last updated Tuesday, December 23, 2014

Please make sure that you read the chapter about how to set up glassware for reaction under inert gas (page 87-97 in reader, Winter 2015) 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 basic training session.

Make sure to check all glassware for crack or star crack prior to assembling the setup and evacuating it!

Step 1: Grignard formation (for pictures check this link)

The setup has to be flame-dried twice and refilled with nitrogen gas. The Mg-turnings are crushed before being placed in the 250 mL three-necked flask prior to the flame-drying heating process for which a heat gun is used as heat source.

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

Like in Chem 30 CL, the bromobenzene has to be dissolved in dry diethyl ether. The best approach is to place the diethyl ether from the solvent still in the addition funnel first before adding the bromobenzene to obtain a more homogeneous solution. If the bromobenzene is added before the diethyl ether, a layering of the two liquids will be observed in most cases. Thus, more or less neat bromobenzene would be added in the beginning, which is not desirable because more biphenyl will be formed :-(

In the beginning, a small quantity (5-8 mL) of the ethereal bromobenzene solution is added to the Mg-turnings. The reaction mixture will turn milky if the reaction initiated. If this does not occur after 10 min, the mixture will have to be heated gently with a heat gun. A clear liquid will drip back from the reflux condenser then. (Note that this is usually diethyl ether and not water unless the Liebig condenser has a pin hole on the inside tube which is very rare!) 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 where this does not lead to success, 1-2 iodine crystals are added and the mixture heated again. At this point, the reaction should initiate. If it does not initiate, the mixture has to be discarded. The student will have to restart the reaction.

After the reaction initiated, the remaining PhBr solution is added continously at a rate that the reaction maintains a gentle boil (it will turn brown at some point, see link above). Upon 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.

PhMgBr + H₂O -----> Ph-H + MgBrOH

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. Note that MgBr(OH) is not very water soluble. The mixture has to be titrated until the solution remains colorless after 10 minutes. Note that often times the mixture will become colorless and turns pink after a few minutes because the MgBr(OH) does not dissolve well in water.

Step 3: Formation of triphenylphosphine (PPh₃)

It is very important to have the correct ratio of PhMgBr to PCl₃. Based on the balanced equation, at least three equivalents of Grignard reagent are required for the formation of the one PPh₃. In the lab, a ratio of PCl₃:PhMgBr=1:3.5 will be used to suppress the formation of other chlorophosphines. Experimentally, it is easier to quench the excess Grignard reagent than separating complicated mixtures of chlorophenylphosphines (Ph_xPCl_3-x).

After drying the organic layer, the solvent and other volatile compunds have to be removed by distillation i.e., biphenyl (b.p.=256 ^oC), dichlorophenylphosphine (PPhCl₂, b.p.= 240 ^oC) under nitrogen. Do not use a Vigreux column here! Other impurities like diphenylchlorophosphine (PPh₂Cl, b.p.: 326 ^oC) and triphenylphosphine oxide (OPPh₃. b.p.= 360 ^oC) will be removed by recrystallization from ethanol (or methanol) as long as they are present in small quantities.


Step 4: Characterization

Infrared spectrum (ATR), m.p., ¹H-, ¹³C- and ³¹P-NMR, mass spectrum (submit a sample 1 mg/mL hexane) to the instructor


Step 5: Problems to be considered

1. How can the student prepare a 0.10 M hydrochloric acid solution quickly from concentrated hydrochloric acid? What molar concentration does concentrated. hydrochloric acid have? How close does the concentration of the solution have to be to 0.10 M?

2. Estimate of the concentration of the Grignard solution assuming that the Mg metal reacted completely and that there are ~70 mL of solution after the reaction. Does your experimentally determined value from above make sense?

3. Why is the formation of Ph_xPCl_3-x (x=1,2) a problem in the reaction? How can these species be detected in the mixture?

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


Hints to the pre-lab questions

ad 3: What is the driving force for the reaction?

ad 4: What determines which solvent should be used for a recrystallization?

ad 5: What determines the splitting in the NMR spectrum?