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This is how I always end my organometallics unit in my advanced inorganic chemistry class. The students have already learned electron counting, the major reaction types (oxidative addition (OA), reductive elimination (RE), 1,1- and 1,2-insertion, β-hydrogen elimination, and [2+2] cycloadditions), and have gone through naming elementary steps in class for some classic catalytic cycles (hydrogenation with Wilkinson's catalyst and the Monsanto acetic acid process).
To end the unit, I have them divide into groups of 2 or 3 and propose mechanistic pathways for the conversion of reactants to products using the elementary mechanistic steps that they have learned. I let them know that this is *hard* and that all steps should be written as equilibria so that if they start going down a non-productive pathway, they can go backwards and try again.
Students propose mechanisms outside on the sidewalk using chalk. This is usually good for a photo shoot for the college catalog too!
The key for this learning object can be found under related activities. It has been posted as a problem set (see related activity Catalytic cycles and artistry: Chalk Drawing 102, below) so that students cannot access it online.
| Attachment | Size |
|---|---|
| catalytic chalk drawings | 30.5 KB |
After completing this exercise, students should be able to:
- understand the cyclic nature of catalysis, in other words, that you end up back where you started;
- apply his/her knowledge of basic mechanistic steps to a new problem;
- work in a small group to discuss elementary steps in organometallic chemistry; and
- propose a realistic mechanism for an organometallic transformation based on elementary reactions.
sidewalk chalk
sidewalk
sun
non-rainy weather
This activity is pretty straightforward. I have sometimes assigned groups and have sometimes let them self select. Groups of two or three seem to work best. I provide the nine problems on the attached worksheet, but more could be added. In years with small enrollments, I have students do a subset of the catalytic cycles, but provide the others for extra practice.
I bounce from group to group, helping students get over hurdles. I encourage the groups to draw out the first mechanistic steps that come to mind for each compound. If they are wrong, they can equilibrate backwards and try another step. I emphasize the principles of the reaction steps that we have learned (such as, no oxidative addition to an 18 electron complex or to one in its highest oxidation state, cis-ligands for reductive elimination, etc.). Some students require some prodding to start writing stuff out on the sidewalk, they want to get it "right" on the first try. I encourage group discussion and help them count electrons and remember subtleties of the organometallic reactions.
Evaluation
This activity is not graded, it is a learning exercise only. However, I almost always put a mechanism proposal on the final exam. I always put a "name the elementary step" question on the final exam, and after this exercise, students find that a lot easier.
I do not require that the students get the mechanism "right." For example, in the cobalt catalyzed hydroformylation, the accepted mechanism from (CO)4CoH is: 1,2-insertion 1,1-insertion, OA of hydrogen and RE of the aldehyde. However, if they propose: 1,1-insertion to acyl, 1,2-insertion, OA of hydrogen and RE of the aldehyde, I would not complain.
I post the key on our course management system after class.
Students have a hard time with this at first. It is hard to propose a mechanism. However, once they start writing, they usually can complete the cycle in about 15-20 minutes. I encourage groups to visit the other chalk drawings and see the other mechanisms.
Students often call the addition of an olefin to a metal (displacement of CO) an oxidative addition; I ask them to count the oxidation state of the metal.
Students have a hard time seeing the regioselectivity of the 1,2-insertion in the hydroformylation reaction; they always want to form the linear isomer and can't see how to produce a branched isomer.
I'm a solid state chemist and spend about 6-8 classes on organometallic chemistry with my students. Originally, I thought that this would be the perfect wrap-up to the unit, but I ended up modifying it because we weren't quite ready to draw mechanisms by ourselves. Here's how I used the activity.
My students and I have spent the last 4 class periods working on electron counting, looking at ligands and their interactions with metal centers, and discussing mechanism types. (We use George Stanley's online book as a resource.) For in class practice, I gave my students copies of five of the mechanism with the electron counting and reaction type name blocked out. I asked my students to count electrons and identify the reaction type in groups of 2 (or 3). While they were doing that, I walked around the room answer questions that arose. When students finished (all groups did) I asked them to propose a mechanism using the basic steps that we had discussed and without looking back at the mechanisms they had just completed for hydroformylation (alkene + H2+ CO -> aldehyde) with Co2(CO)8. This mechanism is close to identical to two the mechanisms provided by Adam. Two of my four student groups who were able to write a mechanism for this in about 10 minutes. During my next class period, I will have my students try another mechanism or two (maybe using the chalk cycles if the weather remains good) and then give some homework to reinforce these ideas.
I left about 35 minutes for this activity. It clearly demonstrated which students were comfortable with electron counting and familiar with the basic reaction types. During and after the activity, the students commented that this practice helped them better identify the reaction types. I'd give the activity 5 d-orbitals for engaging students with the material and for improving their perceived understanding. (We'll have to wait for the exam to see if their understanding is more than perceived.)
I used this activity with different reactions on the last day of class and it was awesome! (I will have other reactions that we used.) The students had good discussions about what you can and cannot do in an organometallic reaction and really liked the fact that we could have class outside. (Having a beautiful fall day helped!)
A student passing by (not a chemistry major) was overheard saying, "Why can't we do stuff like this in my classes?"
Thanks for sharing this Adam.
one of my student groups this year, in addition to drawing mechanisms, also drew this...
I tried this exercise, with relatively minor modifications (number and content of reactions), with my 300 level inorganic class of ~40 students. While 10 groups of 4 take up a large amount of concrete (not to mention chalk) I think they all enjoyed a beautiful Michigan spring day. I bounced between groups to answer questions and give hints--some groups did exceedingly well, completing multiple reactions, while others struggled to get beyond basic electron counting. While I had no formal means of evaluating the effectiveness of this exercise, the final exam included both 'name the fundamental reaction' and 'propose a catalytic cycle' problems. On the whole, students did remarkably well on these questions, no doubt in part due to this exercise.
All in all this was a great exercise, and I'm going to use it, and the general idea, whenever I can. Many thanks!
My students are always asking me to have class outside, especially with our early Spring this year in Michigan. I have never given in until now.
We did this exercise today in my Chem 403 (inorganic II- transition metals) class with a few modifications. First, I had to use about half of the period to finish going over the different types of organometallic reactions which I started in the previous lecture. Since the material was so new, I prepared a handout for the students on the different reactions, and had them use this as we built the catalytic cycles together as a class (only 8 students in this class).
We only worked through the Stille Coupling reaction cycle, but I found it very instructive.
With handout in hand, and a few nudges from me (I kept drawing their attention back to the overall reaction which I wrote on the sidewalk to get us started), the students were able to sort out the entire cycle. Interestingly, for my students, this was the first catalytic cycle that they looked at. We did ot go over one on class before this. The only thing that I told them is that catalytic cycles tend to favor a 16-18-16-18 cycle.
Students commented that they really enjoyed it (although one student was underdressed for the 45° but sunny morning) and most significantly, the weakest student in the class was the one most engaged in this activity.
I will eventually post a handout similar to the one I gave students today ( that one was stolen shamelssly from various sources on the internet at the last minute)
For what it's worth, although I also teach 18-16-18-16 for drawing mechanisms at the "intro level" (ie, advanced undergraduates), in "real life" I believe that many catalytic cycles are really in a 16-14-16-14 manifold, or something similar. The rationale being, 18 electron complexes are stable, isolable, and therefore not catalytically relevant. But that doesn't need to be brought into the discussion at this level, I don't think. Only for practitioners in the field and late night conversations at Salve Regiina. If you simply pop off one ligand from an 18 electron complex, you get to an excited, higher energy 16 electron complex that can then do the same reactions. You could add this step, making an 18e off-catalytic cycle species.... but I've found that when I stray too far from the basics/big picture, the students lose sight of closing the loop and restarting the cycle, so I've kept it simple.
By the way, this and several other organometallics LOs were recently written up in J. Chem. Educ. (dx.doi.org/10.1021/ed200200w)
I used this activity in my organometallic class (31) students to tie together the elementary reactions we discussed in class to catalytic cycles. We did not have time to discuss catalytic cycles this semester so it was a great way to have the students build them and recognize there are a number of ways these cycles could potential go through. We did this in December (which in NJ can be cold a snow) but for one section the weather was 60 and sunny and the other snowing and 30. I will upload pictures to share.
I gave the students about 5 of the cycles to work out (Pd-coupling reaction (we talked about base free/base), RCM, ROMP, hydroformylation (L/B)). Most students were able to complete a good portion of the them before class ended (~2 hour of a lab period). The students needed help from me to start the cycle but once started were able to begin to piece together the cycles. They really enjoyed this assignment because it was hands on and allowed them to apply their knowledge to solve the puzzle. I asked a hydroamination catalytic cycle on my final exam based on these cycles and 27/31 got the question right.
I would say about 85% of the students were fully engaged in the assignment. All students were able to determine the oxidation state and electron count with no problem.
If it is warm, we will be going outside! Love this.
There was a power outage in class yesterday and we were forced to leave the building - my class assembled outside and I decided it would be a perfect time to try this rather than cancelling class. Despite not having finished introducing all the reaction classes, this LO helped my students think through the catalytic cycles wiht a few hints from me. It was great! They found it challenging but rewarding.
I've used some parts of this LO for the better part of the last decade, and have now moved more toward an activity that involves catalytic cycle drawing as the culminating activity (guided inquiry approach). Unpredictable Ohio weather means that I have moved indoors most years, but it works fairly well. I'll try to write up this LO and link it below.