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Chem 226-001

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January 05, 2018

 

Reduction of Benzophenone Using Sodium Borohydride

 

Objective

            The
purpose of this experiment was to synthesize diphenylmethanol from benzophenone
using sodium borohydride as a reducing agent. This is known as a reduction reaction
which reduces ketones or aldehydes to alcohols. A ketone, benzophenone, was
reduced to a secondary alcohol, diphenylmethanol.

 

Balanced
Chemical Equation

       

 

Procedure

            First of
all, the reflux apparatus was set up in the assigned hood. 0.55 g of
benzophenone was weighed on a balance and added to a dry 25 mL round-bottom
flask. Then, 0.07 g of sodium borohydride was weighed and added into the same
round-bottom flask. Added 3 mL of 2-propanol using a 10 mL graduated cylinder
to the same flask. The mixture in the 25 mL round-bottom flask became a
colorless solution. The flask with the solution was clamped to the reflux
apparatus. A condenser was attached to the flask containing the solution. The
heat was turned on with the Variac setting at 50%. About 3 boiling chips were
added to the flask through the condenser because it had been a few minutes
since the heat was turned on. The vapors were observed to be on the bottom
third of the condenser. The solution was allowed to be refluxed for 30 minutes.

Time began when the vapors were observed to drip back into the flask from the
condenser tip. After the 30 minutes, the heating mantle was turned off and pulled
away from the flask with condenser still attached. The solution was allowed to
cool to room temperature. Moreover, 3 mL of 10% aqueous sodium hydroxide were
added to flask using a 10 mL graduated cylinder to decompose the borate ester
complex. A glass stopper was placed on the flask; consequently, the solution in
the flask was swirled vigorously. After adding 3 mL of H2O to the
flask, the solution was vigorously swirled again with the glass stopper on the
flask. The solution in the flask turned cloudy white, and two distinct liquid
layers began to separate in the flask.

The liquid-liquid extraction apparatus was
set up in the hood. Added 5 mL of dichloromethane to the flask. Then added
another 5 mL of dichloromethane to the flask. Carefully poured the solution
into the separatory funnel without getting any of the boiling chips into the
funnel. A glass stopper was placed on the funnel. The separatory funnel was
inverted carefully about 3 times. During each invert, the stopcock was open to
release the pressure. The funnel was placed back on the ring stand while
immediately removing the glass stopper. The layers were allowed to separate
before draining. The bottom layer (organic layer) containing desired product was
drained into a 100 mL beaker which was a cloudy liquid. The other layer in left
in the funnel was properly discarded. Added anhydrous calcium chloride to the
solution that was extracted into the beaker.

The distilling apparatus was set up in the
hood. The solution was transfer from the beaker into a 50 mL round-bottom
flask. The heat was turned to a Variac setting of 50%. The boiling chips were
mistakenly not added into the flask with solution. The 50 mL round-bottom flask
was large enough to not lose too much product due to bumping. The distillation
was stopped when no more dichloromethane solution was distilled off into a 10
mL graduated cylinder. A clean watch glass was weighed on the balance. The
solution in the flask will turned solid as soon as it starts cooling. So, the
flask with solution was pour quickly onto the watch glass when distillation
stopped. The clear solution turned a white cloudy solid. The heat was turned
off. The watch glass containing the solid was weighed on the balance. The
melting point was measured using Mel-Temp. The percent yield for this
experiment was calculated.

 

Results

            The
product obtained from this experiment is a white cloudy solid. The mass of the
product was determined to be 0.44 grams. The melting point of the product that
was measured using the Mel-Temp was 47.2-55.6 °C.

 

 

Discussion/Conclusion

            In this
experiment, the goal was to synthesize diphenylmethanol from benzophenone using
sodium borohydride as a reducing agent. The experiment conducted was a
reduction reaction; therefore, a reducing reagent is able to reduce ketones or
aldehydes to alcohols. A ketone, benzophenone, was reduced to a secondary
alcohol, diphenylmethanol. The initial mixture was a clear colorless solution,
but the final product was a white cloudy solid. The desired product was
synthesized by performing a reflux of the solution. Moreover, the product was
isolated by using extraction and distillation techniques. The measured melting
point of the product is 47.2-55.8 °C while the literature melting point is 65-67 °C for
diphenylmethanol. The measured melting point is broad in range and lower than the
actual melting point. This is a result of the product not being completely pure.

The product most likely contained some impurities due to not completely
isolating diphenylmethanol from the mixture. The percent yield after conducting
this experiment was 79.1% which is a good yield. The yield is less than 100% due
to some experimental errors. During the distillation, boiling chips were not
added to flask resulting in loss of solution due to bumping. Perhaps allowing
the reflux to continue for more than 30 mins would have yield better results. Another
error was that a 100 mL beaker was used instead of an Erlenmeyer flask to drain
solution when using separatory funnel. The wider rim of the beaker caused some
of the solution to splash out of the beaker. Moreover, the solution was
transfer to different containers causing some of the solution to be loss. Overall,
the results demonstrate that diphenylmethanol was successfully synthesized with
the experimental product contained some impurities.

 

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