The migratory insertion reaction is one of the "four" main reactions in organometallic chemistry. It involves the formation of an acyl group by insertion of a CO molecule into a metal alkyl bond. The reaction is sometimes called the carbonyl insertion reaction because the product appears to be a result of direct insertion of the CO into the metal alkyl, but that name implies a mechanistic pathway that may not be in operation.
The reaction of methyl pentacarbonyl manganese(I), MeMn(CO)5, was studied extensively by Calderazzo in the mid 1960s. The use of C13 labeled CO and IR spectroscopy allowed for the identification of the mechanism for the reaction among the likely possibilities of direct insertion, alkyl migration, or carbonyl migration. This guided inquiry exercise presents some of the data from the Calderazzo paper and has students interpret it to determine the mechanism of the reaction in this system.
It should be noted that there are examples of all three mechanisms operating in different chemical systems, so this exercise is specific to the manganese substrate, though it is usually more generally applied.
Students will interpret and analyze IR data of metal carbonyls
Students will calculate IR bands for 13C labeled peaks in the IR
Students will predict product distributions for the three likely mechanisms (direct insertion, carbonyl migration, alkyl migration).
Students will compare expected and observed product distributions and identify the mechanism operating
Students will discover and discuss the concept of "negative evidence."
In my course, we usually cover isotopic labeling and its application to IR spectroscopy. We also use group theoretical methods to predict and assign M-CO stretches the correct symmetry labels and whether they are IR active or not. These two factors could be removed from the guided inquiry and presented as additional data to the students if you don't cover these topics. The rest of the activity is self contained. Access to the paper is not required, as the IR bands are in the document but a reference is provided.
I look to see if students are able to
1) determine the correct number of IR stretches for the compounds,
2) calculate the labeled IR stretches from the unlabeled ones,
3) correctly predict the product distributions expected for the 3 mechanistic pathways
4) understand/explain the importance of experiment 2, negative evidence, and microscopic reversibility
Awaiting assessment data at time of submission; will add ASAP.
based on 3 complete submissions (43% response rate, due to COVID)
students generally had no problems with questions 1 and 2, and were able to determine the number/symmetry of IR stretches using group theory, and to predict a vibrational frequency from a areduced mass calculation.
question 3 gave them a lot of trouble. I would normally do this as an in-class exercise and be able to talk them thorugh problems. students were able to draw some of the correct products for the various mechanisms but did not understand the fact that there would be a statistical distribution of products based on the 13C label. However, I spoke with all 3 students and they said that after priming their brains with the exercise, the reading in the textbook made a lot of sense and they understood what they had missed. Perfect!
Students did not generally understand the concept of negative evidence as hoped.
For future years, if I were unable to do this exercise in class, I would want to provide more guidance to get students to think about product distribution. However, if done in class, I think that watching them struggle a bit before helping them over the hurdle would be good.