Question

QUESTIONS TO ANSWER:

1-What is the role of the silica gel beads in the reaction setup?
2-What is the byproduct of the reaction if the Grignard reaction if these silica beads weren’t there?
3-The sulfuric acid has 2 roles what are they?
4-Why we cant we use hydrochloric acid instead of sulfuric acid? and explain
5. If there was another spot on the TLC plate, what would be that other spot assuming all starting material was consumed? At what stage of the workup do we separate this biproduct from triphenylmethanol?

RXNS OF GRIGNARD REAGENTS Experiment

BACKGROUND INFO:

Procedure:

1. Set up the apparatus using the dry conical vial and a drying tube filled with Drierite and plugged at each end with a piece of cotton. Heating should be done using a stirring hotplate.

2. Transfer 4 mL of anhydrous diethyl ether to larger vial provided in the kit andquickly cap the vial. Use this throughout the experiment when anhydrous ether is needed and be sure to keep the vial capped whenever it is not in use.

3. Accurately weigh about 0.05 g of magnesium turnings. Do not touch them. Use the mortal and pestle to remove any oxidation from the surface of the metal. Add the magnesium to a clean dry 5 mL conical vial. Add the spin vane to the vial with the magnesium.

4. Prepare a solution of 0.24 mL of bromobenzene in 0.5 mL of anhydrous diethyl ether in the small vial provided in the kit. Gently swirl the solution so that it becomes homogeneous.

5. Using a clean vial from your drawer, obtain ~1.5 mL of 3 M Sulfuric acid and place it in an ice bath for later use.

At this point it is important to follow the steps and keep everything capped and use the syringe. DO NOT CLOSE THE NEEDLE UNTIL YOU ARE FINISHED USING IT. It cannot be reopened once closed.

6. Using the syringe provided in your kit, draw up 1.0 mL of the anhydrous diethyl ether. Add this to the conical vial by inserting the needle through the septum cap

7. Draw 0.25 mL of the bromobenzene solution into the syringe and recap the vial. Insert the syringe needle through the septum cap and add the solution to the reaction vial and begin gentle heating (very low heat). Hold the plunger of the syringe to control the rate of addition.

8. Begin slowly stirring the reaction mixture in the conical vial.

9. After a few minutes, the solution will start to boil. You will have to turn off

   the stirrer briefly to determine this.

10. After small bubbles begin to form, immediately lift the vial off the hot plate and turn off the heat. The stirrer should still work.

11. The mixture will start to turn slightly cloudy or chalky once the reaction has started, and bubbling will continue without further heating. You can turn the stirrer off briefly to see clearly what is happening. If this does NOT happen, see your instructor

12. As soon as the reaction begins, add another 0.5 mL of anhydrous diethyl ether to the reaction vial.

13. Add the remaining bromobenzene solution DROPWISE at a rate that does not cause too vigorous a reaction.
    DO NOT LET THE VOLUME IN THE VIAL GET TOO LOW OR EVAPORATE COMPLETELY – add more anhydrous diethyl ether

14. Add 0.5 mL of anhydrous diethyl ether to the vial that contained the bromobenzene solution. Add this solution to the vial in one portion

15. Continue to stir the solution for 15 minutes. DO NOT LET THE VOLUME DROP BELOW 2 MLS. The solution should turn a yellow or brown color

16. During the time that the solution is stirring, prepare a solution of 0.12 mL of methyl benzoate in 0.5 mL of anhydrous diethyl ether in the screw-cap vial that originally held the bromobenzene solution. There is no need to clean the vial before use. Quickly cap the vial.

17. When the Grignard reaction has run 15 minutes, draw the methyl benzoate solution into your syringe and add it dropwise to the reaction vial. This reaction is exothermic, so add slowly enough that the diethyl ether vapors do not reach the Claisen adapter. (The solution should turn pink or red, even if only briefly.)

18. Add 0.3 mL of anhydrous diethyl ether to the vial that contained the methyl benzoate to rinse the vial. Add this solution in one portion to the conical vial

19. Let the reaction mixture continue to stir for 15 minutes. If it stops stirring with the spin vane you can take off the top and occasionally stir with your spatula.

At this point, put away the needle and syringe that came in your kit and do not use them again!

20. Remove the Claisen adapter and drying tube and place the conical vial in an ice bath. Be very careful taking off the drying tube. This point is where most of them are broken! Return your kit to the stockroom at this point.

21. Slowly add about 1.5 mL of ice-cold 3 M sulfuric acid dropwise to the reaction mixture. Add slowly to avoid frothing of the reaction mixture. The contents of the vial may solidify. Use a small glass stirring rod or spatula to break up the solid during the acidification. If needed, close the vial and shake with venting to induce dissolution of all solid materials. Two distinct layers should appear when all the material is in solution.

22. If needed, add solvent grade diethyl ether to get the material into solution. (You may use anhydrous ether if you have some left over, but use solvent grade once you run out of anhydrous at this point.) You may also need to add more 3 M sulfuric acid. Cap the vial and thoroughly mix the two layers. Be sure to vent the vial often. (Usually when the entire solid is dissolved there will be approximately equal ether and aqueous layers and the vial will be nearly full.)

23. Remove the top ether layer and place it into a 10 mL conical tube from your drawer and cap the tube.

24. Wash the remaining aqueous layer with 0.5 mL of diethyl ether. Remove this ether from the aqueous layer and place it in the tube with the first ether layer from step 22.

25. Wash the combined ether layers with 0.5 mL portions of saturated sodium bicarbonate (VENT!!!!). Discard the bottom aqueous layer saving the ether layer.

26. Repeat with another 0.5 mL portions of saturated sodium bicarbonate (VENT!!!!) Discard the bottom aqueous layer.

27. Wash one 0.5 mL portion of saturated NaCl. Discard the bottom aqueous layer.

28. After removing the last saturated NaCl wash, add anhydrous sodium sulfate to the ether solution until there is some rolling sodium sulfate. Allow the solution to dry for a few minutes.

29. Transfer the dried ether solution to a dry screw-cap vial from your drawer. You can add fresh diethyl ether to the sodium sulfate to extract more product and combine with the original dried solution.

30. Place the uncapped vial in a small, labeled beaker and leave it in the hood until the next lab period.

Second Day:

31. Obtain your vial from the fume hood.

32. Add 1.0 mL of petroleum ether to the vial and scrape the sides of the vial with a small stirring rod or spatula. The product will not dissolve. This process is called trituration.

33. Filter the material using vacuum filtration in a Hirsch funnel and allow it to dry for a short time. It will be difficult to remove the entire solid from the vial. The solid that does not come out can be left in the vial and dissolved with hot isopropyl alcohol during the next step.

34. Weigh the centrifuge tube that comes with the kit that you check out.

35. Transfer the crude product from the filter to the centrifuge tube.

36. Heat isopropyl alcohol (~20 mL) to near boiling and add it in small amounts to the crude product until all is in solution (~2-4 mL) while heating. The solid that could not be removed from the vial after trituration can be included by dissolving with a portion of the isopropyl alcohol and added to the product of filtration. (Do not filter hot solution.) Add the minimal amount of isopropyl alcohol needed to dissolve the product

37. When all the solid is in solution, cool to room temperature and then place in an ice bath.

38. Add 3-5 mL of water to the cooled solution in the ice bath. You should see cloudiness.

39. Centrifuge the tube until the solid is plastered against one side and the solution is clear. You will need to find a partner to balance the centrifuge. Your water level should match your partners. If they did not match, you can add more water to the same level as your partner.

40. Pipet off all the liquid.

41. Dry the solid product using a stream of nitrogen. It will be flaky when dry.

42. Determine the weight of the product.

43. Determine the melting point of the product and analyze using thin-layer chromatography. Turn in the remainder of your product as directed.

TLC Plate Preparation

1) Obtain a TLC plate
a. Make sure to handle the plate by the sides or plastic backing only. Do NOT touch

the absorbent (dull) side of the TLC plate

2. 2) Using a pencil (NOT PEN THIS TIME), draw a line 1 cm up from the bottom of the dull

   ide of the TLC plate. Draw VERY lightly as too much pressure will ruin the plate.

3. 3) In the middle of the line draw an “X”

4. 4) Prepare the chromatography jar (or beaker)

   1. Obtain a chromatography jar

   2. Add half a piece of filter paper to the inside of the jar

   3. Saturate the filter paper with dichloromethane and fill the bottom of the jar with

      the mobile phase (dichloromethane), making sure not to fill it higher than the line on the TLC plate. Overfilling the chamber will dissolve your sample instead of eluting on the plate.

5. 5) Prepare your sample – You want to make sure you have everything you need for the TLC before preparing your sample.

   a. Place a small amount of crystals (1-2 crystals) on a clean, dry 3-dram vial.
   b. Place a small amount of acetone (3-4 drops) to the 3-dram vial to dissolve the

   crystals.

6. 6) Loading the TLC Plate

   a. Obtain a TLC capillary tube - it should be open on both ends.
   b. Place the capillary tube into the ethyl acetate solution. The solution will be

   drawn up the tube by capillary action.
   c. Quickly tap the loaded capillary on the middle of the “X” on the TLC plate. This

   can be done by tapping quickly several time on the “X”
   d. You need to check to see if you sample was transferred properly to the TLC plate

   by viewing the plate under the short wave UV lamp in 344 HH. You do not want

   your sample spot to be too big/small or too faint.
   e. If your acetone evaporates, you can add more and continue.

7. 7) The place the TLC plate into the jar (using tweezers) with the mobile phase making sure that the bottom line is above the mobile phase.

8. 8) Place a lid over the jar and allow the mobile phase to move up the TLC plate

9. 9) When the mobile phase stops moving (should be at least half way up the plate), remove

   the plate.

10. 10) Immediately mark the solvent line with a pencil.

11. 11) Allow the plate to completely dry.

12. 12) Take the TLC plate to 344 HH and bring a pencil with you.

13. 13) Place the TLC plate under the UV lamp and turn on the short wave UV. The plate should

    look green with dark colored spots.

14. 14) Draw circle(s) around any spot(s) that appears along the middle of the plate.

15. 15) Measure the distance from the bottom line to the top line (in cm).

16. 16) Measure the distance from the bottom line to the middle of the spot(s).

17. 17) Record this data on your lab report.

18. 18) Calculate the Rf of the triphenylmethanol. If you have two spots, determine which one is

    the triphenylmethanol.

19. 19) Turn in your TLC plate along with the sample to your instructor with your initials on the

    top of the plate.

Answer 1

1. The reaction of bromobenzene with Mg will produce a Grignard reagent Ph-MgBr.

Reaction: Ph-Br + Mg + anhydrous ether (as solvent) Ph-MgBr

This reaction needs anhydrous condition to take place as any amount of moisture would render the reaction useless by reacting with the produced reagent, PhMgBr and will produce benzene. This is why dry solvent and dry reactant are used. But it still may contain a little amount of moisture or during the reaction time it may collect some moisture. This is where silica gel beads come in. Silica gel beads act as desssicant which means it will capture the moisture from around and make the neighboring region moisture free for an efficient reaction.

2. If silica gel beads are not used, some moisture would still be in the reaction mixture and would produce benzene as byproduct.

Reaction: PhMgBr + H₂O Ph-H + Mg(OH)Br

3. The two roles of sulfuric acid are: (a) converting the magnesium alkoxide salt (produced from the reaction of PhMgBr and methyl benzoate, shown below) to the resulting alcohol product.

(b) destroying any remaining unreacted magnesium and converting insoluble magnesium hydroxide salts into soluble magnesium sulfate.

Mg (unreacted) + H₂SO₄ = MgSO₄ + H₂ (g)

Mg(OH)Br + H₂SO₄ = MgSO₄ + HBr +H₂O

4. If we use HCl instead of H₂SO₄, the counter-ion Cl^- is nucleophilic which will attack the carbocation produced due to the protonation of the product, triphenylmethanol. H₂SO₄ creates no such problem as the counter-ion of H⁺ is SO₄^2- which is not nucleophilic at all.