last updated Monday, January 10, 2011

Please make sure that you read the chapter about how to set up glassware for reaction under inert gas (page 81-89 in reader, Winter 2011) with emphasis on the basic reaction setup and the use of the Schlenk line. Those are important topics that should be familiar to you after Thursday's introduction.


Step 1: Grignard formation

The setup has to be flame dried and refilled with nitrogen gas. The Mg-turnings should be crushed before being placed in the (250 mL) three-necked flask.

Dry diethyl ether (from solvent still) is required to get a good yield of the Grignard reagent. The experimenter should make sure to take care about this while preparing the equipment.

Like in Chem 30 CL, the bromobenzene is dissolved in diethyl ether. The solution is then poured into the addition funnel. If the bromobenzene is added first and the ether afterwards, a layering of the two liquids is observed!

The reaction is started by adding a small quantity (5-8 mL) of the ethereal PhBr solution. The reaction mixture turns milky if the reaction initiated. If this does not occur after 10 min, the mixture is heated gently with a heat gun. A clear liquid will drip back from the reflux condenser then. After the heat source is removed, the reaction should maintain a gentle boil on its own. If this does not occur, the heating has to be repeated. In case this does not lead to success, a crystal of iodine is added and the mixture heated again. At this point, the reaction should initiate. If not, the mixture should be discarded.

After the reaction initiated, the remaining PhBr solution is added at a rate that the reaction maintains a gentle boil (it will turn brown at some point). After completion of the addition, the mixture is refluxed for an additional ~15 minutes to complete the reaction.


Step 2: Assay

In order to determine the assay, a small portion of the Grignard reagent formed is hydrolyzed. A 2 mL aliquot is placed into an Erlenmeyer flask with 50 mL of water. The mixture is stirred for 10 minutes. Then three drops of a phenolphthalein solution are added (which color will it have?). As titre, a diluted solution of HCl is used (~0.1 M). The number of moles of Grignard in the prepared solution.


Step 3: Formation of SnPh₄

The number of moles of SnCl₄ required for the reaction are calculated assuming that at least four equivalents of Grignard reagent are required for one Sn-atom. If insufficient Grignard reagent were present, mixtures of Ph_(4-x)SnCl_x would be obtained.

Tin tetrahalides forms 'stable adducts' with Lewis bases like ethers (see project 2.3.). Therefore the addition funnel should be rinsed with 10 mL of dry toluene before the SnCl₄ is added. If this is not done, the outlet of the addition funnel will clog up immediately.

Upon addition of the SnCl₄ solution, a off-white precipitate is usually formed immediately. The mixture has to be stirred vigorously throughout the addition and afterwards. The reflux condenser and the addition funnel can be removed at this point and the mixture can stored under nitrogen (Keck clips should be used to secure the glass stoppers at this point!).

The diethyl ether is distilled off during the next lab period (setup see distillation without the Vigreux column, why?).

The toluene used here does not have to be dry since the excess Grignard reagent is quenched with ice water anyway. The addition of the ice and 2M hydrochloric acid should be done slowly because the reaction is pretty exothermic. The mixture should be acidic afterwards.

After separating the layers, the organic layer is dried over MgSO₄. The mixture is heated to ~90 ^oC and then the liquid part transferred to a clean container. More toluene (~20-30 mL) is added to the remaining solid in order to extract more product. The combined extracts are allowed to cool down and then placed in an ice bath for further crystallization.


Step 4: Characterization

IR spectrum (ATR), ¹H, ¹³C-NMR and ¹¹⁹Sn spectrum (ask your instructor/TA for help)


Step 5: Problems to be considered

1. How can you prepare such a solution fast from conc. HCl? What molar concentration does conc. HCl have?

2. Estimate of the concentration of the Grignard solution assuming that the Mg metal reacted completely and that there are ~120 mL of solution after the reaction. Does your experimentally determined value from above make sense?

3. Why is the formation of Ph_(4-x)SnCl_x (x=1-3) a problem in the reaction?

4. How could you quantify the amount of a Ph₃SnCl in the mixture?

5. Why is it not advisable to get the toluene to a boil during the extraction procedure?

6. Which solvent should be used for the characterization using NMR spectroscopy?