5.2: Practical Considerations, Procedural Changes, Safety Tips
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Refer to pages 317 - 324 of your textbook (4th ed.)
The experimental procedure is presented on p. 320. It is divided into major sections as follows:
Preparation of glassware. The most important thing to note here is that all glassware must be DRY. This means free of water. Water can destroy all or some of your Grignard reagent by acting as an acid. Remember that many good nucleophiles are also good bases. Since water is a strong acid, it can protontate the Grignard reagent to yield its conjugate acid, effectively derailing your reaction. If you have trouble getting your reaction to start, this could be one of the problems.
Grignard reagent resoance forms
The best way to prepare the glassware for the reaction is to wash it well with soap and water first. Then rinse it with acetone until all the water has been washed off. Finally, blow a soft stream of air on the glassware to dry it, or place it in the oven for at least 5 min. to evaporate the acetone.
Formation of the Grignard reagent. Please note that the solvent ether is very volatile and releases noxious vapors at room temperature. Prolonged exposure to these vapors can make you drowsy. If you feel dizzy from breathing ether vapors, notify your instructor immediately and step outside the building to get some fresh air.
If the conditions of the experiment are all in place, you should notice formation of a cloudiness and “boiling” (actually formation of bubbles from the reaction). If you don’t see such signs, heat the reaction pot mildly (to about 50 degrees, no more!), DO NOT ATTEMPT TO CRUSH THE MAGNESIUM as the textbook recommends on p. 296. This is a good way to break your flask and possibly hurt your hand. If heating fails to start the reaction, add a crystal of iodine (ask your instructor for help) and continue heating and stirring. Finally, if all fails, borrow a drop of Grignard solution from another student whose reaction started, and quickly add it to your reaction mixture.
Since ether boils at a very low temperature, you may lose some of it as the reaction proceeds under the action of heat. Keep adding ether to compensate for these loses. You will know when the reaction is complete because all or nearly all of the magnesium has dissolved. If a few bits of metal remain, do not wait for them to dissolve, as they might be impurities. Proceed with the next step.
Addition of benzophenone (p. 323). The only important thing to note here is that after this step you should stop the experiment, since you will not have time to finish it in one session. The following session you will perform the hydrolysis step (bottom of p. 323).
Hydrolysis. In this step you deliberately add a source of protons (dil. HCl) to protonate the conjugate base of the product (tryphenylmethanol) and convert it to the actual alcohol. What follows is a series of extractions and purification procedures to obtain the final solid. This solvent is crystallized from hot isopropyl alcohol (also called isopropanol).
Spectroscopy. After calculating the percent yield, obtain an IR spectrum using the KBr pellet technique that your instructor will demonstrate. This technique is frequently used when the product is a solid.