Lab 3: Grignard Chemistry

Pre-Lab:

1. You should come to the lab being fully prepared on what you need to do.  Write a quick list of what needs to be done 1st, 2nd, 3rd, etc.  If you are not fully prepared, you will not finish on time and/or have very poor yield.  If you are not prepared, it is very likely you will also lose participation points. 

1.) Collect two drying tubes from the oven and allow them to cool to room temperature (approx 20 degrees Celsius)

2.) Collect the remaining hot, dry glassware from the oven and assemble the reaction apparatus as indicated in the laboratory handout.

3.) Measure 1 g of magnesium and add it the two/three-necked round bottom flask.

4.) Add 8 mL of dry ether and 5 mL of bromobenzene to the same round bottom flask.

5.) Gently crush magnesium turnings using a “stirring rod" while holding the bottom of the flask in your hand.  Continue to crush the magnesium until the reaction begins. 

6.)  Once the reaction has begun, add an additional 12 mL of dry ether.

7.) Connect the flask to the refluxing apparatus and set the heat to low.

8.) Draw a line on the outside of the flask marking the level of the ether. Monitor the level of ether within the flask, adding more when the level of ether dips below the line.

9.) Continue the reaction until either all the magnesium has been consumed or only 1 hour and 20 minutes of the class remains (whichever happens first).

10.)  Turn off the heat and allow the reaction to cool to room temperature.

11.) Transfer the entire reaction mixture into an oven dried, cooled 50 mL graduated cylinder.

12.) Bring the total volume up to 26 mL with dry ether.  Gently mix the solution with a dry stirring rod. 

13.) Place 13 mL  into a 100 mL into a dry Erlenmeyer flask using the 10 mL pipette and cover tightly with foil.

14.) Pour the remaining 13 mL into a round bottom flask and seal this flask with a rubber stopper. 

15.) Using the 13 mL of the solution that was previously covered w/ foil, place a magnetic stir bar into the beaker with the phenyl magnesium bromide reagent and place on top of a stirring hot plate. Turn on the stirring and stir at a moderate to fast rate.

16.) Add 8 mL of dry ether and 3 mL of methyl benzoate into the cooled dropping funnel. 

17.) Mix the methyl benzoate and ether with gentle swirling.

18.) Lower the dropping funnel directly into the Erlenmeyer flask and slowly drip the mixture into the reaction flask. 

19.) Add the methyl benzoate mixture to the Grignard reagent one drop at a time.

20.) When all of the methyl benzoate has been added and the reaction flask has again reached room temperature, add an additional 15 mL of dry ether, and continue to stir with the stir bar.  Cover the reaction with foil and let it stir until the end of lab.

21.) At the end of lab, seal the flask with a rubber stopper.   Label the flask with your name and the product description and place it on a cork ring in your drawer. 

22.) Using the 13 mL solution that was sealed with a rubber stopper, carefully pour the solution over 5 g of crushed dry ice that has been placed in a beaker. 

23.) Stir until the dry ice has evaporated. 

24.) Place the beaker in the hood and add 10 mL of 3 M HCl.  

25.) Cover with a kim-wipe and place in your drawer.  

26.) Make a soapy sink of water and place all of the glassware that you used into the sink to soak for a bit of time.  While the glassware is soaking, put away all other materials from the lab. 

27.) Wipe down BOTH the benchtop and the inside of the hood.  When your hood space is tidy, wash all of the glassware.  

2.  Using curved arrow formalism, draw the mechanism of both reactions, the reaction of phenyl magnesium bromide with methyl benzoate and the reaction of phenyl magnesium bromide with carbon dioxide.  You should include all workup steps.  

See Image Labeled "2.)" and "2.) b.)".

3.  Explain with words why must the reaction to synthesize the Grignard reagent be kept perfectly dry?  

Grignard reagents are highly reactive with water. If water is present within the reaction the Grignard reagents will undergo a chemical reaction with water and not form our desired product. 

4.  Explain why can’t we use ethanol as a solvent such as we have done in other labs?

Ethanol can't be used for as a solvent for Grignard chemistry because ethanol is a protic solvent (containing an OH group). Grignard reagents are highly reactive with protic solvents causing the reagent to form chemical bonds with the solvent rather then the desired product of our chemical reaction.

5.  The lab specifies to measure accurately.  Since this wording does not mean that you have to measure exactly 5.0 mL of bromobenzene, for instance, what does it mean?

When measuring accurately, we should attempt to measure as close to the desired amount as possible but because we are human, it is difficult to measure exactly 5.00 mL, so a small margin of error is accounted for when measuring. For example, measurements such as 4.90 mL or 5.10 mL are measured accurately but not exactly. 

6.  In the synthesis of triphenylmethanol, it is highlighted that you must slowly add a reagent to keep the temperature low otherwise the correct product will not form.  If your reaction gets too hot, other products will form.  Why is it expected for nearly all reactions that if the temperature is high, multiple products will form?

As the temperature of a chemical reaction increases the speed at which the molecules involved in the chemical reaction move and interact with other chemicals also increases. This allows for reactions that are generally energetically unfavorable to occur as the energy of activation is overcome leading to the formation of both energetically favorable and unfavorable products.

In some cases, the main reaction may proceed too quickly, leading to excess heat generation and promoting secondary reactions such as rearrangements.

7.  List the “accessory glassware” that you will need to retrieve out of the oven for the synthesis.  

Two drying tubes, a dropping tube, a condenser, a two-necked or three-necked round bottom flask, an Erlenmeyer flask, another round bottom flask, and a graduated cylinder.

 Results: 

Triphenylmethanol:

Theoretical Yield: 6.2 g

Actual Yield: 4.67 g 

% Yield:  4.67 g/ 6.2 g = 75% yield

Melting Point: 143.7-152.2  Celcius

TLC: Sample Preparation: Dissolved the student-prepared sample in methanol. No lab provided standard for comparison.

TLC solvent: 5 mL methanol and 5 mL hexane for 50:50 ratio.

*Attempted several TLC plates in different solvents to determine the ideal polarity ratio.

Triphenylmethanol TLC:

Discussion: 

This experiment resulted in a 75% yield of triphenylmethanol and a 93% yield of benzoic acid. The % yield for both of these experiments is considered to be successful due to their high yield.

While there was no lab grade standard for comparison, TLC analysis for the triphenylmethanol product indicates a likely pure product, as there is only one distinct spot noted on the TLC plate with an RF value of 0.85.

The TLC plate for the benzoic acid leaves some room for question, as the product prepared in the lab today revealed one distinct spot with an RF value of 0.62. The lab provided standard taking from the "student prepared benzoic acid"; however, it had three distinct TLC spots, each having an RF value of 0.62, 0.75, and 0.89, respectively. This indicates that while there is likely relative purity in the product formed in the lab today, some impurities are likely found within the student-prepared benzoic acid standard provided for comparison. The two samples having a spot with an RF of 0.62 could indicate a presence of benzoic acid within both samples.

The melting point of the triphenylmethanol product formed in the lab was 143.7-152.2 Celsius. The high of this temperature range is 10.8 degrees Celsius below the reported literature value of 163°C. his is greater than that expected margin of error for melting point analysis, indicating that impurities are likely contained within the triphenylmethanolsample. he melting point of the benzoic acid formed in the lab was 108.4-111.2degrees Celsius; the high of this temperature range is also 10.8 degrees Celsius below the reported literature value of 122 degrees Celsius. This indicates that there are also impurities within the benzoic acid sample. The temperature difference in both samples being 10.8 could also be an indication of a calibration error with the melting point machine.

The [M]+ value obtained from the mass spec of triphenylmethanol was 243.11. The known literature value for a mass spec analysis of pure triphenylmethanol is 260.33 g/mol. For the Westminster mass spec machine, we would expect a result of the literature value of pure triphenylmethanol with a +1 value. There are several prominent peaks around the [M]+ values, indicating there are likely impurities within the sample, which is consistent with the finding from the melting point analysis. While it is possible that a smaller ion than expected was picked up, this lower-than-expected [M]+ value could indicate that there was no triphenylmethanol synthesized. The [M]+ value obtained for benzoic acid was 123.04. For the Westminster mass spec, we would expect a value of 122 g/mol (the literature value for benzoic acid) with a +1 value. This indicates that there is likely benzoic acid present within the product. There is one prominent peak that could indicate impurities within the sample.