Download Quantitative Microscale Hydrogenation of Vegetable Oils

In the Laboratory
edited by
The Microscale Laboratory
R. David Crouch
Dickinson College
Carlisle, PA 17013-2896
Quantitative Microscale Hydrogenation of Vegetable Oils
W
Daniel E. Blanchard
Department of Physical Sciences, Kutztown University of Pennsylvania, Kutztown, PA 19530; [email protected]
A major challenge in teaching organic chemistry is that
the students are primarily nonchemistry majors who see no
connection between chemistry and any other facet of their
lives. Student interest, and success, can be increased by showing how organic concepts relate to everyday life. The laboratory experiment presented in this article is intended to
connect a standard organic reaction, catalytic hydrogenation,
to issues of nutrition and health.
Using vegetable oil as a starting material provides an
opportunity for the discussion of many related topics. Presenting the structure of fatty acids could be a start to a discussion about unsaturation and the relation between melting
range and the way molecules pack together. The relative
health benefits of saturated and unsaturated fats and the toxicity of trans-fatty acids are topics that allow a connection
between the structure of a compound and its biochemical
role. These topics all relate to the use of “partially hydrogenated vegetable oils” that are found in most baked goods sold
today. This experiment provides a valuable opportunity to
look at a common consumer product through the lens of organic chemistry.
Most modern hydrogenation experiments are carried out
on commercially available compounds to yield a single
isolable product (1–4). Instead of using hydrogenation as a
synthetic reaction this experiment endeavors to measure the
efficiency of hydrogenation using vegetable oil. While examples exist for preparing partially hydrogenated vegetable
oils (5), the opportunity to employ stoichiometry and the
ideal gas law in making the hydrogenation quantitative has
not been explored. This is an occasion to show how basic
concepts from general chemistry can be used in studying organic reactions. In addition, the apparatus used in this experiment is simpler than those employed elsewhere for
quantitative hydrogenation (3, 4).
Procedure
All reagents were obtained from commercial sources and
used as received. Vegetable oils (canola, corn, olive, peanut,
safflower, and sunflower oils) and balloons were purchased
from a supermarket. All of the glassware used was standard
laboratory equipment. All of the oils listed underwent hydrogenation using the following procedure and gave comparable results.
A 250-mL beaker filled with water was heated on a stirrer hot plate to maintain a steady temperature between 40
and 60 ⬚C. The apparatus was assembled as shown in Figure
1 except for attaching the 5-mL conical vial.
To the vial was added 0.10 g of vegetable oil, 3.0 mL of
isopropanol (solvent), and a spin vane. A small amount of
544
10% palladium-on-carbon (approximately 15 mg) was added
to the conical vial and the vial was secured to the take-off
adapter. The vial was then clamped above (not in) the water
bath.
A 20-mL syringe fitted with a small gauge needle was
used to remove air from the system. The septum on the takeoff adapter was pierced and enough air was removed from
the system to draw water up into the graduated pipet to the
0.0-mL mark (pipet mostly filled with water). The syringe
was then removed. The constancy of the water level demonstrated that the system was leakproof. The system was then
charged with hydrogen using a balloon filled with hydrogen
secured to a hose barb-to-luer lock adapter with a stopcock
and small gauge needle attached (Figure 2).
The needle of the balloon assembly was pushed through
the septum on the take-off adapter and the stopcock was
opened. The flow of gas was used to push all the water out
of the pipet and was continued for an additional 20 seconds
to ensure excess hydrogen for the reaction was in the system.
The balloon assembly was then removed. Using the syringe
some hydrogen was then removed from the system to bring
the water level up to the 22.0-mL mark (pipet nearly empty
of water). The conical vial was lowered into the water bath
and the stirrer was turned on to a moderate speed.
septum
vacuum
take-off
adapter
Tygon tubing
25-mL
graduated
pipet
5-mL conical vial
spin vane
250-mL beaker
with water
stirrer/hotplate
150-mL beaker
with water
Figure 1. Quantitative microscale hydrogenation apparatus
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
In the Laboratory
The water level in the pipet was measured every five minutes while the reaction was stirred and heated. When the uptake of hydrogen had slowed to less than 0.2 mL per
five-minute period (usually within two hours) a final reading was made and the conical vial was removed from the water bath. The reaction mixture was disposed of in an
appropriately labeled container.
of the acids as triglycerides. The number of moles of each
fatty acid can then be calculated and converted to the number of moles of carbon–carbon double bonds in the original
sample. From this amount a theoretical volume of hydrogen
consumed can be calculated using the ideal gas law. By comparing the actual to the theoretical a percent efficiency can
be calculated.
Hazards
Discussion
Hydrogen is a flammable gas and forms explosive mixtures with air. Palladium-on-carbon is an extremely flammable
solid. Sparks and open flames should not be allowed in the
laboratory during this experiment. The risk of fire is kept
low as the amount of hydrogen required for a 16-student
laboratory section is only what is needed to fill two mediumsized balloons. Also, the catalyst is not weighed out in order
to minimize its spread and contact with air. No incidents of
fire have occurred in carrying out this experiment at the
author’s institution.
This experiment has been successfully run for the last
four years by three instructors. Student results range, depending on the vegetable oil used, from 5 to 15 mL of hydrogen
consumed. Calculated efficiencies are typically 100–120%.
Hydrogen appears to be adsorbed by the catalyst or lost
through the Tygon tubing or the septum. The loss is typically in the range of 2 to 3 mL. This could be taken into
account by having students work in pairs with each pair running both a “real” hydrogenation and a control with everything except the oil. While this and fluctuating ambient lab
temperatures adversely affect accuracy, they also provide an
opportunity to talk about experimental design.
In conclusion, a new hydrogenation experiment for the
introductory organic chemistry laboratory is described. It
provides an opportunity to discuss fats and fatty acids, their
physical behavior, and their nutritional value. The ideal gas
law is used to determine the efficiency of the hydrogenation.
While results are routinely above theoretical expectations, a
way of correcting for this is proposed. With this experiment
links are made between the organic chemistry laboratory and
the local fast food restaurant or the dormitory vending machine.
Data Analysis
The goal of this experiment is to calculate the efficiency
of the reaction by comparing the actual amount of hydrogen
consumed with a theoretical value calculated from the fatty
acid composition recorded on the nutrition labeling. The label identifies the number of grams of saturated, monounsaturated, and polyunsaturated fats in a serving size of the oil.
Calculations can then be made of the mass of each type of
fatty acid in the sample, the number of moles of each, the
number of moles of carbon–carbon double bonds, and finally the volume of hydrogen necessary to reduce all the
double bonds. While the calculations are somewhat lengthy,
most students can accomplish them if the tasks are described
stepwise. Sample calculations and an Excel spreadsheet that
could be provided to students are included in the lab documentation for this article.W
Percent composition of each type of fatty acid can be
calculated from the nutrition label and then used to determine the mass of each kind of fatty acid in the sample. The
major monounsaturated fatty acid in all of the vegetable oils
listed above is oleic acid (6, 7, 8). With the exception of
canola oil, the major polyunsaturated fatty acid is linoleic
acid.1 By adding 12.68 g兾mol to the molar masses for each
of the free fatty acids one can compensate for the presence
Acknowledgments
The author would like to thank Edward Vitz for the
original inspiration and Thomas A. Betts and Beth A. Kroa
for their help in refining this experiment.
W
Supplemental Material
Instructions for the students, including a prelab assignment and lab write-up, and notes for the instructor, including sample calculations, are available in this issue of JCE
Online.
Note
1. Canola oil is approximately 58% oleic acid, 26% linoleic
acid, and 10% linolenic acid. Since most nutrition labels do not
differentiate polyunsaturated fatty acids it is necessary to use these
percentages when calculating hydrogen consumption by canola oil.
Literature Cited
Figure 2. Hydrogen balloon assembly for charging hydrogenation
apparatus.
1. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Microscale Approach, 3rd ed.; Harcourt Brace: Fort Worth, 1999; pp 244–248.
2. Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry:
A Miniscale and Microscale Approach, 2nd ed.; Harcourt Brace:
Fort Worth, 1998; pp 456–461.
JChemEd.chem.wisc.edu • Vol. 80 No. 5 May 2003 • Journal of Chemical Education
545
In the Laboratory
3. Williamson, K. L. Macroscale and Microscale Organic Experiments, 3rd ed.; Houghton Mifflin: Boston, 1999; pp 334–337.
4. Landgrebe, J. A. J. Chem. Educ. 1995, 72, A220.
5. Schoffstall, A. M.; Gaddis, B. A.; Druelinger, M. L. Microscale and Miniscale Organic Chemistry Laboratory Experiments;
McGraw-Hill: Boston, 2000; pp 225–229.
6. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G.
546
Introduction to Organic Laboratory Techniques: A Microscale
Approach, 3rd ed.; Harcourt Brace: Fort Worth, 1999; p 241.
7. Ruchi Infotech Limited. e-Sunrich.com-Shopping online for
sunflower oil. http://www.ruchihealth.com/esunrich/product.htm
(accessed Jan 2003).
8. Fazio Foods International Ltd. http://www.faziofoods.com/htm/
frame5.htm (accessed Jan 2003).
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu