Computational Chemistry Studies of [1,7] Sigmatropic Hydrogen Shift In Z,Z-1,3,5-Heptatriene Systems and Esterification Reactions

Macharia, Kibe Godfrey (2009) Computational Chemistry Studies of [1,7] Sigmatropic Hydrogen Shift In Z,Z-1,3,5-Heptatriene Systems and Esterification Reactions. PhD thesis, Kenyatta University.

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Abstract

Density functional theory has been used to study mechanisms of the reactions involving [1,7]-sigmatropic hydrogen shift in Z,Z-1,3,5-heptatriene systems, esterification of acetyl chloride with methanol and acid-catalyzed esterification of carboxylic acids with methanol. The calculations employed the B3LYP functional, with 6-31+G(d) basis sets. Mechanism routes were computed, with complete optimization of all intermediates and transition states. The effect of electron-donating and electron-withdrawing substituents in Z,Z-1,3,5-heptatriene systems was investigated. Analysis of the geometries, energies and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted cases provided insights into substituent effects of the reactions. The study revealed that the inductive and mesomeric effects of heteroatoms or heterosubstituents are of great importance in the energetics of the transformation. Steric effects also play an important role due to the geometrical constraints of the reaction. Generally increasing the electron density of the  system, decreases the electron density on the protropic proton resulting in the decrease of the activation energy. For the esterification of acetyl halides with methanol, the calculations suggest that the reaction proceeds through a loose transition state with a concerted SN2 mechanism. The nucleophilic methanol attacks the carbonyl carbon at Bürgi-Dunitz trajectory and the interaction is subject to general base catalysis, either by a second molecule of methanol or a solvent molecule. This explains the mixed reaction kinetics observed from experimental data. The calculations suggest that the acid-catalyzed esterification of carboxylic acids with methanol involve interaction of both the proton and the counter ion. The calculated activation parameters in the solvent model are in excellent agreement with experimental results. The computational results for the methanolysis of acetyl chloride and acid-catalyzed esterification reactions call for a re-evaluation of some commonly accepted classical mechanistic pathways. For the reactions investigated in this study, the calculations revealed a new mechanistic insight of the reaction pathway.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QD Chemistry T Technology > TP Chemical technology
Divisions:	Africana
Depositing User:	Tim Khabala
Date Deposited:	12 Sep 2017 11:38
Last Modified:	12 Sep 2017 11:38
URI:	http://thesisbank.jhia.ac.ke/id/eprint/2008

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