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Influence of Intramolecular Hydrogen Bonding on Ability of Leaving Group in SN2 Reactions

Ahmadi, Aliakbar | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43914 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Fattahi, Alireza
  7. Abstract:
  8. Many of the significant reactions in chemistry and biology are related to bimolecular nucleophilic substitution (SN2) reactions which have been studied first by Hughes and Ingold in the 1930s. Afterward, many organic chemists have continued this work. Up to now, numerous experimental kinetic and theoretical researches have studied the mechanism of these reactions. Since most SN2 reactions occur in solution, a large number of these studies have been devoted to the solvent effects. One of these effects that has been examined extremely is H-bonding via protic solvents that lowers the energy of an anionic nucleophile relative to the transition state, in which the charge is more diffuse, and results in an increase of activation energy.
    In this study, computational investigations on the gas phase SN2 reactions of X ions ( X = OH , F , Cl , Br , and I ) with fluoroethane and 2-fluoroethanol (β-hydroxyl ethyl fluoride) were performed by the B3LYP/6-311++G** level of theory. To better understand the nature of the corresponding intermolecular interactions, we performed natural bond orbital (NBO) analysis and Bader’s quantum theory of atoms in molecules (QTAIM). In our calculations, we took into account two possible pathways for these reactions. In path (i), β-hydroxyl group participates in hydrogen bonding (H-bonding) with fluoride ion as leaving group in transition state. However, in path (ii), transition state is including H-bonding between hydroxyl group and nucleophile X . Results indicate that in comparison with absence of β-OH substituent, the activation barrier has decreases in path (i) but increases in path (ii). The progress of each pathway depends on H-bonding character of nucleophile and leaving group. In addition, for X = Cl we compared β-OH group effect in the gas phase with that dimethyl sulfoxide (DMSO)
  9. Keywords:
  10. Natural Bond Orbital (NBO) ; Density Functional Theory (DFT) ; Atom in Molecule Method ; Quantum Theory ; Bimolecular Nucleophilic Substitution ; Fluoroethanol

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