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.
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.
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.
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.