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Adam R. Johnson, Harvey Mudd College
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Joined: 11/17/2007 - 10:55am

Help with Co(en)3 prep

Hi everyone. I'm troubleshooting the [Co(en)3]3+ prep from Girolami, Rachfuss and Angelici. The prep, briefly, is to take 6 g Co salt, 13 g en·2 HCl, 25 mL water, 8 g NaOH and then 20 mL of 3% H2O2. Then it says to dilute to 50 mL and heat to boiling. However, at the end of the addition, the volume is 70+ mL. My student yields were really low this spring (much lower than usual) so I am wondering if anyone else has worked on this. I just set it up for the first time ever (I did the old "pull air through the solution" from the previous edition by Angelici 25 years ago...) and the reaction works fine, its just too dilute to get a good yield.  Thoughts? Ideas? thanks.

Adam R. Johnson, Harvey Mudd College
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Joined: 11/17/2007 - 10:55am

ok. Crude yield of first crop is 68%, which beats all my students from last spring, so I don't think the prep is bad. However, the "dilute" the solution at 70 mL to 50 mL doesn't make a lot of sense. Based on some googling, I think the biggest issue is making sure the recrystallization is ice-cold, as opposed to typical student ice baths which are basically RT water baths with some ice thrown in for a feel-good measure.

I'm playing around with this experiment a bit this summer so I'll report back in a blog when I have something interesting to report.

Chip Nataro, Lafayette College
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Joined: 04/14/2008 - 9:10am

Have you thought about using an ice-salt bath as opposed to just an ice bath? It might aid in the process.

John Lee, University of Tennessee Chattanooga
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Joined: 06/28/2012 - 2:47pm

Adam, I have had some issues with this experiment as well, and like you I have been able to reproduce the experiment when using the exact same reagents (that the students use). One thing I have done is to use 30% hydrogen peroxide (purchased through Fisher) instead of the 3%. I first noticed a couple years ago that the student polarimetry data was consistently showing a racemic mixture. That was the first time I reproduced the experiment and was able to get excellent optical purity for both the tartrate diasteromer and the lambda (+) enatiomer and good purity for the delta (-) enantiomer as expected. As a result I started having the students balance all chemical equations and obtain correct molar masses before the experiment as often times they would not obtain a mass at or above what they should start according to the prep (and would have to adjust quantities). This helped and the success rate was approximately 50% in each class. Two years ago I brought in our benchtop diffactometer as a part of the experiment as the tatrate diasteromer gives consistently massive high quality crystals. Here I found that the student samples were correct up until that point (i.e., got the correct structure) but their polarimetry data was still split regardless of the crystal structure. I assumed here that this was likely due to impatience in dissolving the compounds while making the highly concentrated samples for the polarimeter(?). This year however I discovered a new issue. Even though every group of students obtained beautiful crystals of the tartrate diasteremer not a single one of them resulted in a solved structure. Moreover, the X-ray data resulted in a unit cell that is not even the correct complex! I'm still needing to carve out some time to explore if there is anything in their data sets I can solve as it appears to be something new! And, of course with not making the tartrate diastereomer all polarimetry data showed a racemic mixture. I went back to the lab over spring break, using the exact same reagents they used, and like before was able to not only make the tartrate diastereomer and solve the structue but also obtained good optical purity data.  There appears to be something subtle in this procedure that gets the students but you and I are able work out? Anyway, maybe the 30% hydrogen peroxide will be of help to you, and maybe buried in in this post you will see something that might be impacting your students as well (or know exactly what my students are doing wrong!). One important side note, two groups out of seven this past semester did obtain "decent/okay" optical purity data, which again suggests there is something very subtle in the procedure that the students are missing.

 

Todsapon Thananatthanachon, University of Evansville
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Joined: 05/01/2012 - 1:44pm

I followed the prep published by Janet Morrow (J. Chem. Ed, 2016, 93, 1115), and the percent yields have always been in 60 - 90%.  The procedure calls for mixing CoCl26H2O, HCl, en, H2O2, in H2O.  The volume of the solution was then reduced to boiling, and Co(en)3Cl3 solids were crashed out upon after adding conc. HCl and EtOH, and cooing in ice bath.  I have used this prep for 2 years now, and it has worked for every student.

 

So, I agree that in your case the solution was too dilute.  You will probably get higher % yield if you reduce the volume of the solution.