EXPERIMENT 4
THE WITTIG REACTION.
SYNTHESIS OF TRANS-9-(2-PHENETHYLETHENYL)ANTHRACENE AND
CHEMILUMINESCENT REACTION
ITS USE IN A
Reading Assignment: Smith Sections 21.10, 21.4A, 16.6, 16.15
Pre-lab Questions, Week 1:
1. Prepare a flow diagram showing the procedure for isolation and purification of
the product. Where does the triphenylphosphine oxide byproduct end up?
2. Show how the starting phosphonium reagent could be prepared.
3. Sodium hydroxide is a very mild base for deprotonation of the phosphonium
salt. For methyl triphenylphosphonium chloride, n-butyllithium is the typical
base of choice. Why is possible to use NaOH here? (To state it another way,
why is this phosphonium salt so acidic?)
The Wittig reaction is a reliable and general synthetic method for the conversion of a
ketone or aldehyde into an alkene. The active reagent in this reaction is a phosphonium ylid,
which, because of its instability, is generated in situ (in the presence of the carbonyl compound
that it will react with.) In this experiment, you will generate the phosponium ylid from a
phosphonium salt using concentrated sodium hydroxide as the base. When the ylid is generated
in the presence of 9-anthraldehyde, trans-9-(2-phenylethenyl)anthracene is formed.
CHO
NaOH
P
H
P
C
C
H
Cl
trans-9-(2-Phenylethenyl)anthracene
While the details of the reaction mechanism are dependent on several variables, it is
accepted that the carbanion attacks the carbonyl group to yield an alkoxide-phosphonium
intermediate that collapses to the four-membered oxaphosphetane ring. This four membered ring
is unstable and spontaneously undergoes fragmentation to give the alkene and
triphenylphosphine oxide.
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Ph
Ph P
Ph
R
R'CHO
Ph
Ph P
Ph
Ph Ph
Ph P
O
R
R'
R
R
Ph3P O +
R'
R'
O
phosphonium
ylid
oxaphosphetane
intermediate
trans-9-(2-Phenylethenyl)anthracene produces a green fluorescence when combined with
the chemiluminescent compound Cyalume. Chemiluminescence is the process in which light is
produced by a chemical reaction. Most exothermic reactions produce energy in the form of heat,
but a few produce light and release little or no heat. Chemiluminescence is what causes fireflies
to glow and light sticks to light. Chemiluminescent processes occur when the energy given off
in a chemical reaction produces molecules in an electronically excited state. As these molecules
drop down to the ground state, the light emitted is visible. Many such reactions occur in nature
where they are referred to a biolumenescence. Lightsticks contain a solution of Cyalume and a
fluorescer. Bending the light stick activates it (by breaking a small glass sealed tube filled with
hydrogen peroxide, which mixes with the Cyalume/fluorescer solution) to give off a bright glow
in a variety of colors. The different colors are created by using different fluorescers. The
observed color depends on the structure of the fluorescer.
In the light producing reaction, the ester of Cyalume reacts with hydrogen peroxide to
produce a peroxyoxalic acid. Intramolecular displacement generates a small intermediate
molecule which has been suggested to have the structure, A (this small molecule remains elusive,
and has not yet been detected by IR, CNMR, or MS.) A then transfers its energy to the
fluorescer, which is excited to the singlet excited state. When the excited state relaxes back
down to the ground state, it releases light.
Cl
Cl
O O
Cl
OC CO
Cl
Cl
Cl
H2O2
Cl
O O
OC COOH
Cl
Cl
Bs(2,4,6-trichlorophenyl)oxalate
Cyalume
O
Cl
O
+ 2 Cl
OH
O O
A
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Cl
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O
O
O O
H
C
C
H
+
charge transfer complex
A
trans-9-(2-Phenylethenyl)anthracene
2-CO2 + trans-9-(2-Phenylethenyl)anthracene*
(singlet excited state)
hn + trans-9-(2-Phenylethenyl)anthracene
(ground state)
Safety Precautions:
Sodium hydroxide is extremely damaging to the eyes and skin. Keep organophosphorous
compounds off the skin. Avoid skin contact with DMF. It can go through gloves, carrying with
it residues of other compounds that are on the gloves onto your skin, where it is readily absorbed.
You must wear gloves and eye protection at all times.
Part A: Synthesis of trans-9-(2-phenylethenyl)anthracene:
To a 10 mL round bottom flask equipped with a small stir bar, add benzyl
triphenylphosphonium chloride (200 mg, 0.514 mmol), 9-anthraldehyde (115 mg, 0.558 mmol),
and N,N-dimethylformamide (DMF) (0.5 mL). While vigorously stirring the solution, carefully
add 50% NaOH (50% w/w) (0.25 mL) dropwise. If necessary, rinse any solid of the sides of the
walls with additional DMF (about 5 drops). Continue the vigorous stirring for 30 minutes. At
the end of the stirring period, add 4 mL of a 1:1 mixture of 1-propanol/H2O to precipitate the
product. Collect the crude product by vacuum filtration, and TLC using 8:2 Hex:Ethyl Acetate.
The product should be bright fluorescent blue, and the starting material, if present, yellow when
irradiated with long-wave radiation.
Transfer to a 10 mL Erlenmeyer and recrystallize using about 4 mL 1-propanol. The
product crystallizes as thin yellow plates, mp 131 –132°C.
Calculate the yield of the purified trans-9-(2-phenylethenyl)anthracene and obtain a
melting point and UV spectrum. A typical concentration for the UV spectrum is 0.050 g/L.
In your laboratory writeup, include and interpret the UV spectra for trans-9-(2phenylethenyl)anthracene.
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Part B: Chemiluminescent Experiment
In a scintillation vial, dissolve 50 mg Cyalume [bis(2,4,6-trichloro)oxalate] and 3mg of
the trans-9-(2-phenylethenyl)anthracene synthesized in Part A in 5 mL diethylphthalate by
warming in a hot water bath. In anther scintillation vial, mix 0.2 mL of 30% hydrogen peroxide
with 5 mL diethyl phthalate, cap, and shake to mix. Add the peroxide suspension dropwise to
the Cyalume solution in a darkened hood. Warm the solution and then cool the solution to see
the effect that this has on the rate of reaction. If additional trans-9-(2-phenylethenyl)anthracene
is available, add 2 mg more along with 0.2 mL additional peroxide. What is the effect of adding
the additional fluorescer and peroxide? Submit the remaining trans-9-(2-phenylethenyl)anthracene in a scintillation vial, and the capped vial with your chemilumenescent reaction
mixture to your TA.
References:
1. The procedure for Part A adapted from: C. Jaworek, S. Iacobucci. J. Chem Ed. 2002,
79(1), 111.
2. The procedure for Part B is adapted from: Kenneth L. Williamson, Organic Experiments,
9th Ed. Houghton Mifflin Company, 2004, p. 558-563.
Post-lab Questions:
1. Who received the Noble Prize for the discovery of the Wittig reaction?
2. The Wittig reaction usually gives a mixture of cis and trans isomers, yet in this
reaction, only the trans isomer is formed. The stereochemistry of product is set
in the first step of the reaction of the ylid with the aldehyde. Draw the
intermediate oxaphosphetane that leads to the trans-alkene geometry, and the
one that would lead to the cis - alkene geometry, showing the stereochemistry at
all of the chiral centers. Can you suggest a reason for the fact that only the trans
alkene is formed?
3. trans-9-(2-phenylethenyl)anthracene can be easily distinguished from cis-9-(2phenylethenyl)anthracene by looking at the HNMR. Indicate how you could tell
the difference between the two by HNMR.
4. It is often possible to make an alkene from two different combinations of a
carbonyl group and a phosphonium salt. What is the other combination of
carbonyl compound and phosphonium ylide that could be used to make the
product in this reaction?
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