CHM 2202
Organic Chemistry Lab II
Spring 2014
Department of Chemistry
Villanova University
EXAMPLE OF LABORATORY WRITE-UP
Experiment Title: Reduction of Benzophenone
Date: September 2, 2013
Name: Ima Chemist Course: 2201
Section: 4 Lab Partner: Ben Zene
Pre Lab
(Completed before you come to lab)
Statement of Purpose: The purpose of this experiment is to reduce benzophenone to
diphenylmethanol using sodium borohydride. The reaction outcome will be assayed by melting
point analysis and IR spectroscopy. This experiment is described on page 216 of Bell, Clark &
Taber.
•
Reaction Sequence/Theoretical Yield
A
(Limiting Reagent)
B
C
O
OH
+
4
NaBH4
CH3OH (4 mL)
4
MW = 37.83
Wt. = 0.064 g (0.0017 mol)
C13H10O
C13H12O
MW = 184.24
Ther. Yield: 0.64 g
MW = 182.22
Wt. = 0.64 g (0.0035 mol)
(To ensure that you understand how to calculate the theoretical yield of product based on the
above reaction sequence and stoichiometry, calculate the Theoretical Yield using quantities
specified in text or handout)
Calculation of Theoretical Yield:
0.64 g A x 1 mole A
x
182.22 g A
4 moles C x
4 moles A
184.24 g C
1 mole C
= 0.64 g C
The following calculation shows that reagent B is not the limiting reagent
0.064 g B x 1 mole B
37.83 g B
x
4 moles C
1 mole B
x
184.24 g C
1 mole C
Page 1 of 5
= 1.25 g C formed
CHM 2202
Organic Chemistry Lab II
Spring 2014
Department of Chemistry
Villanova University
EXAMPLE OF LABORATORY WRITE-UP
Pre Lab (continued)
•
Table of Physical Constants
Table of Physical Properties of Reagents and Solvents
Reagent/Solvent
Molecular
Molecular
Melting/Boiling Density, g/mL
Formula
Weight
Point, °C
Benzophenone
C13H10O
182.22
mp = 47 - 51
Methanol
CH3OH
32.04
bp = 65
0.791
Sodium Borohydride NaBH4
37.83
mp > 300
Diphenylmethanol
C13H12O
184.24
mp = 65 - 67
•
Safety
Sodium Borohydride is moderately unstable to water, so protect from moisture while
weighing. Sodium borohydride reacts vigorously with acid with the evolution of
hydrogen gas, so do not expose to acid. Use the NaBH4 immediately after weighing.
Benzophenone is an irritant.
Methanol is an irritant and is flammable.
•
Procedure Outline
1. Place 0.64 g of benzophenone and 4 mL of methanol in a 25 mL Erlenmeyer
flask.
2. If needed, warm the mixture to dissolve the benzophenone and then cool to room
temperature.
3. To the benzophenone solution, add in one portion 0.064 g of sodium borohydride.
4. Swirl the flask and allow the reaction mixture to stand at room temperature for 20
min.
5. Add 2 mL of cold water and warm the reaction mixture on a steam bath for 5
minutes.
6. Cool the resulting mixture in an ice bath to precipitate a solid.
7. Collect the solid using vacuum filtration and air dry.
8. Determine the weight and melting point of the solid.
9. Calculate the % yield of product and compare mp with literature mp of
diphenylmethanol.
10. Obtain and interpret an infrared spectrum of product.
Page 2 of 5
CHM 2202
Organic Chemistry Lab II
Spring 2014
Department of Chemistry
Villanova University
EXAMPLE OF LABORATORY WRITE-UP
Laboratory Observations
(Completed during lab)
(Redraw the reaction sequence and use actual quantities employed in the reaction to
recalculate the theoretical yield – calculations shown in the conclusions section)
A
(Limiting Reagent)
B
C
O
OH
+
4
C13H10O
NaBH4
CH3OH (8 mL)
4
MW = 37.83
Wt. = 0.128 g (0.0034 mol)
MW = 182.22
Wt. = 1.28 g (0.0070 mol)
C13H12O
MW = 184.24
Ther. Yield: 1.28 g
Actual Yield: 1.04 g (81%)
Experimental Procedure and Observations:
Modification from pre-lab procedure: All amounts of chemical reagents and solvents
were doubled for this laboratory session (see the above reaction sequence).
1.
2.
3.
4.
5.
6.
To a 25 mL Erlenmeyer flask was added 1.28 g (0.0070 mole) of benzophenone and 8
mL of methanol.
The solid did not completely dissolve, so the flask was swirled and gently warmed on a
steam bath to produce a transparent solution. The flask was set aside to cool back down
to room temperature.
After cooling to room temperature, 0.128 g (0.0034 mole) of sodium borohydride was
added in one portion to the flask and it was swirled briefly until the solids dissolved.
The reaction mixture was allowed to stand at room temperature for 20 minutes with
occasional swirling. The reaction mixture bubbled slightly when the solution was
swirled.
After the 20 minute reaction period had elapsed, 4 mL of cold water was added to the
reaction mixture and the resulting solution was warmed intermittently on a steam bath for
5 minutes. Care was taken to not overheat the flask to ensure that the methanol was not
boiled off.
(a) The resulting warm solution was then allowed to cool to room temperature with
occasional scratching with a glass rod to stimulate crystallization of the product. The
flask was then cooled in an ice bath and scratching with the glass rod continued. A small
amount of white crystalline solid precipitated.
Page 3 of 5
CHM 2202
Organic Chemistry Lab II
Spring 2014
Department of Chemistry
Villanova University
EXAMPLE OF LABORATORY WRITE-UP
Laboratory Observations (continued)
7.
8.
9.
10.
(b) Modification from pre-lab procedure - After chilling in an ice bath for 10 minutes,
there did not appear to be a significant amount of precipitated solid so 1 mL of cold water
was added to the mixture to attempt to force more solid to precipitate. The resulting
mixture was chilled in an ice bath for another 10 minutes and the flask’s contents were
stirred/agitated with a glass rod to facilitate crystallization. A significant amount of
additional solid formed during this process
The solids were then collected using a Hirsch funnel and vacuum filtration. The
collected solids were allowed to remain on the funnel and air was drawn through the
solids to air dry the material. (During this drying process, an additional quantity of white
solid precipitated in the filtrate in the filtration flask. This newly precipitated material
was not pursued and was eventually discarded.)
After the collected solids were dry and free flowing, they were weighed to provide 1.04 g
of the product as a white crystalline solid, mp 64 – 66 °C. The chosen melting point
apparatus indicated that no melting point correction was needed (the apparatus reads
“actual” values).
For percent yield calculation and melting point discussion, see the conclusions section.
An infrared spectrum (attach spectrum to the write-up that you hand in to be graded) of
the product was obtained and the characteristic carbonyl absorption (1670 cm-1) for
benzophenone starting material was absent indicating that the reduction to the alcohol
product (strong OH absorption bands present at 3300-3400 cm-1) was complete.
Conclusions
(May be completed outside of lab)
This reaction of benzophenone with sodium borohydride to form diphenylmethanol is an
example of a sodium borohydride reduction of a ketone to an alcohol. The reaction was
relatively easy to run and provided a fairly good yield (81%) of the desired product.
The reaction appeared to completely convert most, if not all, of the benzophenone to the desired
diphenylmethanol. This conclusion is based on:
1. The absence of a carbonyl absorption (observed at ~1720 cm-1 in the benzophenone
starting material) and the appearance of strong O-H absorption bands at ~3300 – 3400
cm-1 in the infrared spectrum of the purified product.
2. The appearance of an aliphatic (sp3) C-H absorption band at ~2900 cm-1 in the infrared
spectrum of the product.
3. The melting point (64 – 66° C) of the obtained product compares favorably with the
literature mp value (65 - 67° C) of the expected diphenylmethanol.
According to the above equation for the borohydride reduction of benzophenone, there is an
excess of borohydride in this reaction; consequently, benzophenone is the limiting reagent (LR)
and it is used to calculate the theoretical and actual % yields as follows:
Page 4 of 5
CHM 2202
Organic Chemistry Lab II
Spring 2014
Department of Chemistry
Villanova University
EXAMPLE OF LABORATORY WRITE-UP
Conclusions (continued)
1.28 g A x 1 mole A
x
182.22 g A
4 moles C x
4 moles A
184.24 g C
1 mole C
= 1.28 g C (Theoretical
Yield)
Actual % yield = 1.04 g product obtained x 100 = 81% yield
1.28 g (Ther. Yield)
While the yield of desired product was relatively good in this experiment, the yield could have
possibly been increased by the addition of slightly more water to force more of the product out of
solution during the crystallization procedure; however, this may have caused more impurities to
co-precipitate with the product and decreased its purity slightly. Overall, the yield and purity of
product from this experiment is completely satisfactory for a reaction of this type.
TMBare (8/2011) – revised DLZubris (1/2013)
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