Synthesis of an Alkyne: Addition-Elimination

Method:

·         Add 0.099 g of pyrdinium perbromide, 0.045g of E-stilbene and 1ml of acetic acid to a test tube from the microscale kit.

·         Heat the reaction in a boiling hot water bath under the hood for 5 minutes while stirring.

·         After 5 minutes, allow the test tube to cool for two minutes at room temperature and then place in an ice-water bath to ensure crystallization.

·         Weigh the filter and filter paper, then collect the crystals using suction filtration.

·         Rinse the crystals with 2 ml of ice cold methanol, then allow the crystals to dry on the suction filtration for two minutes.

·         Weigh the filter, filter paper and crystals of meso-stilbene dibromide.

·         Preheat a sand bath by setting the variac box to 2.

·         Place 0.040g of meso-stilbene dibromide, 1 pellet of potassium hydroxide and 0.25ml of triethylene glycol into a new test tube from the microscale kit.

·         Use a thermometer to monitor the temperature while heating the tube in a sand bath until it reaches 160-170°C.

·         Allow the reaction to proceed for 5 minutes while stirring with the thermometer.

·         After 5 minutes, remove from sand bath and allow to cool until comfortable to touch.

·         Slowly add 2.5 ml of water to the mixture and stir with the thermometer while in an ice bath for 5 minutes.

·         Use suction filtration to filter crystals after reweighing the filter and filter paper.

·         Dry the crystals using 5 drops of methanol while the crystals are still on suction.

·         Weigh the filter, filter paper and crystals.

·         Collect crystals and find the melting point and melting point range to determine the purity.

Data and Results:

            Crystals of meso-stilbene dibromide = 0.0734g

            Crystals of diphenylacetylene = 0.0174g

            Melting Started = 58°C

            Melting Stopped = 60°C

Calculations:

8.8483g – 8.7749g = 0.0734g of meso-stilbene dibromide

Percent Yield

0.045 g E-stilbene (1mol/180g)(1mol meso-stilbene dibromide/ 1mol E-stilbene) = 2.5E-4 moles expected

Crystals of meso = 0.0734g (1mol/340g) = 2.258E-4 moles meso-stilbene dibromide

(2.258E-4 moles meso-stilbene dibromide / 2.5E-4 moles expected) *100 = 86.32% yield

8.7975g – 8.7801g = 0.0174g Diphenylacetylene

Percent Yield

0.040 g meso-stilbene dibromide (1mol/340g)(1mol Diphenylacetylene / 1mol meso-stilbene dibromide) = 1.176E-4 moles expected

Crystals of Diphenylacetylene = 0.0274g (1mol/178g) = 9.775E-5 moles Diphenylacetylene

(9.775E-5 moles Diphenylacetylene / 1.176E-4 moles expected) *100 = 83.12% yield

Conclusions:

For the experimental results the percent yield of meso-stilbene dibromide was 86.32%. This is a decent percent yield for the addition portion of the reaction. The percent yield may not have been higher because we had trouble removing all of the crystals from the small test tube without using too much methanol. The percent yield of diphenylacetylene was slightly lower at 83.12%.  The percent yield was not higher for the elimination portion of the reaction because we had a small spill when we tried to stir with the thermometer. We had trouble removing all of the crystals from the test tube, just as we did with the addition portion of the reaction. We concluded that our crystals were slightly impure because they had a melting range of 2°C and a melting depression of 1°C. Overall, this was a successful lab because we obtained percent yields above 80% and the final crystals were only slightly impure.