
PURPOSE:
You will use a reduction reaction in the synthesis of
hydrobenzoin from benzil using sodium borohydride as the
reducing agent and characterization of the product by melting
point.
IMPORTANT REACTION:
C
O
C
O
Benzil
M.Mass 210
NaBH4
H
C
OH
H
C
OH
(1R,2S)-(meso)-Hydrobenzoin
M.Mass 214, mp 137oC
+
H
C
OH
HO
C
H
(1R,2R)-and (1S,2S)Hydrobenzoin
mp 120oC
BACKGROUND INFORMATION:
Oxidation and reduction reactions are very important in
organic chemistry in the synthesis of desired compounds. There
are many oxidizing and reducing agents that can be used to
achieve the desired product. Some of these redox reagents are
general in that they can react with a large number of functional
groups. Other redox reagents are very specific and have limited
but specific reactivity and can cleanly do the conversion that
is desired. Sodium borohydride is such a specific reducing
reagent. It can be safely used to reduce an aldehyde or ketone
to an alcohol. Hydride type reagents are rather basic and can
often be vigorously decomposed in water or alcohol. Extreme
care should be exercised whenever you work with any type of
hydride. However, sodium borohydride is reasonably safe to use.
It does not undergo the decomposition as vigorously as other
hydrides. Further, the reaction can be run in alcohol and
yields are very good. The reaction that will be done in this
experiment is generally fast and almost quantitative.
EXPERIMENTAL PROCEDURE:
Dissolve 0.5 g of benzil in a 25 ml Erlenmeyer flask using
5 ml of 95% ethanol. Slight heating may be required but cool to
room temperature before adding 0.1 g of sodium borohydride. In
about 3 minutes the reaction is complete as noted by the
disappearance of the yellow color of benzil. After about 5 more
minutes, add 5 ml of water and heat the solution to decompose
the large excess of sodium borohydride. While still warm, add
approximately 10 ml of water. Let the solution cool to allow
the hydrobenzoin product to crystallize. Suction filter the
product, dry and weigh it and take its melting point.
IMPORTANT INFORMATION ABOUT THE REPORT:
This experiment is another example of a synthesis reaction.
You must calculate the percent yield as you did last week. Be
sure to use the correct number of moles from the balanced
equation in your calculation to determine the theoretical yield.
Be sure to record both the observed and reported melting point
ranges for your product.
END OF EXPERIMENT.
© 2007 Stephen Anderson and Robert Shine