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William F. Coleman Wellesley College Wellesley, MA 02481
Diels-Alder Reactions and the Structure of Transition States William F. Coleman Department of Chemistry, Wellesley College, Wellesley, Massachusetts 02481
[email protected] w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jchemeduc. n
JCE Featured Molecules for November 2010
The JCE Featured Molecules (1) for this month come from the paper by Tevye Celius, in which he introduces several interesting Diels-Alder reactions that proceed sufficiently quickly to allow for running the reactions and performing spectroscopic characterizations in a single lab period (2). Included in the molecule collection are PTAD (the dieneophile in the reactions), 4-phenylurazole (the starting material for the synthesis of PTAD), cycloheptatriene, norcaradiene, cyclopentadiene, and molecules 2, 3, and 4 in the article (2). Table 1 lists the 3D, rotatable images in MOL format of the molecules available in the HTML version of this column. Scheme 1 shows the reaction of PTAD with cyclopentadiene and cycloheptatriene.(2) An interesting way to extend Celius's experiment would be to consider follow-up exercises or problems in a subsequent physical chemistry or computational chemistry course. Some examples of such problems might include the following: 1. The rings in 4-phenylurazole and PTAD are not coplanar. How does the dihedral angle between the two rings vary with the level of calculation from semiempirical to Hartree-Fock to density functional? Within HF and DFT methods, what are the effects of basis set on that angle? 2. The product of the reaction between PTAD and cycloheptatriene may have come as a surprise. Why is molecule 4 in the paper the observed product? Compute the energies of molecules 3 and 4 at a moderately high level of theory. Can the reaction be explained based on these energies? Compute the transition states for the two reactions pathways. What are the relative activation energies? Can you confirm that your computed transition state seems appropriate for the reaction that you are considering? How
would you describe the reaction between cycloheptatriene and PTAD in terms of the roles played by thermodynamics and kinetics?
Figure 1. The valence tautomerism of cycloheptatriene was studied at the HF-6311þþG(d,p) level and the results of the calculation are shown. This reaction offers a useful introduction to transition state calculations and verification of the transition state, as the reaction is sufficiently simple for students to make good a priori predictions. Scheme 1. Reaction of PTAD with (top) Cyclopentadiene and (bottom) 1,3,5-Cycloheptatriene from Ref 2
Table 1. JCE Featured Molecules for November 2010 Featured Molecule
Description
PTAD
dieneophile in the reactions; molecule 1 in ref 2
4-phenylurazole
starting material for the synthesis of PTAD
molecule 2
molecule 2 in ref 2
molecule 3
molecule 3 in ref 2
molecule 4
molecule 4 in ref 2
cycloheptatriene cycloheptatriene transition state
structure of the computed transition state of the tautomerism of cycloheptatriene
norcaradiene cyclopentadiene
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Vol. 87 No. 11 November 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed1009078 Published on Web 09/13/2010
On the Web
The valence tautomerism of cycloheptatriene was studied at the HF-6311þþG(d,p) level. This reaction offers a useful introduction to transition state calculations and verification of the transition state, as the reaction is sufficiently simple for students to make good a priori predictions. The results of the calculation are shown in Figure 1, and the structure of the computed transition state is included in the molecule collection. The transition state is a saddle point on the potential energy surface of the reaction, and, as such, has a single imaginary frequency associated with it. The vibrational mode corresponding to that imaginary frequency should be consistent with the reaction being studied. That mode is shown for this particular
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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reaction in the video embedded in the HTML version of this column. The mode is clearly moving in the direction of the formation of the three-membered ring in norcaradiene. Literature Cited 1. JCE Featured Molecules from Jun 2002 through Dec 2009 are available at the JCE Digital Library, http://www.jce.divched.org/ JCEWWW/Features/MonthlyMolecules/ (accessed Sep 2010). JCE Featured Molecules from Jan 2010 to the present are available in the HTML version of each column. 2. Celius, T. C. J. Chem. Educ. 2010, 87, DOI: 10.1021/ed100344g.
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Vol. 87 No. 11 November 2010
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