Chemistry 351 Laboratory - Spring Semester 2009
Relevant textbook readings - Lab Manual, expt 38A, pp 141-148. Technique 11 (pp 189-210). 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 144-148 of the lab manual.
Preparation of Grignard Reagent. Carry out the procedures given on pp 144-146 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 145.
Reaction of Grignard with Benzophenone. The procedure is as specified on page 147 in Pavia. However, after the benzophenone addition has been completed and the reaction starts to cool, replace the Claisen adapter/drying tube attachment with a reflux condenser and reflux gently with a warm water bath for 15 min. Then cool the reaction to room temperature, at which point a solid may form and you may have to commence with manual stirring as specified in the book procedure.
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 204) use only 2 mL and do not heat. Instead stir and manually mash with a glass rod or spatula the solid under the petroleum ether. (This type of purification procedure in which an impure solid is washed with a solvent in which it is insoluble is referred to as "trituration".) 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. Repeat this whole step as many times as is necessary to obtain a lightly colored, powdery solid crude product. (Prior to this step the product will likely be a dark red, oily semi-solid material.)
Crystallization. We will need to do small scale tests to find an appropriate solvent to use for the crystallization procedure. See technique 11.6. 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 204.
Characterization. Obtain the mp as well as 1H, 13C NMR, and IR spectra of the crystallized product. .
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 could one most efficiently prepare 4,4'-difluorotriphenylmethanol using a Grignard reaction?
For writing up the Grignard experiment and interpreting your NMR data, it will be helpful to know what an ideal spectrum of the product looks like. You can use the spectra files indicated below to figure out the spectra and assign peaks. Then also figure out and label all of the peaks on your own spectra (which for the C-13 may not be showing all of the peaks due not having run enough scans).