Theoretical Study On The Bimolecular Nucleophilic Substitution Reaction At Nitrogen

Nucleophilic substitution reaction is one of the most fundamental chemicalreactions. Most work published so far were the nucleophilic displacementreactions at aliphatic carbon. In the last decade, SN2 reactions at neutral nitrogenbecame the focus of attention because of their synthetic, biochemical andtheoretical importance. SN2 reaction at nitrogen was studied theoretically andsystematically using the DFT methods. This paper is to explore the reaction rule,reveal some similarities and differences between SN2 reactions at nitrogen andcarbon, research the reaction mechanisms and investigate each of the reactionpaths. At the same time, energy barrier curves were estimated so that dynamic andthermodynamic parameters of chemical reactions and the relationships betweenenergy parameter and structure could be obtained. We hope the results can providesome theoretical bases for experimenters. According to the difference of the nucleophiles and the substrates involved inthe reactions, we divide our research project into three parts(In the followingcontext, Nu and Lg are used to denote nucleophile and substrate in order to beconvenient for depiction): First part is the reactions of halogen anion Nu- acted as nucleophile and neutralsubstrate NH2Lg. The nucleophile and leaving-group are all F-, Cl-, Br-, or I-.Theequation can be described as: Nu-+NH2Lg NH2Nu+Lg-(Nu, Lg= F, Cl, Br, I) iiThe second part, the reactions of halogen ion Nu- acted as nucleophile andN(CH3)2Lg. The nucleophile and leaving-group are also F-, Cl-, Br-, or I-. Theequation is: Nu-+N(CH3) 2Lg N(CH3)2Nu+Lg-(Nu, Lg= F, Cl, Br, I) The last, neutral nucleophile Nu reacts to neutral substrate NH2Lg. In this part,the nucleophile is ion-pair and the Lg concludes: F-, Cl-, Br-, and I-.The expressionis as the following: MX+NH2Lg NH2X+MLg(X, Lg= F, Cl, Br, I M=Li) All calculations were performed using GAUSSIAN–98 package. Allelectron(AE) calculations were carried out for the first- and second- periodelements, while effective core potentials(ECP) were used for third- andfour-period elements. The molecular structures were optimized using the DFTmethods. In specific, we studied the bimolecular nucleophilic substitution reaction atnitrogen from the following aspects: 1. Fix the substrate and change the nucleophile so as to investigate thenucleophilicity of nucleophile. 2. Fix the nucleophile and change the substrate to investigate the leaving abilityof the leaving-group. 3. Change the mediums of the reactions and research how the different solventsinfect the reaction path and the nucleophilicity of nucleophile and the leavingability of the leaving-group. 4. Make researches on the steric effects of the nucleophilic displacement atnitrogen through changing the substitution group. 5. Compare some similarities and differences between SN2 reactions involvedanionand ion-pair, such as reactions X-+NH2Lg and LiX+NH2Lg.