CHem 2211L Lab 8- Preparation of Diphenylacetylene PDF

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Professor: Richard Hubbard...

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Aspen King TA: David Fischler CHEM 2211L March 25, 2019 Preparation of Diphenylacetylene Introduction: The purpose of this week’s lab is to form a carbon-carbon triple bond using two separate reactions. The first reaction involves the halogenation of an alkene, and the second reaction involves dehydrohalogenation of an alkyl halide. An alkene halogenation typically involves the use of bromine as a reagent. However, bromine is highly toxic and corrosive, so this particular lab will substitute bromine with pyridinium hydrobromide perbromide (in glacial acetic acid), which is much safer. The second step will use potassium hydrobromide (in ethylene glycol) and heat to double dehydrohalogenate the intermediate (stilbene dibromide) formed in step one. This will create the final product, diphenylacetylene. Reflux via a condense will be used to react the stilbene dibromide intermediate crystals with KOH in ethylene glycol. Recrystallization and suction filtration will be used to isolate and dry both the intermediate and final product crystals. Melting point determination will be used to confirm identities of each product. The formation of carbon-carbon triple bonds is important in the production of plastics. It can also be important in creating the fuel for an acetylene torch, which cuts and welds steel. Diphenylacetylene, in particular, is used as a building block in organic synthesis.

Balanced Equation:

Reaction Mechanism:

Table of Reagents: Compound Name

Trans-Stilbene

Structure

MW

MP

BP

Densit

(g/mol

(C)

(C)

y

) 180.25

122

----

(g/mL) ----

125 Glacial Acetic

60.06

----

118.1 1.05

319.82

101

----

----

Acid Pyridinium Hydrobromide

-

Perbromide

103

Sodium Bisulfite

104.06

150

----

----

Methanol

32.04

----

64.7

0.792

Stilbene

340.06

239

----

----

Dibromide

241

Potassium

56.11

360

----

----

62.07

----

197.

1.11

Hydroxide

Ethylene Glycol

3 78.4

Ethanol

46.07

----

Diphenylacetylen

178.24

62.5 ----

-------

e

Safety Information: General safety guidelines should be followed, as well as individual chemical safety. Use caution around glassware, chemicals, and toxic/caustic liquids. Users should avoid contact or inhalation of all hazardous or toxic chemicals, wear appropriate protective wear (gloves, lab coat, safety glasses), as well as avoid sparks, flames, or hot surfaces, especially when handling flammable substances. Trans-stilbene is acutely and chronically toxic, non-flammable, and harmful if swallowed. It can also cause severe eye irritation. Glacial acetic acid is non-toxic, but flammable and caustic. It can cause serious eye damage and skin corrosion. Pyridinium hydrobromide perbromide is non-toxic, not flammable, but is caustic and can cause severe skin burns and eye damage. Sodium bisulfite is acutely toxic, not flammable, harmful if swallowed, and can cause serious eye damage. Methanol is acutely toxic, flammable, harmful if swallowed, and can cause organ damage. Stilbene dibromide is acutely toxic, not flammable, harmful if swallowed, and can cause severe eye damage and skin corrosion. Potassium hydroxide is acutely toxic, not flammable, harmful if swallowed, and can cause severe eye damage and skin corrosion. Ethylene glycol is acutely toxic, not flammable, harmful if swallowed, and can cause serious organ damage. Ethanol is not toxic, but highly flammable, and can cause serious eye irritation. Diphenylacetylene is not toxic, hazardous, or flammable, though precaution should be taken and contact should be avoided. Experimental Procedure:

Step one: 

Add 2.0 grams of trans-stilbene and 40 mL of glacial acetic acid to a 125 mL Erlenmeyer flask.



Place flask on a hotplate and gently heat to dissolve the trans-stilbene into the solvent.



Once the trans-stilbene is fully dissolved, slowly add 4.0 grams of pyridinium hydrobromide perbromide to the flask and swirl contents.



Gently heat on the hotplate for approximately five more minutes.



Cool the flask in an ice water bath and collect the solids via suction filtration.



Wash the collected crystals with 10-12 mL of methanol.



Dry the crystals through the Buchner funnel for approximately five minutes.



Air dry crystals on a watch glass for an additional ten minutes.

Step two: 

Weigh out 1.5 g of potassium hydroxide (KOH) in a 100 mL round bottom flask.



Add 20 mL of ethylene glycol to the flask and place it in a heating mantle.



Gently warm and swirl the mixture until all KOH dissolves.



Slowly add all of the dibromostilbene crystals, along with several boiling chips to the flask.



Attach a reflux condenser and reflux the solution for twenty minutes.



Once the reflux is finished, pour the hot solution into a 250 mL Erlenmeyer flask.



Allow the flask to cool to room temperature for ten minutes, and cool in an ice bath for an additional five minutes.



Collect product via suction filtration and use 5-10 mL of cold water to rinse flask and wash crystals.



Transfer the crystals to a 100 mL beaker and add 10-12 mL of warm ethanol.



Once the crystals have dissolved, add approximately 0.5-1 mL of water to the beaker drop-wise until the solution becomes cloudy.



Allow the solution to cool and collect the crystals via suction filtration.



Air dry the product, weigh it, and determine a melting point....