Mount Holyoke Organic chemistry Mechanism Of The Wittig Reaction Lab Report Organic chemistry lab: Do pre lab and use the photos and given data do post lab. …. Experiment 9
Spring 2020
Experiment 9 —
Wittig Reaction
_____________________________________________________________________________
Pre-lab preparation: Read this document carefully. This reaction is covered briefly in
Ch 9 of your text (pp 237-8).
(1) Briefly state the purpose of this experiment. (2) Stand in front of a mirror and
pronounce the following words: Wittig, ylide, zwitterion, 9-(2-phenylethenyl)anthracene. (No
written answer is required here, but you may be asked to stand in front of the class and recite
these words in lab.) (3) Write the mechanism of the Wittig reaction that you will do, including
specific structures and reagents. Don’t just copy the generic mechanism in the introduction
below. Start with deprotonation of the benzyltriphenylphosphonium salt with the base you will
be using (not nBuLi!) (4) (a) Calculate the quantities of benzyltriphenylphosphonium chloride
and of 9-anthraldehyde that you will need to measure out, and (b) calculate the theoretical yield
of the product. (5) (a) What is the molarity of “50% aq. NaOH”? (b) How many mmol of 50%
aq. NaOH will you use? (c) What is the mol ratio of NaOH to your limiting reagent? (6) (a)
What byproducts and (b) what excess reagents need to be removed after the reaction is
complete? (7) At the end of the reaction, you dilute the mixture with water and CH2Cl2, so the
aqueous layer will then have a density similar to pure water. (a) Which layer is on top and which
is on the bottom at this point? (b) What’s dissolved in each layer? (8) (a) Why is it important to
wear goggles when visualizing a TLC plate with 254-nm light? (b) Why is it a bad idea to tape a
TLC plate into your notebook? (9) What’s the bp of isopropyl alcohol, and why does it matter?
And, as always, write a concise procedure and work from that, not from this document.
You’ll need to rearrange a few things in the procedure section of this document. And if you don’t
remember how to recrystallize, review your previous procedures before lab.
The Wittig (das ist Deutsch; it’s pronounced “VIT-ik”) reaction is one of the classic
methods for making new CC bonds. It was discovered by the German chemist Georg (GAY-org)
Wittig in 1954, and it earned him a share of the Chemistry Nobel prize 25 years later. This is a
reaction (re-AK-shun) that you may not have studied yet in lecture (LECT-shoor); the overall
transformation is fairly simple and is shown below.
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Experiment 9
Spring 2020
In this reaction a carbonyl (carPh3P
bo-NEEEL) compound reacts with the
“Wittig reagent”, which is also referred
+
O
to as a phosphonium ylide (ILL-id). An
ylide is a compound that has adjacent
Ph3P
ketone or
aldehyde
positive and negative charges on two all-
“Wittig reagent”
(an ylide)
octet atoms, and this term obviously
refers just to the second of the two resonance structures shown. The Wittig reagent is more often
written as the uncharged structure (note the 5-bond P), although its reactivity is best understood
in terms of the ylide (+ –) resonance form. Of course the actual molecule is somewhere between
these two structural extremes.
The reaction begins with a nucleophilic addition of the partially negative carbon of the
Wittig reagent to the carbonyl group, as illustrated below. The result of this step is a zwitterion
(tsvIT-er-I-on), an overall neutral compound that has separate + and – charges. This intermediate
is thought to snap shut to
form a 4-membered ring (a
Ph3P
PPh3
Ph3P
phosphaoxetane), and this
strained intermediate quickly
O
O
O
expels triphenylphosphine
oxide to make the final
alkene product.
+
PPh3
O
PPh3
O
You’ll learn in class that phosphonium ylides are usually prepared by reaction of an alkyl halide with
triphenylphosphine (Ph3P:). The resulting phosphonium salt is then deprotonated with a strong base like
butyllithium (byoo-til-LITH-ee-um). However, we’re skipping this step (awwwww) and starting with a
commercially available phosphonium salt.
And, your phosphonium salt can be
PPh3
nBu–Li
deprotonated with a much milder base,
PPh3
PPh3
Br
R
R
R
HO–, so we won’t need to use the superBr
basic (and air- and water-sensitive) nBuLi.
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Experiment 9
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We’ll be using the Wittig reaction to make 9-(2-phenylethenyl)anthracene, starting with
9-anthraldehyde and benzyltriphenylphosphonium chloride. The latter will
be deprotonated with hydroxide to produce the Wittig reagent, and that will
react with the aldehyde carbonyl. (Notice the historically quirky
numbering of the anthracene
O
8
7
9
1
2
6
5
10
4
PPh3 Cl
3
that assigns the central Cs as
9 and 10.)
Light absorption and emission. Later in the course you may learn about light absorption
and emission by organic molecules. You already know that when a molecule absorbs a photon
of light, an electron is promoted from a lower energy orbital to a higher energy orbital. The
energy difference between the two orbitals determines the energy of the photon that can be
absorbed. The smaller the orbital energy gap, the lower the energy of the photon needed to cause
the electron to hop into the higher energy orbital.
In most cases, the electronic transition responsible for a molecule’s color and
photochemical reactivity is the one between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied MO (LUMO). In organic molecules, one important factor that affects the
HOMO-LUMO energy gap is conjugation of p-bonds. In general, if we compare otherwise
similar structures, the more p-bonds in conjgation, the smaller the HOMO-LUMO gap, and the
lower the energy of the light that the molecule can absorb.
This trend is seen in many series of structurally related organic molecules. One such
series is the linear acenes, the first few of which are below.
tetracene
470
orange
benzene naphthalene anthracene
315
360
abs λ: 255
—— c o l o r l e s s ——
E:
112
91
79
61
3
pentacene
595 nm
purple
48 kcal/mol
Experiment 9
Spring 2020
We can see light that has a wavelength (l) between about 400 and 700 nm. Most small
organic molecules absorb only in the ultraviolet (UV) region, below 400 nm, and they appear
colorless under white light. The first three acenes (above), for example,
absorb only UV light. Benzene is a colorless liquid; naphthalene and
anthracene are white (i.e. colorless) solids. As the length of the
conjugated p-system increases, the HOMO-LUMO energy gap decreases,
and the absorption wavelength gets longer. Tetracene’s absorption,
around 470 nm, is in the blue region of the spectrum, so under white light,
it appears orange. Pentacene, an organic semiconductor that has been
used in solar cells and organic electronics, has its strongest absorption in
the yellow-orange region (595 nm), so it looks purple-blue.
White light is a
combination of all
visible wavelengths.
Blue is absobed by the
compound, and the other
wavelengths reach our
eyes. Those remaining
wavelengths are centered
in the orange part of the
spectrum, so the
compound appears
orange. In general, the
color we see is the
complement of the color
absorbed.
Your starting material, 9-anthraldehyde, has a carbonyl group conjugated with an
anthracene unit, so it absorbs light at a longer l than anthracene itself (the lone pairs on the O
have something to do with that as well). Anthracene is colorless because its absorption is
entirely within the UV region (360 nm), although it is close to the visible range. 9Anthraldehyde is yellow. This means it must be absorbing purple light, somewhere between 400
and 450 nm. The product of your Wittig reaction also has more extensive conjugation than
anthracene, so you should not be surprised if it turns out to be visibly colored as well.
Some molecules will also emit light. Light emission is accompanied by an electron
dropping from a higher- to a lower-energy molecular orbital (the opposite of absorption). The
simplest (and fastest) type of emission is fluorescence. Anthracene, for example, is fluorescent.
It absorbs light in the near-UV and it then emits light in the purple region of the spectrum, so it
appears to “glow” purple when irradiated with UV light. Of course, in keeping with The Laws
Of Physics, a molecule can’t produce more energy in the form of emission than it absorbs, so
fluorescence must always occur at a longer l than absorption.
It’s difficult to predict whether a molecule will be fluorescent or not. Rigid molecules
like the acenes tend to be emissive; more flexible molecules can often find ways to wiggle
around conformationally and dissipate their absorbed energy as heat rather than light. Will your
product fluoresce? You’ll have to make it and find out.
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Experiment 9
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Procedure. Overview — We first need to make the 9-(2-phenylethenyl)anthracene
(Anth–CH=CH–Ph) by linking the Anth–CH=O to Ph3P=CH–Ph (the Wittig reagent that’s
generated in situ when the base deprotonates the phosphonium salt). We’ll fish out and clean up
the product by extraction, then recrystallize it.
Running the reaction. Dissolve 1.16 mmol of 9-anthraldehyde in about 6 mL of CH2Cl2,
then add a 6% excess of benzyltriphenylphosphonium chloride (did that dissolve? Should you
make a note of that?). Carefully add 600 µL of 50% aqueous NaOH, with stirring. (“50%” in
this case is weight-to-volume, i.e., 0.50 g of NaOH per 1 mL of solution. This is the conversion
factor that you need to covert from volume to mass, not the density value that Google spits out
— that’s for solid NaOH. Or you can use the molarity you calculated to go directly from volume
of solution to mmol of NaOH.)
Safety issues. Dichloromethane is toxic and a suspected carcinogen. The aldehyde and
phosphonium salt are toxic irritants. Wear gloves and work in your hood. The extremely
concentrated NaOH solution is very caustic. In addition to handling this with gloves, it is
imperative that you not dribble this solution all over the outside of the bottle and the
surrounding benchtop — immediately clean up any drips, dribbles, drools, spills, splashes, or
splorts so that you or others don’t accidentally come in contact with the NaOH!
Notice that the aqueous layer isn’t where you probably expected it to be — water is less
dense than CH2Cl2, right? But this aqueous solution contains such a high concentration of Na+
and HO– ions that it’s actually more dense than CH2Cl2. That’s just plain weird.
Now, the base is in the aqueous layer needs to react with the stuff dissolved in the organic
layer, so we need good contact between the two liquid phases. In other words, this mixture
needs to be stirred vigorously. Don’t go crazy, but make sure that your magnetic stirrer churns
the two layers together so the reagents can find each other. Vigorous stirring is generally more
effective in a small r.b. flask than an Erlenmeyer.
How do we know when it’s done? Hmmmm… That’s an important question for just
about every reaction. We can often monitor reactions by NMR, IR, GC, TLC or other methods.
TLC would be easy, so let’s try that. We’ll need to be able to see reactants disappear and/or
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Experiment 9
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products appear. If we just run a TLC during the reaction, we may see a bunch of spots, but we’ll
have no idea who’s who. We obviously need to start with a TLC of the reactants alone. The
easiest way to do that is to sample a bit of the solution in the reaction flask as we add reagents.
So as you start setting up, first, spot the solution of anthraldehyde in CH2Cl2, then spot
the solution again after you’ve added the phosphonium salt. After you get the reaction going,
take samples after a total of 2, 5, and 15 min. To get a TLC sample, pause the stirring to allow
the phases to separate, then dip a capillary spotter directly into the CH2Cl2 layer. (But we want
the reaction to continue as we make our TLC spots, eh? So get that stirrer going again!) Line up
all the spots on one TLC plate. (Two pre-rxn spots plus the ones during the rxn… how many
spots in all? Plan ahead so you can space them out evenly on one TLC plate.) Then develop the
plate with 5% Et2O/hexane, and visualize the spots by using a 254-nm UV lamp. Try 365 nm as
well. Can you see one or both reactants? Any new spots that might be the product? Is anything
changing with time? Maybe it’s done, or maybe it needs to run longer. You may need to run
another TLC or two to figure out when the reaction is complete. Don’t start the work-up until the
reaction is finished!
TLC refresher. A TLC reminder sheet will be posted in the lab — a quick summary: (a)
forceps not fingers (oink). (b) Spot once, tiny tiny spot, (c) let solvent climb up half way; mark
solvent front. (You can turn the plate upside down and use the clean half for a second TLC!) (d)
254-nm light is extremely bad for your eyes, as is 365-nm, so keep your goggles on! (e)
Circle/trace everything, then copy the plate directly and accurately into your notebook, noting
relative intensities of spots, colors, how you saw them, etc. (f) Save your plates until the end of
the period, then dump them in the TLC plate waste bottle, not in the trash! (g) Silica dust is toxic
once it leaves the TLC plate, so don’t rub or scrape the plates, and don’t put them in your
notebook, your backpack, or your mouth. And (h) keep the TLC plate away from your work area
so you don’t drip, dribble, splash, or spray anything onto it. Don’t sneeze on it either. Ick.
Work-up. After you have TLC results that indicate that the reaction is over, transfer the
mixture into your sep funnel — use about 10 mL of water and 10 mL of CH2Cl2 to rinse all the
goodies out of the flask and into the sep funnel. Separate the layers, then extract the aqueous
layer with a few more mL of CH2Cl2 (why?). Wash the combined CH2Cl2 solution with 2 x 10
mL of water (why?), pre-dry it with brine (sat’d NaCl; why?), and dry it over MgSO4. Filter by
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Experiment 9
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gravity (plug of cotton in your powder funnel), and rinse rinse rinse so you don’t leave half of
your product entrained in the glob of MgSO4. (Rinse with what? Think about what you’re
filtering and what you’re trying to accomplish!) Use the rotavap to remove the solvent. How
will you know when it’s done? Is your product a liquid or a solid? If you’re not sure, keep
reading, and be sure you have that in your procedure! Determine the mass. This is the “crude”
yield. Again, think ahead — how will you determine the mass of stuff in the flask?
Recrystallize the crude material from isopropyl alcohol (aka, iPrOH, 2-propanol,
“rubbing alcohol”). You’ll probably need about 10 mL per gram of crude product. (Hint: you
didn’t make a gram, did you? So scale down accordingly.) Start with about 2/3 of the
recommended amount of solvent, then add more if necessary. Remember how to recrystallize?
Get the stuff into solution, get it back out of solution. When it’s in solution you have an
opportunity to get rid of insoluble junk; as it comes out of solution by crystallizing slowly, most
of the soluble junk will stay dissolved. The product is much more soluble in hot iPrOH than in
cold. Isolate the solid in the usual way; the residual iPrOH should evaporate quickly. (If you
need a refresher on recrystallization, review your procedure from a few weeks ago). Measure the
yield of your purified solid. (We’re going to skip the mp this time and do TLC instead.)
Analysis. TLC can give us an idea of what we accomplished by working up the reaction
and then recrystallizing the product. Run TLCs of the material before and after recrystallization,
side-by-side on the same plate. Also spot a sample of the filtrate from which you isolated your
final crystals. (let’s travel back in time… you will have planned ahead by preparing a TLC plate
and grabbing a tiny bit of that sample before recrystallizing it all… it needed to be in solution to
get into the capillary and onto the plate… so when it was in solution after the work-up but before
the recrystallization, you took a wee bit of that solution and spotted it… and you remembered to
do this because you had made a note in your procedure. Right? … and now we return to the
present…) Did the TLC show that anything changed?
Finally, place your sample in one of the vials provided. The sample must be labelled with
the initials of each group member, the date, and the complete and correct structure of the
compound. Put it in the usual location (sample storage shelf). You’ll have some fun with it later.
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Experiment 9
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Also, clean that reaction flask thoroughly with water! Don’t leave conc. NaOH in there!
Over time, the conc NaOH will etch the glass.
Discussion. (1) (a) Compare what you did with the procedure in this write-up, and note
any significant errors or deviations. (2) (a) At what point in the procedure do you think the
excess phosphonium salt was removed? (b) When was the excess NaOH removed? (c) When
was the NaCl byproduct removed? (d) Were there any other byproducts? When do you think
they were removed? Is there TLC evidence to support your answer?
(3) (a) At about what Rfs did you find the reactants and product(s) when you ran TLC?
Indicate clearly which compound is which. Did any other compounds show up? (b) Based on
your TLC data, approximately how long did the reaction take, and what key bit of evidence
allowed you to reach that conclusion? (c) Based on the TLC data, were any compounds removed
in the extraction? Were any compounds removed in the recrystallization?
(4) (a) The compound you made is structurally similar to trans-stilbene, Ph–CH=CH–Ph.
But trans-stilbene is a white solid, and the compound you made in this experiment is colored.
What’s the structural reason for the difference in color? (b) Suppose you used a Wittig rxn
reaction to link two anthracenes with an etheno unit to make Anth–CH=CH–Anth. Would you
expect that compound to absorb light at a longer or shorter l than the compound you made?
(5) EC (a) The anthracene group in your product came from an aldehyde, and the phenyl
group came from the Wittig reagent, which you made by deprotonating a phosphonium salt. In
principle, you could have make the same product the other way around — with an anthracenecontaining Wittig and a Ph-containing aldehyde. Write that reaction (not the mechanism, just the
overall rxn), starting from the appropriate aldehyde and phosphonium salt (assuming we could
have bought that). (b) The Management recently ran a Wittig reaction analogous to yours to
make 9,10-bis(2-phenylethenyl)anthracene. Write the phosphonium salt and the di-aldehyde
reactants that were used to make that compound. (c) Your compound is yellow. What color do
you think the compound referred to in b is? (pick one) colorless, yellow, orange.
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Data to Calculate Yields
These are the numbers you need to calculate the crude and re-crystallized product yields.
Mass of crude product: 0.843 g
Mass of recrystallized product: 0.181 g
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