Cindy Lee
1041791
Chem 363
Synthesis of Azulene – Why Questions
1. Explain the acidity of cyclopentadiene. (p. 190)
When cyclopentadiene is deprotonated, it becomes aromatic (6 pi electrons = 4n +2). It has a few
resonance forms to delocalize the electrons, which make it stable and hence, makes deprotonation
favorable and the compound acidic.
2. Thiophene dioxide is very reactive and cannot be isolated because it undergoes Diels-Alder
dimerization. Show the products that would form if the solvent is removed. (p. 191)
3. The dehydrobromination reaction requires the use of powdered sodium hydroxide, since the
product (thiophene dioxide) is susceptible to attack by base. Show the products that would
form if aqueous base is used. (p. 191)
4. In the final step of the synthesis, fulvene 4 and diene 7 are combined. They first react by a [6+4]
cycloaddition reaction, followed by the cheletropic extrusion of SO2. Finally, to establish
aromaticity in the molecule, dimethylamine is expelled from the compound. Aromaticity is a
major driving force for this reaction, but what other phenomena favour the reaction? (p. 191)
The reaction is entropically favored as well. There are 3 products resulting, compared to 2 starting
materials, and one intermediate. Also there is gas released, SO2,which drives the reaction forward by
Le Chatelier’s principle, and contributes to the increase in entropy.
5. Why are you filling a balloon with N2(g) and placing it on your apparatus? (p. 193)
To establish an inert environment for the reaction, the nitrogen displaces the air in the apparatus,
which may contain traces of water and other gases that may interfere with the reaction.
6. Why is it necessary to dry all your glassware? What could happen if you didn’t dry your
equipment before performing the reaction? (p. 193)
The reactants, bromine and the 2,5-dihydrothiophene, and intermediates (brominium ion) may react
with any water present in the apparatus, which would increase the chance of side reactions and
decrease the yield.
7. Why is it necessary to take special safety precautions when working with bromine? Why do we
try to work with it only in the hood? (p. 193)
Bromine is a very corrosive and volatile substance. We must handle it with care and can be easily
contained within the fumehood should there be an accidental spill. We do not want any contact with
the skin and do not want to inhale its vapors.
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Cindy Lee
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8. Why is it important to add the bromine to the thiophene dioxide slowly, over 30 min? (p. 194)
We want to control the rate of reaction and maintain a gentle reflux.
9. What is the purpose of allowing the reaction to continue for another 1-2 hours after adding the
bromine? (p. 194)
To ensure the reaction goes to completion.
10. Why are you washing the product with cold CHCl3 until it is white (hint – what does the colour
imply)? (p. 194)
The chloroform washes the bromine and other impurities away. The colour (red, yellow) implies
there is bromine present, when the crystals are white, that indicates the bromine is gone.
11. How many isomers of the product will be formed? What type of isomers are they? (p. 194)
2 isomers that are enantiomers.
12. Why is cracked cyclopentadiene (the monomer) stored at -80oC? (p. 194)
Cyclopentadiene dimerizes readily at warm temperatures. Storing at cold temperatures prevents this.
13. Why must you flush your apparatus with nitrogen? (p. 195)
To establish an inert environment for the reaction, the nitrogen displaces the air in the apparatus,
which may contain traces of water and other gases that may interfere with the reaction. (Water reacts
with the dimethylformamide diethyl acetal.)
14. Why is it important to reflux for at least 3 hours? (p. 195)
To ensure the reaction goes to completion.
15. Why is it important to make sure all the joints of your apparatus are well greased? (p. 195)
Grease seals the joint of the apparatus so that gases or the solvent cannot escape and air cannot get
enter during reflux.
16. Write out the mechanism that illustrates the side product that will form if water is present in
your system. (p. 195)
17. After preparing the dimethylaminofulvene, you remove “volatiles” use rotary evaporation.
What are the volatiles that you are removing (ie: what is being rotovapped off)? If you need
help answering this question, look carefully at the mechanism of the reaction – do you recognize
any solvents that are made during the reaction? (p. 195)
Ethanol that was produced in the reaction (this is with the diethyl acetal, it would be methanol for the
dimethyl acetal).
18. Why is it important to store the fulvene in the fridge? (p. 195)
The fulvene may decompose at warmer temperatures. It is light sensitive and the fridge is dark, so it
is stored in the fridge, wrapped in aluminum foil.
19. For the dehydrobromination reaction, why is it important to use powdered KOH (as opposed to
just throwing in pellets of KOH)? (p. 196)
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Cindy Lee
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Powdered KOH has an increased surface area. Since it is used in THF and isn’t soluble in this
solvent, a greater surface area will be necessary for the KOH to react.
20. Why is it important to store the powdered KOH under an inert atmosphere? (p. 196)
KOH is hygroscopic, so if left in air, it will absorb water. (We do not want water because it reacts
with the thiophene-1,1-dioxide product.)
21. Why is benzophenone ketyl used in the THF still? What does the benzophenone ketyl do?
What do the colours indicate? (p. 196)
Benzophenone ketyl is used as an indicator of water. In the absence of water, it is blue, in the
presence of water it is white. We want anhydrous THF so we use benzophenone ketyl to show there
isn’t water present in the still.
22. Why is it important to add the powdered KOH quickly? (p. 196)
KOH must be added quickly because it is hygroscopic and can absorb moisture from the air. (We do
not want water because it interferes with our reaction
23. How will you filter the reaction mixture? (p. 196)
We used a sintered glass funnel and a celite pad. (A vacuum was used to pull the filtrate through.)
24. Why are you filtering through a celite pad? (p. 196)
Celite pads can adsorb colloidal material and prevent it from passing through the filter paper or
clogging its pores. (It is finer than filter paper and we won’t have pores getting clogged by fine
particles.)
25. Why must you flush the system with N2(g)? (p. 197)
To establish an inert environment for the reaction, the nitrogen displaces the air in the apparatus,
which may contain traces of water and other gases that may interfere with the reaction.
26. After adding your cold thiophene-1,1-dioxide solution to your 6-dimethylaminofulvene, the
mixture will become red. As the solution warms up to room temperature, it becomes yellow.
Upon reflux, the colour will start to change again – to what? (p. 197)
The colour will change to blue, which is the colour of our azulene product.
27. What does it mean to triturate? Why must you triturate your product? (p. 197)
Triturate means to crush and grind. We must do this because our product, azulene, must be in
solution with the solvent to be run on the column, and crushing it will help it dissolve.
28. What is the purpose of the alumina column? (p. 197)
For purification of our product. The column separates compounds based on different affinities for the
column.
29. Why are you only collecting the blue band? (p. 197)
Our product is blue. Anything that is not blue is not our product.
30. After recrystallizing the azulene and removing the recrystallization solvent, you are told to
wash the crystals with ice cold ethanol. What is the purpose of this step and why must the
ethanol be ice cold? (p. 197)
The azulene is soluble in warm ethanol but not in cold (assuming this because a one solvent
recrystallization method is being used).
31. Why is it important to not dry your crystals too long under vacuum? (p. 198)
Azulene is volatile and can sublime, so you would lose product if dried for too long.
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