Theoretical Study Of Nucleophilic Substitution At Sulfur And Oxygen

The nucleophilic substitution reactions at sulfur and oxygen have been studied theoretically and systematically. This paper is to research the reaction mechanisms, investigate each of the reaction paths and reveal some similarities and differences between these reactions. According to the difference of the nucleophiles and the substrates involved in the reactions, we divide our research project into three parts: The first part: the gas-phase reactions X─+ CH3SY (X, Y = F, Cl, Br, I) have been investigated in chapter 2. In the gas-phase reactions X─+ CH₃SY (X, Y = Cl, Br, I), attacking at the central sulfur is kinetically and thermodynamically preferred over attacking at the terminal carbon and the nucleophilic substitution reactions at sulfur happen, following an addition-elimination (A-E) pathway. The fluorine differently behaves in many respects from the other halogens and the reactions F─+ CH₃SY (Y = F, Cl, Br, I) correspond to deprotonation instead of substitution. However, the reactions X─+ CH₃SF (X = Cl, Br, I) follow an A-E pathway other than the last two out going steps which proceed via a deprotonation. The second part: the gas-phase reactions LiX + CH3SY →CH3SX + LiY (X, Y = F, Cl, Br, I) were studied in chapter 3. Two possible reaction pathways (inversion and retention) were discussed. As a result, inversion pathway is implied to be much more favorable for all halogens, which is consistent with ion-pair SN2 reactions at nitrogen. The third part: the gas-phase reactions X─+ CH3OY →Y─+ CH3OX (X, Y = Cl, Br, I) have been investigated in chapter 4. Calculated results indicate that X─(X = Cl, Br, I) attacks preferably oxygen atom instead of carbon atom of CH3OY and nucleophilic substitution reaction at oxygen happens via a SN2 mechanism. We hope the results can provide some theoretical bases for experimenters.