Preparation of Styrene and Phenylacetylene from Polystyrene

Introduction:

Phenylacetylene is a flexible organic intermediate with a myriad of potential uses. As a terminal alkyne, it undergoes many of the characteristic reactions undergone by acetylene, but with the convenience of being an easily measured liquid. Terminal acetylenes are weakly acidic and thus, it can be deprotonated by strong bases such as NaNH2 or Grignard reagents for use a nucleophile, such as in the preparation of rubrene (1). Phenylacetylene can be hydrated using strong acids, mercury, gold, or other catalysts to give acetophenone or phenylacetaldehyde (2,3) It can be homocoupled forming diphenyldiacetylene (4) or cross coupled with other alkynes via a Cadiot-Chodkiewicz coupling (5). Phenylacetylene is also amenable to Sonogashira coupling.

Phenylacetylene has been prepared by a relatively small number of methods in the literature. Typically, either (1,2-dibromoethyl)benzene or β-bromostyrene is treated with a strong base, inducing dehydrohalogenation(s) to form phenylacetylene. This can be a solution or melt of alkali hydroxide (6,7) or a solution of sodium amide in anhydrous ammonia (8,9). Molten alkali hydroxide rapidly destroys glassware. Sodium amide is difficult to prepare for the amateur, requiring sodium metal and liquid anhydrous ammonia. The latter is especially difficult to work with, and dangerous without a fume hood. For this reason, a solution of alkali hydroxide was chosen for the production of phenylacetylene.

As an immediate precursor, (1,2-dibromoethyl)benzene can be easily prepared by simple addition of Br2 to styrene (10). β-bromostyrene can be prepared from cinnamic acid by bromination and reflux with an alkali carbonate in aqueous acetone (11). The Takai olefination utilizing CrCl2 and haloforms can also be used to make β-halostyrenes from benzaldehydes (12). As this reaction is very reagent intensive, it is not worthwhile for making unsubstituted phenylacetylene but may be useful for substituted phenylacetylenes. Due to its simplicity and high yield, the bromination of styrene was selected for this preparation.

Styrene can be prepared by a number of methods. Due to the great variety and frequent complexity of the preparations, most will not be discussed here. Cinnamic acid can be decarboxylated to yield styrene, but the preparation takes a long time and yield is poor (13). Direct conversion to β-bromostyrene as mentioned above would be a more efficient use of cinnamic acid. Styrene is present in significant amounts in easily available products such as fiberglass resins (14). This can most likely be easily vacuum distilled from the other components of the resin. Another simple route to styrene is the thermolysis of polystyrene (15). The polymer undergoes thermally induced bond homolysis and styrene distills over. Because the cost of the reagents for this last preparation is negligible, it was selected for production of styrene.

Experimental:

Styrene

A 100ml RB flask (that the experimenter is not emotionally attached to) is charged with 52.08g (0.50mol) of polystyrene. A convenient source of fairly pure and dense polymer is clear, colorless plastic cutlery. Pure polystyrene is brittle and wire cutters easily break the utensils into small pieces. Tapping may be necessary to settle the polystyrene into the flask so that it will all fit. No headspace is necessary. The flask is set up for simple distillation with an air condenser and stopper in the 3-way adapter where a thermometer adapter would usually go. A heat-proof clamp must be used to hold the flask. Do not use keck clips. The receiving flask should be 100ml in volume or larger and submerged as much as possible in an ice bath.

The distillation flask is heated directly with a flame. The polystyrene will begin to melt and a thick white fume will start distilling over. It condenses with astounding efficiency considering the high temp and lack of a water cooled condenser. With continued heating, the polystyrene will form a melt. Apply heat until nearly the entirety of the plastic has decomposed and distilled over. This will only take a few minutes. A very small amount of dark residue remains in the distillation flask and is difficult to remove. As soon as cooled, rinse all parts of the apparatus with acetone or toluene to remove styrene before it polymerizes. The distillate consists of a yellow mobile liquid. If you must stop at this stage, store the crude product in the dark and as cold as possible. A small amount of hydroquinone or other polymerization inhibitor may be added.

The crude product is purified by setting up the flask with a stirbar for simple vacuum distillation with a water aspirator or roughly equivalent vaccum source. Lower pressures will make the styrene difficult to condense efficiently. The styrene cannot be distilled at atmospheric pressure or significant losses due to polymerization will occur. The condenser should be pumped with icewater and the receiving flask (250ml, in preparation for the next step) should be immersed in an ice bath. The flask is heated with stirring in a hot water or air bath. Pure styrene distills over at 47-48°C (32torr) as a clear, colorless mobile liquid. Distillation is ceased when the stillhead temperature exceeds 50°C, yielding 34.96g of styrene (67.1% yield). If the styrene is to be stored at this point, a polymerization inhibitor must be added. It is preferable to make the styrene immediately before use.

(1,2-dibromoethyl)benzene

The styrene from the previous step is diluted with 35ml of dichloromethane and chilled in an ice bath, or preferably a freezer. A seperatory funnel or addition funnel is charged with solution consisting of 53.86g of bromine (1eq) in 35ml of dichloromethane. A stirbar is added to the styrene solution and the funnel set up over it. Bromine is dripped in rapidly with stirring at first. When the solution becomes lukewarm, the addition rate is slowed down and the reaction flask is immersed in an ice bath. Toward the end of the bromine addition, a yellow color will remain in solution. If addition is stopped, it fades. When a pale orange color that does not fade on standing remains in the reaction flask, cease addition. A small amount of bromine solution may remain. Distill or rotovap off the dichloromethane to yield crude (1,2-dibromoethyl)benzene. CAUTION: This material is a fairly potent skin sensitizer. This may be used directly in the preparation of phenylacetylene.

If purification is desired, the crude product is broken up and dissolved in 600g of 67% aqueous isopropanol. Allow to cool to room temperature, then chill in an ice bath. The mass of crystals that forms is stirred into a slurry and vaccum filtered. Air is drawn through the product to help dry it. Removal of solvent is completed by placing in a dessicator over sulfuric acid until constant weight is achieved. The product is a fluffy, pearly white powder weighing 73.17g (82.6% from styrene, 55.4% from polystyrene) and melting at 69-71°C (uncorrected).

Phenylacetylene

A 500ml, 2-neck RBF (WARNING: The flask and thermometer are superficially etched by this procedure) is charged with 250ml of glycerol. A large stirbar is added. One neck is fitted with a thermometer adapter so the bulb is fully immersed in the glycerol. Strong heating is applied to reduce the viscosity of the glycerol and stirring is begun. 40g (1mol) of NaOH is added. At roughly 125°C, the NaOH will be almost fully dissolved forming a pale yellow-tan solution. 65.99g (250mmol) of (1,2-dibromoethyl)benzene is added and the flask set up for simple distillation. Stirring must be efficient. The melted (1,2-dibromoethyl)benzene and intermediate β-bromostyrene are immiscible in the glycerol and good contact between the phases is essential. At ~144°C, the reaction mixture begins boiling vigorously from phenylacetylene distilling. Two phases collect in the receiving flask, clear droplets of water and a cloudy organic phase The temperature is increased gradually until it reaches 200°C. There should be almost no distillate coming over at this point. Heating is ceased.

The distillate is transferred to a small seperatory funnel. The phases do not separate particularly well. The receiving flask is rinsed with 20ml of diethyl ether, and this is added to the distillate. After mixing, the phases readily separate and the lower, aqueous layer is drained off. The organic phase is rinsed with 30ml of distilled water, and the slightly cloudy organic phase transferred to an Erlenmeyer flask along with a small amount of anhydrous MgSO4 to dry it. When the organic phase is clear, it is filtered through a fluted filter paper into a 100ml RBF. The Erlenmeyer is rinsed with 5ml of ether and added to the filter paper. Once the solvent has stopped passing through, the filter paper and dessicant is rinsed with another 5ml of ether to prevent loss. A chaser solvent is added to the flask. 20g of diethylene glycol was used by the author. A stirbar is added and the flask set up for vacuum fractionation. A controllable air bleed is a necessary addition.

The ether is removed first under reduced pressure. It shouldn’t condense, even at ice water temperature due to the reduced pressure. The next fraction to come over is the phenylacetylene. The air bleed should be adjusted so this fraction comes over at ~80°C. When the phenylacetylene is depleted, the stillhead temperature will rise rapidly. The vacuum should be shut off when this occurs, immediately halting the distillation. The distillate is a clear, colorless liquid weighing 15.30g; 59.9% yield from (1,2-dibromoethyl)benzene (49.5% from styrene, 33.2% from polystyrene). Unlike styrene, phenylacetylene is not terribly polymerization prone and does not need a stabilizer added. Store in the dark and cold.




References

1) User Brillantschwarz of Versuchschemie.de’s post at http://www.versuchschemie.de/topic,10433.html; based on Vogel's Textbook of Practical Organic Chemistry, Fifth Edition
2) Organic Syntheses, Vol. 83, p.55 (2006).
3) Synthesis, 2007, 1121-1150
4) Organic Syntheses, Coll. Vol. 5, p.517 (1973)
5) Synthesis, 2011, 1541-1546
6) Modified procedure from Organikum, post on Sciencemadness.org by Garage Chemist Thread ID=14935
7) Organic Syntheses, Coll. Vol. 1, p.438 (1941)
8) J. Am. Chem. Soc., 1934, 56 (10), pp 2120–2122
9) Organic Syntheses, Coll. Vol. 4, p.763 (1963)
10) Vogel, Arthur. A Text-Book of Practical Organic Chemistry, 3rd edition. Pg. 900
11) J. Chem. Educ., 1991, 68 (2), p 161
12) J. Am. Chem. Soc., 1986, 108 (23), pp 7408–7410
13) Organic Syntheses, Coll. Vol. 1, p.440 (1941)
14) www.jamestowndistributors.com/userportal/pdfs/MSDS/bondo/401... accessed 1/28/2012
15) The Chemical Educator, Vol. 8, No. 5, Published on Web 8/30/2003

[Edited on 1-29-12 by UnintentionalChaos]

Great post

I have also performed the third step (styrene dibromide/(1,2 dibromoethyl)benzene to phenylacetylene) using potassium hydroxide in methanol according to:

Fiesselmann and Sasse, Chem. Ber., 89, 1775 (1956).

Quote: Originally posted by plastics

Great post I have also performed the third step (styrene dibromide/(1,2 dibromoethyl)benzene to phenylacetylene) using potassium hydroxide in methanol according to: Fiesselmann and Sasse, Chem. Ber., 89, 1775 (1956).

Quote: Originally posted by UnintentionalChaos

Quote: Originally posted by plastics   Great post I have also performed the third step (styrene dibromide/(1,2 dibromoethyl)benzene to phenylacetylene) using potassium hydroxide in methanol according to: Fiesselmann and Sasse, Chem. Ber., 89, 1775 (1956). What did you use the phenylacetylene for? Some of my batch is destined for a small (<50mmol) prep of 9,10-bis(phenylethynyl)anthracene. I want to try out the Sonogashira coupling. Or maybe I'll attempt rubrene. Either way, it's going to glow.

Quote: Originally posted by plastics

Great post I have also performed the third step (styrene dibromide/(1,2 dibromoethyl)benzene to phenylacetylene) using potassium hydroxide in methanol according to: Fiesselmann and Sasse, Chem. Ber., 89, 1775 (1956).

What was your yield? It seems weird that people choose to carry out this reaction at 200 degrees and etching their precious flasks if it can be carried out in refluxing methanol. Seeing as I am emotionally attached to most of my flasks, this route would be desirable...

Quote: Originally posted by benzylchloride1

Nice synthesis, reminds me of some good times in my own lab several years ago. I used styrofoam as the polystyrene source and an old flat bottomed 24/40 flask for the pyrolysis reaction. I used a mixture of KOH and ethylene glycol for the dehydrohalogenation. I got a decent amount of phenylacetylene, along with one of the partial dehydrohalogenation products. I have IR spectra of these compounds at my lab, but I am at graduate school several hundred miles away currently. I think that I will have to make some more phenylacetylene since I still have about 60 mL of the styrene from the pyrolysis reaction. I am thinking about making some cobalt tetraphenylcyclobutadiene complexes and I will need some phenylacetylene to couple with bromobenzene via a Sonogashira reaction. I could also make some from trans-stilbene which I synthesized during Christmas break from benzaldehyde via a cyanide catalyzed benzoin condensation followed by a Clemmensen Reduction, following some early Organic Syntheses procedures.

Quote: Originally posted by Enkii

would it be possible to use a poly-resin; styrene + maleic anhydride? if so how would one remove the maleic anhydride? and could one possibly retain the MAA for future use?

Quote: Originally posted by subsecret

Instead of Br2/DCM, could sodium hypochlorite solution be used to give chlorine instead of bromine?

Quote: Originally posted by byko3y

I see no reason why chlorine won't work. Of course you need to acidify the hypochlorite with HCl and wait till all the chlorine moves to the top (non-polar) layer. Chlorine has solubility of 0.06 mole/L (2g/L) in HCl solution.