28 Dec 2016

Isotope Effects in Arene C-H Bond Activation by Cp*Rh(PMe3)

Literature Discussion

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Description: 

This literature discussion is based on a paper by Bill Jones and Frank Feher (J. Am. Chem. Soc., 1986, 108, 4814-4819). In this paper, they study the activation of aromatic C-H bonds by a rhodium complex. Through careful experimental design, they were able to examine isotope effects on the selectivity of the reaction. Analysis of the rate data allowed them to prepare a reaction coordinate free energy diagram. This paper also introduces the effects of C-H bond breaking in early or late transition states on the vibrational energy spacing at both ground and excited states. The paper is a good way to bring kinetic isotope effects into the classroom. The paper also introduces the concept of deuterium labeling experiments and what that information can tell you.

An important aspect of this paper, and what makes it so interesting, is that they are able to get two kinetic isotope effects, one for each step of the reaction. From these two KIEs alone they are able to determine the unexpected rate-determining step of the reaction. It is a triumph of mechanistic investigation into intermediates that are undetectable.

This LO presents a series of guided reading questions that help a student approach and understand the material presented in the paper in a more thorough way. Part one is a guided inquiry that allows the students to derive and understand differing zero point energies for proteated and deuterated compounds. Part two guides students  through the results presented in the paper to help them better understand how experimental data can be used to understand the mechanism of a chemical reaction. There is more to the paper than kinetic isotope effects, but that is the focus I chose while developing it. The LO is suitable for junior or senior undergraduates in an organometallics course or unit within an inorganic course.

I would like to acknowledge Ryan Pakula and Joanne Redford from my Chem 165 course in 2008 who wrote early versions of some of the questions about vibrational states, and a careful critical read by Nancy Williams, who understands this stuff at a much deeper level than I do.

AttachmentSize
File Jones and Feher questions85.35 KB
Learning Goals: 

upon completing this LO students should be able to
:
1. calculate and interrelate reduced mass, vibrational frequency, force constant, and zero point energies for vibrational states of bonds
2. define kinetic isotope effects (normal, and inverse)
3. calculate/predict/estimate a normal and inverse KIE for a chemical reaction from IR data.
4. interpret and describe a reaction coordinate diagram for a chemical reaction
5. count and classify metal complexes using CBC method

Implementation Notes: 

I used this LO as a guided reading handout for a senior-level organometallics class. The questions and the paper were provided to the students a week in advance and the in-class activity was a student-led discussion of the paper.

Time Required: 
1 50-75 minute class period for discussion
Evaluation
Evaluation Methods: 

This LO was used in class to help a student guide a discussion of the paper. We did not cover all of the LO in a 75 minute class period, as we let the discussion take us where it wanted to. 


A better way to ensure student preparation would be to collect the questions at the beginning (or end, if they wanted to use their notes in class) of class to ensure that they had really studied the paper.

Evaluation Results: 

I used this LO as a guided reading handout and did not collect the answers so I do not have any assessment data at this time.

Although my students found this paper to be relatively dense and hard to follow at times. The paper separates out the Results and Discussion sections, so at times it seems repetitive. However, once they had worked their way through the paper, the students found the results interesting and the methods informative. We were able to discuss it at a high level in class.

Creative Commons License: 
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