Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
MODULE 3: Organic Chemistry … Now I See the Light
(Mar 14 - Apr 9)
Introduction
The reactions you will perform over the next three weeks may seem like an unusual
clustering of reactions, but they all have one thing in common – their unique relationship to light.
Collectively, these experiments will demonstrate protection from hυ, synthetic uses of hυ, and
emission of hυ. In the first experiment, you will perform an Aldol reaction in order to synthesize
dibenzalacetone, a compound that absorbs UV radiation and was once used as an active
ingredient in some sunscreen formulations. In the second experiment (which is actually started
during the first week), you will prepare a thin solid sample of cinnamic acid, which will be left
out for two weeks in the natural sunlight so that a solventless [2+2] photochemical dimerization
can take place. In the second week, working in pairs, you will make two different derivatives of
the cinnamic acid photodimer. The physical properties of the derivatives formed will be used to
determine the original stereochemistry of the product. In the third week, you will perform a
chemiluminescence experiment – one that actually converts chemical energy into light energy.
In this experiment, you really will see the light, and not just in the proverbial sense!
Part A: Protection from Light: Synthesis of Dibenzalacetone1
Although the sun is necessary for life, too much sun exposure can lead to adverse heath
effects including skin cancer. More than 1 million people in the US are diagnosed with skin
cancer each year, making it the most common form of cancer in the country. Sunscreen alone
may not be effective in 100% protection from skin cancer, but when used properly, it can
certainly help protect human skin from some of the sun’s damaging UV radiation.
There are usually a variety of active ingredients in commercial sunscreens.2 The
inorganic ingredients – titanium dioxide and zinc oxide - reflect, scatter and absorb both UVA
and UVB rays and typically do not cause allergic reactions. From a cosmetic standpoint these
have not been the most ideal compounds, (i.e. painted white noses) but newer technologies have
allowed for a reduction in the particle sizes of these compounds, making them more transparent.
Most broad spectrum sunscreens also contain a variety of organic ingredients that absorb
UVA and UVB radiation. Many contain UVA-absorbing avobenzone or a benzophenone (such
as dioxybenzone, oxybenzone, or sulisobenzone) in addition to UVB-absorbing ingredients.
Sunscreens also typically contain alcohol, fragrances or preservatives. You may notice that
many sunscreens are marketed as “PABA-free” – which means they do not contain paminobenzoic acid. This is because while PABA is a good absorber of UVB radiation, PABAbased sunscreens have been known to cause a number of skin reactions (e.g. acne, itching, rash,
redness, swelling etc.).
You will synthesize dibenzalacetone via a base-catalyzed Aldol condensation reaction
between acetone and benzaldehyde (For more information on this type of reaction, see Wade
Chapter 22). Base-catalyzed condensations involve carbanion intermediates that are formed via
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
abstraction of an α-hydrogen and stabilized by resonance (keto-enol tautomerism). The
carbanion can then react with electropositive carbon atoms, such as those in carbonyl groups, to
create new carbon-carbon bond.
When the pH is considerably acidic or basic, water is
spontaneously eliminated to yield an α,β-unsaturated ketone. This dehydration occurs readily
when it leads to a product with a highly conjugated system.
O
O
H3C
OH
CH3
Ph
OH
H
O
Ph
OH
O
H
Ph
Ph
Ph
Figure 1. Synthesis of dibenzalacetone.
Procedure
Prepare the cinnamic acid samples (part 1) for the next photochemistry experiment. Make sure
the solvent has evaporated completely before the test tube is covered. The TA will place them on
the roof.
1.
2.
3.
4.
5.
Place 2.5 mL (density is 1.05g/mL; 2.6g) of benzaldehyde, 0.8 (density is 0.87g/mL;
0.7g) of acetone, and 20mL of ethanol in a 125mL conical flask.
Add 25 mL of 10% sodium hydroxide solution, stopper with a cork and swirl the flask for
at least 15 minutes. The solution will initially be clear, but should quickly become
cloudy as the yellow crystals of the product precipitate.
Collect the product by vacuum filtration, using about 30 mL of water to transfer and
thoroughly was the crystals. Press the solid product against the filter paper with the glass
stopper to remove as much water as possible.
Turn off the suction and add a solution of 1 mL of glacial acetic acid in 20 mL of 95%
ethanol. Allow the crystals to soak for a minute or so, reapply the vacuum and drain off
as much liquid as possible.
Recrystallize the product from 20 mL of ethanol, determine the melting point and
calculate the percent yield.
CLEANUP:
Organic waste should be disposed in the bottles in the back hood.
PRE-LAB QUESTIONS:
1. Why it is important that the cinnamic acid samples be completely free of solvent before they
are covered and placed on the roof?
2. Why is the suction turned off before the glacial acetic acid is added?
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Part B: Synthetic Uses of Light: Photochemical Reactions3
A number of cycloaddition reactions have proven particularly useful in organic synthesis.
In such reactions, multiple new bonds are formed in a single step, and the products form often
reflect a high degree of stereochemical control in the transition state. Cycloaddition reactions
may be classified according to the number of electrons contained in the bonds being broken in
each of the reacting partners. For example, the Diels-Alder reaction is referred to as a [4+2]
cycloaddition, since the diene contains four π electrons, while the dienophile contains two. In
this reaction two π bonds are broken and three sigma bonds are formed.
Cycloaddition reactions between two 2-π electron species do not occur under thermal
conditions because the phases of the HOMO and LUMO orbitals that must overlap to form the
new bonds are mismatched. However, [2+2] cycloadditions are symmetry allowed under
photochemical reaction conditions. When a photon of the correct energy strikes an alkene, one
of the π electrons from the HOMO is excited(*) into the next higher antibonding molecular
orbital (what used to be the LUMO). What used to be the LUMO in the ground state is now the
same as the HOMO* in the excited state. Now the phases of the excited LUMO state (containing
1 electron) and the HOMO of the other alkene (containing 2 electrons) are matched and the
cyclization reaction can occur.
In this experiment, you will carry out a “simple” photochemical [2+2] cycloaddition
reaction – the dimerization of cinnamic acid (trans-C6H5-CH=CH-CO2H). The reaction
conditions you will use here are a bit different from all of the other experiments you have
performed in that this time the reaction will be done in the complete absence of solvent. (“Green
Chemistry” at its finest!) If one were to attempt to do this reaction in solution, there would be a
large number of different cyclobutane isomers formed, each with a different stereochemistry
and/or regiochemistry. “Head-to-head” dimers yield truxinic acids, while “head to tail” dimers
result in truxillic acids. Both types of acids are natural products found in the leaves of the coca
plant (from which cocaine is derived) grown near Truxillo, Peru. Amazingly, if the reaction is
done in the crystalline solid state without solvent, the reaction yields just a single stereochemical
product.4 Why is this? In solution, cinnamic acid molecules are free to move around and can
potentially dimerize when the molecules are in a number of different orientations. Cinnanic acid
molecules in a crystal are trapped in a single specific orientation and cannot freely move around.
A [2+2] reaction can only occur in the solid when the alkene bonds of neighboring molecules are
close enough together and in precisely the right geometry to react. The stereochemistry of the
final product therefore is determined by the alignment of molecules in the crystal.
The structure of the cinnamic acid crystals you will prepare is intentionally not provided.
The objective is to work backwards to determine the molecular alignment in the crystals by
determining to stereo- and regiochemistry of the dimmer product. The [2+2] dimerization will
(hopefully) be complete after two weeks of photolyzing the solid in the sun. In the next lab
period, you will pair up with a partner to make a total of two derivatives - a dimethyl ester and an
anhydride. Based on the physical characteristics of the derivatives and the data in Table 1, you
should be able to figure out which isomer corresponds to the original photodimer.
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Table 1. Physical characteristics of some truxinic and truxillic acid isomers and their derivatives.
ISOMER
mp acid (˚C)
mp anhydride
(˚C)
mp dimethyl
ester (˚C)
1
2
3
4
5
6
7
8
9
10
11
286
285
266
245
239
228
210
209
196
192
175
(cannot form)
(cannot form)
287
N/A
150
191
116
(cannot form)
(cannot form)
N/A
(cannot form)
174
112
104
133
116
127
76
127
199
64
77
# H-NMR peaks
from (CH3O-)
observed in Ester
1
2
1
1
2
1
1
2
1
1
1
Procedure
Part 1: Cinnamic Acid Dimerization (everyone – 1st week)
O
OH
HO2C
hv
Ph
Ph
Ph
CO2H
CO2H
Ph
CO2H
truxillic acids
truxinic acids
Figure 2. Possible products of cinnamic acid photodimerization.
1.
2.
3.
4.
5.
Weigh out 1.5 g of trans-cinnamic acid (C6H5CHCHCO2H) and transfer it to a wide test
tube.
Dissolve the acid in approximately 2 mL of THF by heating in a water bath.
Remove from the steam bath, and while the solution is still hot, rotate the test tube to coat
the walls with the crystallizing acid. If a very uneven coating is obtained, reheat to
redissolve the acid and repeat the process.
When the residue is sufficiently dry, clamp the test tube in an inverted position for 30
minutes or more to permit all of the solvent vapors to evaporate out.
Stopper or Parafilm the test tube, label it clearly with your name and section number and
the TA will place it up on the roof for the next two weeks.
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Part 2a: Fisher Esterification of the Cinnamic Acid Dimer (half of the class – 2nd week)
HO2C
H3CO2C
Ph
Ph
H+
Ph
CO2H
Ph
CO2H
CH3OH
Ph
CO2CH3
Ph
CO2CH3
Ph
CO2CH3
H+
Ph
1.
2.
3.
4.
5.
6.
CO2H
CH3OH
Figure 3. Esterification of truxillic and truxinic acids.
.
Transfer the acid from the photoreaction to a 25 mL RBF and add 10 mL of methanol,
2-3 drops of concentrated sulfuric acid, and a boiling stone. Reflux the mixture gently on
a steam bath for 1 hour.
Cool the solution, add 25 mL each of water and diethyl ether, shake well and separate the
aqueous layer.
Wash the ether layer with NaHCO3 solution, the dry it over magnesium sulfate, and
gravity filter.
Concentrate the solution to a small volume, then transfer it to a small Erlenmeyer flask
and evaporate to dryness.
Recrystallize the residue from a minimum amount of hexane or methanol. Collect the
crystallized diester and report the yield and melting point.
Determine the stereochemistry of the original photodimer based on its melting point, and
the melting points of its dimethyl ester (your product) and the anhydride (your partner’s
product).
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Part 2b: Epimerization and Anhydride Formation (half of the class – 2nd week)
O
Ph
HO2C
O
O
Ac2O
Ac2O
HO2C
Ph
NaOAc
CO2H
Ph
Ph
Ph
cis
Ph
CO2H
trans
O
Ph
CO2H
Ac2O
Ph
H
O
CO2H
Ph
cis
1.
2.
3.
4.
5.
6.
7.
8.
9.
H
CO2H
Ph
CO2H
Ac2O
NaOAc
Ph
Ph
O
trans
Figure 4. Epimerization of cinnamic acid dimer.
Transfer the acid from the photoreaction to a 10 mL flask/test tube with 0.3 g of sodium
acetate and 2 mL of acetic anhydride.
Reflux the mixture gently with a heating mantle or sand bath for 10 minutes.
Add 1 mL of water dropwise to the warm mixture. After the exothermic hydrolysis of
the excess acetic anhydride is complete, add another 5 mL of water.
Transfer the mixture to a small separatory funnel, and rinse the flask/test tube with 20 mL
of water and 10 mL of methylene chloride. Shake well and separate the layers.
After washing the organic layer with NaHCO3 solution, and drying over magnesium
sulfate, gravity filter the solution.
Concentrate the solution to approximately 2 mL on the steam bath (use a boiling stone).
Add ethanol to the refluxing solution (2-4 mL) until the crystals begin to form. Cool and
collect the anhydride. Rinse the crystals with 2-3mL ethanol and air dry. A second crop
may be obtained by concentrating the filtrate.
Weigh the product and determine the yield and melting point.
Determine the stereochemistry of the original photodimer based on its melting point, and
the melting points of its dimethyl ester (your partner’s product) and the anhydride (your
product).
CLEANUP:
All organic waste should be disposed in the bottles in the back hood.
PRE-LAB QUESTIONS:
1. When extracting the diester product in part 2a, is the product in the aqueous layer or the ether
layer? Which layer is on top?
2. When extracting the anhydride product in part 2b, is your product in the aqueous layer or the
methylene chloride layer? Which layer is on top?
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Part C: Generating Light - A Chemiluminescence Experiment5
Chemiluminescence is the process whereby light is produced by a chemical reaction with
the evolution of little or no heat. The periodic flashes of the male firefly in quest of a mate and
the glow of light seen in the wake of a boat, under a rotten log, or among the many organisms
found at great depth in the ocean are examples of natural chemiluminescence. The interaction of
luciferin from the firefly, the enzyme luciferase, adenosine triphosphate (ATP), and molecular
oxygen is the most carefully studied of these reactions. Another common chemiluminescent
material is cyalume. The “light sticks,” which are sold at carnivals and in toy stores contain
solutions of cyalume. This material is also used as emergency lights on life vests, as roadside
markers, on fishing lures, and even in specialty golf balls! Bending the light stick activates it (by
mixing hydrogen peroxide with the cyalume) to give off a bright glow in a variety of colors.
Allowing the synthetic cyalume to interact with a variety of fluorescers (also included in the
stick) produces different colors.
In this experiment, the chemiluminescent material luminol will be synthesized. Luminol
(4) is not a commercial product, but its synthesis is relatively easy. Thermal dehydration of a
mixture of 3-nitrophthalic acid (1) and hydrazine (2) yields a nitro derivative,
nitronaphthalhydrazide, (3) which can subsequently be reduced to luminol (4) in high yield. An
earlier procedure for luminol synthesis called for addition of hydrazine sulfate to an alkaline
solution of the acid, evaporation to dryness, and baking the resulting mixture of the hydrazine
salt and sodium sulfate at 165oC. While effective, this method required a total of 4.5h for
completion. Louis Fieser reduced this working time drastically by adding high-boiling
triethylene glycol (bp 290oC) to an aqueous solution of the hydrazine salt, distilling the excess
water, and raising the temperature to a point where dehydration to (3) is complete within a few
minutes. Though insoluble in dilute acid, (3) is soluble in alkali solution by virtue of enolization,
and can be conveniently reduced to luminol (4) in alkaline solutions of sodium hydrosulfite
(a.k.a. sodium dithionate).
When an alkaline solution of (4) is allowed to react with a mixture of hydrogen peroxide
and potassium ferricyanide, a strong emission of blue-green light results. The dianion (5) is
oxidized to the singlet excited state (two unpaired electrons of opposite spin) of the amino
phthalate ion. This slowly undergoes intersystem crossing to the triplet excited state (two
unpaired electrons of identical spin), which decays to the ground state ion with the emission of
one photon (a quantum of light) per molecule. In dilute, weakly acidic or neutral solution,
luminol exists largely as the dipolar ion, which exhibits beautiful blue color. An alkaline
solution contains the doubly enolized anion and displays particularly marked chemiluminescence
when oxidized with a combination of hydrogen peroxide and potassium ferricyanide.
NO2
NO2
COOH
COOH
(1)
NH2 ∆
NH2
(2)
NH2
O
N
N
(3)
H
N
Na2S2O4
N
H
O
Figure 5. Synthesis of luminol.
7
O
(4)
O
H
H
Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Procedure
Part 1: Synthesis of Luminol (everyone)
1.
Heat a flask containing 15mL of water on the steam bath.
2.
Using a sand bath, heat a mixture of 1g of 3-nitrophthalic acid (1) and 2 mL of an 8%
aqueous solution of hydrazine (2) in a 20 X 20-mm test tube with side tube until the solid
is dissolved.
3.
Add 3 mL of triethylene glycol, and clamp the tube in a vertical position in a hot sand
bath. Carefully insert a thermometer into the hole in a rubber stopper (TA will
demonstrate proper technique). Add a boiling chip to the solution, attach the rubber
stopper to the top of the test tube, and connect an aspirator tube to the aspirator. Boil the
solution vigorously to distill the excess water (110-130oC).
4.
Let the temperature rise rapidly until (3-4 min) it reaches 215oC. Remove the burner,
note the time, and by intermittent gentle heating maintain a temperature of 215-220oC for
~2 min.
5.
Remove the tube, cool to about 100oC (crystals of the product sometimes appear), add 15
mL hot water, cool under the tap, and collect the light yellow granular intermediate nitro
compound (3). Dry weight ~ 0.7g. (The reason for adding hot water and then cooling
rather than adding cold water is that the solid is then obtained in more easily filterable
form.) The nitro compound need not be dried and can be transferred back to the
uncleaned test tube in which it was prepared.
6.
Add 5 mL of 3 M sodium hydroxide solution, and stir with a stirring rod. When the
resulting solution turns deep brown-red in color, add 3g of fresh sodium hydrosulfite.
7.
Wash the solid down the walls with a little water. Heat to the boiling point, stir, and keep
the mixture hot for 5 min, during which time some of the reduction product may separate.
8.
Add 2mL of acetic acid, cool under the tap, and stir; collect the resulting precipitate of
light yellow luminol (4). IMPORTANT: If your test tube does not say “Pyrex” on it, cool
it first to room temperature before cooling it under the tap.
Part 2: The Light Producing Reaction (everyone)
NH2
O
NH2
N
N
O
H
2 OH
H
O
NH2
O
N
N
N
N
(5) O
etc.
O
H2O2, K3Fe(CN)6
NH2
COO-
triplet
state*
ISC
(slow)
singlet
state*
COO-
+ hv
Figure 6. Light emitting reaction.
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
1.
2.
3.
Prepare stock solution (A) by dissolving the first crop of moist luminol (dry weight about
40-60 mg) in 2 mL of 3 M NaOH solution and 18 mL of water.
Prepare stock solution (B) by mixing 4 mL of 3% aqueous potassium ferricyanide, 4 mL
of 3% hydrogen peroxide, and 32 mL of water.
Now dilute 5 mL of solution A with 35 mL of water, and in a dark place (the IR room),
pour this solution and solution B simultaneously into an Erlenmeyer flask. Swirl the
flask and see the light. To increase the brilliance you can gradually add further small
quantities of luminol and ferricyanide crystals. (This reaction can be run on a scale five
times larger if students wish to pool samples.)
SAFETY INFO:
Hydrazine is a carcinogen – handle with care. Wear gloves and carry out this experiment in the
hood.
CLEANUP:
Combine the filtrate from the first and second reactions and put into the luminol waste container.
[TAs: the waste can be neutralized by diluting with a few milliliters of water and adding sodium
carbonate and 40mL of household bleach (5.25% sodium hypochlorite solution). The mixture
can be heated to 50oC for 1 hr. This will oxidize any unreacted hydrazine and hydrosulfite.
Dilute the mixture and flush it down the drain.]
PRE-LAB QUESTIONS:
1. Why it the ethylene glycol heated in a sand bath instead of a water bath?
2. Why is it important to cool non-Pyrex test tubes in stages (e.g. first to room temperature then
under cold tap water) instead of just rapidly reducing the temperature?
Lab Report
Short writing assignments (typed)
1.
For the syntheses performed in parts I - III, write up succinct experimental procedures for
the syntheses you performed as though they were experimental sections of journal articles you
plan to submit for publication. (400 words max)
2.
Write a summary of the lab work you performed in part II as though it were an abstract of
a journal article you plan to submit for publication. It should be succinct (250 words max), but
should address all of the most important concepts and conclusions drawn from the experiments.
3.
If there were an extra week in the semester to perform a related experiment, what
experimental question would you be most interested in asking? (there is a pretty broad range of
questions one might ask for this one given the range of experiments performed) Briefly describe
an experiment(s) that could be performed to address the question you asked. (250 words max)
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Chem 118 Organic Lab II (Spring 2005)
Prof. Swift
Questions (handwritten or typed)
1.
Draw a step-by step (arrow pushing) mechanism for the synthesis of dibenzalacetone.
2.
Write out the structures of all isomers of the truxinic and truxillic acids, showing the
stereochemistry. Don’t confuse geometric isomers and enantiomers – write only one
enantiomer of each acid.
3.
Which of the acids in question 2 can form cyclic anhydrides with acetic anhydride alone
(i.e. no base added)? Which of the acids in question 2 give identical methoxyl groups in
their H-NMR spectra?
4.
Provide a plausible chemically-based explanation for how cyalume light sticks can come
in different colors.
1
Adapted from Organic Chemistry Laboratory Manual 2nd Ed. by P. Svoronos, E. Sarlo and R.J.
Kulawiec (McGraw Hill, New York) 1997.
2
United States Environmental Protection Agency, “Sun Screens: The Burning Facts” EPA430-F-01015, May 2001.
3
Adapted from Organic Chemistry Laboratory Standard & Microscale Experiments 2nd Ed. by C.E.
Bell, A.K. Clark, D.F. Taber and O.R. Rodig (Saunders, New York) 1997
4
Did you really think I would give you the primary reference so you could just look it up?
5
Adapted from Macroscale and Microscale Organic Experiments 3rd Ed. by K.L. Williamson
(Houghton Mifflin, NY) 1999.
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