Chemistry 351 Laboratory - Spring Semester 2010
Expt #6. Grignard Synthesis of 4-Fluorotriphenylmethanol.
Relevant textbook readings - Lab Manual, expt 38A, pp 141-148 (pp 67-74 in the 2009-2010 lab manual). 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 to form 4-fluorotriphenylmethanol.
Procedures
We will adapt the Pavia procedure for the preparation of trifluoromethanol.
Preparation of the Grignard Reagent. The procedures are the same as given in the lab manual (pp 144-146 or 70-72 depending on which edition of the manual that you have). However, substitute an equimolar amount of 1-bromo-4-fluorobenzene for the 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 (71).
Reaction of the Grignard Reagent with Benzophenone. Again, the procedures are mostly the same as given in the lab manual for Expt 38A. However, do the ether rinse and addition (paragraph 1, 6th to last sentence) before cooling to room temperature. After stirring for 15 min at r.t. proceed to the hydrolysis procedure regardless of whether a solid forms or not.
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, use a glass rod or spatula to stir and manually mash 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 the trituration/filtration procedure 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.)
Recrystallization. Recrystallize the crude solid from petroleum ether.
Characterization. Obtain the mp as well as 1H, 13C NMR, and IR spectra of the final 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?