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Diphenylacetylene From Stilbene

U
University of Houston

Be very careful when handling these chemicals and heating apparatus. Protect yourself at all times with proper gloves, eyewear and clothing. Let me know if you have any other questions!

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Diphenylacetylene from Stilbene
Overview Addition Elimination X2 H H Ph H Br 2 - OH C C Ph Ph Ph Ph H Ph Br Br Alkene Dibromide Alkyne
Electrophilic Addition The most common applications of alkene halogenation are chlorination and bromination, which are electrophilic addition reactions. General mechanism of electrophilic addition: E+ Nu- Nu- E Nu E C=C Carbocation intermediate
Bromination of Alkenes 隆+ 隆 Br --- Br Minor Shifts in Bromonium ion intermediate formation Electron Density Anti-addition of nucleophilic Br Pyridinium tribromide (PyH+Br3) is used as a source for bromine. Br Br Bromonium Ion Br Br Br b Br Br b a Br a
Bromination of trans-Stilbene Ph H Ph Br Br Br H C C C C H Ph H a b Ph N Br - Br3- H a b rotate Ph Br Br H H Ph C C C C H Ph Br Br Ph H Br Br 1R, 2S 1S, 2R C C H H Ph Ph Meso Compounds: Identical An internal plane of symmetry
Elimination of Dibromide H H OH H Ph OH Ph Ph Ph Ph Ph Br Br Br E2 E2 Diphenylacetylene E2: Bimolecular Elimination 1-step process. Rate = k[RX][Base] Strong base takes away H, a pi bond forms, and X leaves at the same time. H and X have to be anti-coplanar.
Part A Changes Use 400 mg of (E)-Stilbene, 25-mL round bottom flask, 8 mL of glacial acetic acid, 800 mg of pyridinium tribromide (Weigh in the hood!). Reflux for at least 15 min instead of 5-10 min. Add 8 mL of distilled water instead of 4 mL. Cool in an ice bath. Collect crystals and use cold water to rinse flask. Wash crystals twice with 1 mL each of cold methanol. Suction dry as much as possible. Do NOT heat to dry. Measure both mp and mass.
Reflux Apparatus Set up the reflux apparatus same as the figure shown in the book using a long condenser with the sand bath as a heat source. (Use a thermometer to Sand bath measure temperature on top of in the sand bath. magnetic Circulate the water stirrer through the condenser. Caution: Reflux gently because the overheating may allow bromine to escape. Observe a reflux ring!!!
Part B Use 400 mg of meso-dibromide, 25-mL round bottom flask, 2 mL of triethylene glycol, 2 pellets of KOH. Heat to 185-195 尊C with sand bath (thermometer!!). When mixture turns dark (after 1-2 min), swirl, heat for 15 more min instead of 5 min. Add 12 mL of distilled water instead of 6 mL. Rinse the flask with cold distilled water. Recrystallize from 95% methanol, except do NOT heat to dry. Let crystals dry in drawer for a week. Then take mp and mass.
WARNING!!! Heating sand is very HOT! Glacial acetic acid, pyridinium tribromide and KOH are all caustic and corrisive. Use Gloves!

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Diphenylacetylene From Stilbene

  • 2. Overview Addition Elimination X2 H H Ph H Br 2 - OH C C Ph Ph Ph Ph H Ph Br Br Alkene Dibromide Alkyne
  • 3. Electrophilic Addition The most common applications of alkene halogenation are chlorination and bromination, which are electrophilic addition reactions. General mechanism of electrophilic addition: E+ Nu- Nu- E Nu E C=C Carbocation intermediate
  • 4. Bromination of Alkenes 隆+ 隆 Br --- Br Minor Shifts in Bromonium ion intermediate formation Electron Density Anti-addition of nucleophilic Br Pyridinium tribromide (PyH+Br3) is used as a source for bromine. Br Br Bromonium Ion Br Br Br b Br Br b a Br a
  • 5. Bromination of trans-Stilbene Ph H Ph Br Br Br H C C C C H Ph H a b Ph N Br - Br3- H a b rotate Ph Br Br H H Ph C C C C H Ph Br Br Ph H Br Br 1R, 2S 1S, 2R C C H H Ph Ph Meso Compounds: Identical An internal plane of symmetry
  • 6. Elimination of Dibromide H H OH H Ph OH Ph Ph Ph Ph Ph Br Br Br E2 E2 Diphenylacetylene E2: Bimolecular Elimination 1-step process. Rate = k[RX][Base] Strong base takes away H, a pi bond forms, and X leaves at the same time. H and X have to be anti-coplanar.
  • 7. Part A Changes Use 400 mg of (E)-Stilbene, 25-mL round bottom flask, 8 mL of glacial acetic acid, 800 mg of pyridinium tribromide (Weigh in the hood!). Reflux for at least 15 min instead of 5-10 min. Add 8 mL of distilled water instead of 4 mL. Cool in an ice bath. Collect crystals and use cold water to rinse flask. Wash crystals twice with 1 mL each of cold methanol. Suction dry as much as possible. Do NOT heat to dry. Measure both mp and mass.
  • 8. Reflux Apparatus Set up the reflux apparatus same as the figure shown in the book using a long condenser with the sand bath as a heat source. (Use a thermometer to Sand bath measure temperature on top of in the sand bath. magnetic Circulate the water stirrer through the condenser. Caution: Reflux gently because the overheating may allow bromine to escape. Observe a reflux ring!!!
  • 9. Part B Use 400 mg of meso-dibromide, 25-mL round bottom flask, 2 mL of triethylene glycol, 2 pellets of KOH. Heat to 185-195 尊C with sand bath (thermometer!!). When mixture turns dark (after 1-2 min), swirl, heat for 15 more min instead of 5 min. Add 12 mL of distilled water instead of 6 mL. Rinse the flask with cold distilled water. Recrystallize from 95% methanol, except do NOT heat to dry. Let crystals dry in drawer for a week. Then take mp and mass.
  • 10. WARNING!!! Heating sand is very HOT! Glacial acetic acid, pyridinium tribromide and KOH are all caustic and corrisive. Use Gloves!