Chemistry 351 Laboratory - Spring Semester 2007
Relevant textbook readings - Pavia, expt 38, technique 11. Smith Chap 20, especially 20.9-20.11, 20.13.
Overview - You will prepare a Grignard reagent from 1-bromo-4-fluorobenzene (eq 1) and then react it with benzophenone (Ph2C=O) to form 4-fluorotriphenylmethanol.
Procedures
We will adapt the procedures given on pp 320-324 in Pavia.
Preparation of Grignard Reagent. Carry out the procedures given on pp 320-322 simply substituting an equimolar amount of 1-bromo-4-fluorobenzene for bromobenzene. (Calculate the moles of bromobenzene called for in Pavia and use that number of moles of the fluoro compound.) The reaction apparatus to be used is exactly the same as the figure on page 321.
Reaction of Grignard with Benzophenone. The procedure is as specified on page 323 in Pavia. Do not cool the reaction until a solid forms and stirring is no longer effective...this make take longer than implied in Pavia.
Washing with Petroleum Ether. Where the book procedure says to use 3 mL of petroleum ether to wash the crude solid (midway through top paragraph on p 324) use only 2 mL and do not heat. Instead stir and manually mash with a glass rod or spatula ("triturate") the solid under the petroleum ether. Then go on to the vacuum filtration as described in the text. When you rinse the filtered solid with pet ether, only use a very small amount of ice-cold pet ether.
Crystallization. We will need to do small scale tests to find an appropriate solvent to use for the crystallization procedure. See procedure 11.6 on p 662. Test acetone, methanol, ethanol, isopropyl alcohol, and petroleum ether. After deciding the best solvent based on these tests, carry out the crystallization of the rest of your crude product as described on page 324.
Characterization. Obtain the mp as well as 1H and 13C NMR spectra of the crystallized product. An IR spectrum is optional.
Report
Feel free to integrate the answers to the assigned questions into your results and conclusions section for this report (rather than answering each sequentially).
Assigned Questons:
1. Fluorobenzene, 4,4'-difluorobiphenyl, and 4-fluorobenzoic acid are possible as side products in this reaction. How does each form (show equations)? How is each removed? Do the obtained spectra show any evidence for the presence of any of these in the final products?
2. Do the spectra show any evidence of unreacted starting materials or solvents as impurities? Elaborate.
3. Calculate the C-F coupling constants (J values) observed in the 13C NMR? How do they compare with literature values for C-F coupling constants?
4. Compare the chemical shift of the carbinol carbon (the one with the OH) to the literature value for the same carbon in triphenyl methanol. Predict the chemical shift of the carbinol carbon in 4,4', 4''-trifluorotriphenylmethanol.
5. How would one prepare 4,4',4''-trifluorotriphenylmethanol using a Grignard reaction?