Date of Award

Spring 2013

Document Type

Restricted Thesis

Terms of Use

© 2013 Brett D. McLarney. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts

Department

Chemistry & Biochemistry

First Advisor

Paul R. Rablen

Abstract

SN2 and E2 reactions are among the most studied and most important reactions in organic chemistry. In this study, energies and optimized geometries for the reactants, products and transition state species of cyanide and chloride promoted SN2 and E2 reactions were obtained via ab initio calculations on a set of ~35 alkyl chlorides. The data were examined through the lenses of Marcus theory and Hammond's postulate to rationalize trends in activation energy (ΔG‡) and the placement of the transition state along the reaction coordinate with geometric properties of the transition state. It was found that while Marcus theory is highly accurate in predicting SN2 activation energies and, in some instances, transition state geometries, Hammond's postulate provided a qualitative rationalization for only a loose, positive correlation between ΔG‡ and ΔG° in the E2 reactions. The wealth of data collected also allowed for the examination of SN2/E2 competition and the benchmarking of effects on the ΔG‡ by methylating, fluorinating, chlorinating, and cyanating at the alpha carbon (Cα). It was found that in the sequence {MeCl, EtCl, i-PrCl, t-BuCl}, with each successive methylation, the E2 reaction was promoted by an energetic amount of 3.6 kcal/mol, a value tantamount to the magnitude by which the SN2 reaction was hindered (3.5 kcal/mol). Fluorination and chlorination at Cα destabilized SN2 transition states to a similar degree as methylation, while cyanating at Cα stabilized SN2 transition states by 2.6 kcal/mol per cyanide on average.

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