CHEM2211 Experiment 8 Preparation of Diphenylacetylene Lab Report PDF

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Introduction The objective of this experiment is to examine the formation of a carbon-carbon triple bond, or alkyne, using the halogenation of an alkene and the dehydrohalogenation of an alkyl halide. Double bonds can be broken by the addition of two halogens because they are polarizable. Dehydrohalogenation involves the addition of a strong base to synthesize an alkyne. Alkene halogenations typically use bromine as a reagent, but in its liquid phase, it is highly toxic and corrosive. In this lab, pyridinium hydrobromide perbromide (PHPB) will be substituted for Br2. It delivers a single equivalent of bromine. The second step of the reaction involves the double dehydrohalogenation of the dibrominated intermediate (stilbene bromide) formed as a result of step one. This double elimination reaction will be achieved using potassium hydroxide and will create the final product, diphenylacetylene. The techniques that will be used during this experiment include refluxing the solution to prevent solvent loss, recrystallization and suction filtration to collect and dry the solid products, and melting point determination to confirm the identities. Balanced Equation

N H

Br3

Br

KOH Br

Trans-Stilbene

(meso)-Stilbene Dibromide

heat Diphenylacetylene

Reaction Mechanism

Br

Br Br H

H

Br

H

OH

H

Br

H

Br

Br

OH

Table of Reagents for Step 1: Compound Name

Structure

Molecular Weight (g/mol) 180.25

Trans-Stilbene

Glacial Acetic Acid

Melting Point (°C) 122°C 125°C

60.05

O

Boiling Point (°C)

Density (g/mL)

117°C

1.05

64.7°C

0.791

OH Pyridinium Hydrobromide Perbromide

Br-

O

128°C 136°C

134.1

200°C

Na+

S O-

HO

Methanol

319.8

Br-

NH+

Sodium Bisulfite

Br-

OH

32.04

Table of Reagents for Step 2: Compound Name

Structure

Stilbene Dibromide

Br

Molecular Weight (g/mol) 340.1

Melting Point (°C) 235°C 241°C

56.11

360°C

Boiling Point (°C)

Density (g/mL)

Br

Potassium Hydroxide Ethylene Glycol

K

O

H

62.07

HO

196°C - 1.113 198°C 78°C

OH

Ethanol

46.07 OH

Diphenylacetylene

178.2

58°C 62°C

Safety Information Safety glasses and lab gloves must be worn at all times. Trans-stilbene is harmful if swallowed and causes serious eye irritation. Ethanol and glacial acetic acid are flammable and should be kept away from open flames, sparks, and hot surfaces. Ethylene glycol and methanol are highly toxic and should not be ingested. Methanol can cause organ damage. Position and operate the snorkels properly to avoid breathing any fumes. Avoid allowing any liquid to contact the skin. Pyridinium hydrobromide perbromide, glacial acetic acid, and stilbene dibromide are caustic and will cause severe burns if they contact the eyes or skin. Any vapor or liquid exposure should be reported immediately. Dispose of all liquid and solid waste in their appropriate containers in the fume hood.

Procedure: Step 1 • Add 2.0 grams of trans-stilbene and 40 mL of glacial acetic acid to a 125 mL Erlenmeyer flask • Place the flask on a hot plate and gently heat • Once the trans-stilbene has been fully dissolved into the solvent, slowly add 4.0 grams of pyridinium hydrobromide perbromide to the flask. Carefully swirl the contents • Continue to gently heat the flask for approximately 5 more minutes • Once the heating is complete and product crystals have formed, cool the flask in an icewater bath • Collect the solid product via suction filtration • Wash the collected crystals with 10-12 mL of methanol to remove any soluble impurities • Dry the crystals by drawing air through the Buchner funnel for 5 minutes • Air dry the crystals on a watch glass for an additional 10 minutes. During this time, prepare for the second step of the reaction • Weigh the dibromostilbene crystals prior to adding them to the reaction flask in step 2 • The filtrate from the product filtration must be diluted with water. Add an appropriate amount of sodium bisulfite to the aqueous solution to remove the orange color • Once the solution has been approved, the filtrate may be discarded in the lab sink Procedure: Step 2 • Weigh out 1.5 grams of potassium hydroxide 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 • Once all of the KOH has dissolved into solution, slowly add all of the dibromostilnene crystals from step 1 to the flask • Add several boiling chips and attach a reflux condenser to the apparatus. Reflux the solution for 20 minutes • When the reflux is finished, pour the hot solution into a 250 mL Erylenmeyer flask and allow it to cool • After 10 minutes, cool the solution in an ice-water bath for an additional 5 minutes • Once cooling is complete, collect the impure product via suction filtration • Transfer the product crystals into a 100 mL beaker and dissolve them in 10-12 mL of warm ethanol. Keep the solution warm using a hotplate • Once the impure crystals have completely dissolved in the ethanol, add 1 mL of water to the beaker dropwise until the solution becomes cloudy • Allow the solution to slowly cool to room temperature and observe the gradual formation of the pure diphenylacetylene crystals • Once the crystal formation is complete, collect the solid product via suction filtration • Air dry the product, weigh it, and determine a melting point

Data and Observations The trans-stilbene used in step one was a fine white powder. The glacial acetic acid was a clear solution with a pungent odor. It took about 10-15 minutes to dissolve the trans-stilbene in the acetic acid. The pyridinium hydrobromide perbromide was a vibrant red, chunky solid. When it was added to the dissolved solution, the clear color turned to orange. As the flask was gently heated for 5 minutes, a yellow solid began forming at the bottom. When the solid product was collected during suction filtration, the crystals were soft and white, and the impurities were caught as a yellow solution in the flask. Some of the crystals were lost while transferring to a watch glass. The KOH used in step two was a white solid. The ethylene glycol was a clear liquid. The solution was refluxed for 20 minutes, and as it cooled, the solution became cloudy light brown in color. While transferring the solid to the Buchner funnel, some crystals were lost in the beaker. When the impure product was collected, the crystals were white and yellow. After the addition of ethanol, the solution was a light yellow. There was immediate crystal formation with the addition of water. These crystals were a fine white, with minimal yellow impurities. Results 1.98 grams of trans-stilbene and 4.10 grams of PHPB were dissolved in 40.0 mL of glacial acetic acid to form 3.29 grams of stilbene dibromide intermediate. 1.51 grams of potassium hydroxide was dissolved in 20 mL of ethylene glycol, then the intermediate crystals were added. 12 mL of ethanol was used to crystallize the solution and produce 1.59 grams of diphenylacetylene. The limiting reagent of the first step was trans-stilbene. 1.98%𝑔%𝑡𝑟𝑎𝑛𝑠 − 𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒% ×

1%𝑚𝑜𝑙 = 0.0110%𝑚𝑜𝑙%𝑡𝑟𝑎𝑛𝑠 − 𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒 180.25%𝑔

4.10%𝑔%𝑃𝐻𝑃𝐵% ×

1%𝑚𝑜𝑙 = 0.0128%𝑚𝑜𝑙%𝑃𝐻𝑃𝐵 319.8%𝑔

Trans-stilbene is the limiting reagent because less moles of it were available. The moles of stilbene bromide is equal to the moles of trans-stilbene since it’s produced in a 1:1 mole ratio. The theoretical yield of stilbene dibromide is 3.74 grams. 0.0110%𝑚𝑜𝑙%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒%𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒 ×

340.1%𝑔 = 3.74%𝑔𝑟𝑎𝑚𝑠%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒 %𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒 1%𝑚𝑜𝑙

3.29 grams of stilbene dibromide were recovered in the experiment, so the percent yield is 88%. 3.29%𝑔%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒%𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒 × 100 = 87.99% 3.74%𝑔%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒%𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒

The limiting reagent of the second step was stilbene dibromide. 3.29%𝑔%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒%𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒% ×

1%𝑚𝑜𝑙 = 0.00967%𝑚𝑜𝑙%𝑠𝑡𝑖𝑙𝑏𝑒𝑛𝑒%𝑑𝑖𝑏𝑟𝑜𝑚𝑖𝑑𝑒 340.1%𝑔

1.51%𝑔%𝐾𝑂𝐻% ×

1%𝑚𝑜𝑙 = 0.0269%𝑚𝑜𝑙 %𝐾𝑂𝐻 56.11%𝑔

Less moles of stilbene dibromide were available so it’s the limiting reagent. Stilbene dibromide also reacts to form diphenylacetylene in a 1:1 mole ratio so the theoretical yield of diphenylacetylene is 1.72 grams. 178.2%𝑔 = 1.72%𝑔%𝑑𝑖𝑝ℎ𝑒𝑛𝑦𝑙𝑎𝑐𝑒𝑡𝑦𝑙𝑒𝑛𝑒 0.00967%𝑚𝑜𝑙%𝑑𝑖𝑝ℎ𝑒𝑛𝑦𝑙𝑎𝑐𝑒𝑡𝑦𝑙𝑒𝑛𝑒% × 1%𝑚𝑜𝑙 1.59 grams of diphenylacetylene were recovered in the experiment so the percent yield is 92%. 1.59%𝑔%𝑑𝑖𝑝ℎ𝑒𝑛𝑦𝑙𝑎𝑐𝑒𝑡𝑦𝑙𝑒𝑛𝑒 × 100 = 92% 1.72%𝑔%𝑑𝑖𝑝ℎ𝑒𝑛𝑦𝑙𝑎𝑐𝑒𝑡𝑦𝑙𝑒𝑛𝑒 The melting point range of the purified diphenylacetylene was 60-61.5°C. Discussion/Conclusions The melting point range obtained during the experiment was in the same range as known value for diphenylacetylene, which is 58-62°C. Any melting point depression could be due to impurities from dirty glassware, incomplete reflux, or solvent still left in the product. Given that the experimental melting point is close to the actual value, and the percent yield was 92%, the final product was most likely pure. The intermediate had a relatively low percent yield, which could be attributed to the loss of crystals while transferring between glassware. Since the percent yield of diphenylacetylene was still high, any loss must’ve occurred in the first step of the experiment. If more time had been allowed for the experiment, the percent yields could be easily increased. The solution could cool in an ice bath for longer to crystallize, and filtration could be completed without any loss of product.

Post-lab Questions 1. Br (R)

PHPB (S)

Br

With the addition of pyridinium hydrobromide perbromide, there are two chiral centers in this compound. One has an S configuration, and the other is R configuration. The compound is not optically active because it has a plane of symmetry, making it a meso compound. Br Br

PHPB

Br (S)

(R) (R)

(S)

When the second compound reacts with PHPB, it produces two enantiomers, both with two chiral centers. They are both optically active since they lack planes of symmetry and therefore, aren’t meso compounds. 2. The IUPAC name for the product in the Part 1 reaction is (1R,2S)-1,2-dibromo-1,2dicyclopentylethane. The name of the first enantiomer in Part 2 is (5S,6S)-5,6-dibromo-3,8-diethyldecane, and the name of the second is (5R,6R)-5,6-dibromo-3,8-diethyldecane.

Br

3. 1. 9-BBN,THF 2. H2O2, -OH/H2O

OH

O

H

O

H2O, HgSO4, H2SO4

Li, NH3...