Theoretical studies of thermal syn elimination reactions
Erickson, Jon A.
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Permalink
https://hdl.handle.net/2142/21747
Description
Title
Theoretical studies of thermal syn elimination reactions
Author(s)
Erickson, Jon A.
Issue Date
1992
Doctoral Committee Chair(s)
Kahn, Scott D.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
An ab initio molecular orbital study of two classes of thermal syn elimination reactions was undertaken in order to provide insight into the mechanism of these reactions. In general, thermal syn elimination reactions involve a one step process with a cyclic transition structure. The first class studied is termed 1,5 thermal eliminations and involve a six-membered ring transition structure. In particular, the reactants, transition structures and product geometries of the ethyl formate, phosphinate, and xanthate were optimized at the RHF/6-31G* level. In addition, MP2/6-31G* energies and RHF/6-31G* electrostatic potentials were calculated. Trends in calculated activation energies and electrostatic potentials agree with experimental results. It was determined that the C$\sb\alpha$-O bond appears to be breaking first in the reaction. It was also found that the effect of polar substituents agree with experimental results. Electron withdrawing groups in the $\beta$ position were found to change the order of bond breaking to C$\sb\beta$-H bond dissociation while $\alpha$ electron donating groups significantly increase the C$\sb\alpha$-O bond length in the transition structure.
The second class of elimination reactions involves those which go through a five-membered ring transition structure and are termed 1,4 thermal eliminations. These reactions were studied in a similar manner as the 1,5 class. The reactions examined were the thermal eliminations of amine oxides, sulfoxides, selenoxides and telluroxides at the RHF/6-31G* and/or the RHF/3-21G(*) levels. Again the calculated activation energies and electrostatic potential agreed with experimental observations. Unfortunately, rate trends did not correlate with transition structure properties. It was found, however, that polar substituents had the same effect on the transition structures as in the six-membered ring transition structure class.
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