A Mechanism Study On The Nitration Of Arenes In HFIP

Organofluoro compounds are physically and chemically different from the corresponding hydrocarbons.For example,the use of polyfluorinated alcohols(PFAs)as a solvent will affect the efficiency and rate of organic reactions,as well as the formation of intermediates such as cations,anions,and free radicals.It has been found that PFAs play a significant role to promote some types of organic reactions.Therefore,it is of great scientific significance to study the mechanism of traditional organic reaction in PFAs.The characteristics of PFAs are analyzed and summarized in this dissertation.For example,PFAs have the special aggregation effect and electronic effect,resulting in being a very strong hydrogen bond donor to prolong the stability of cation greatly.On the other hand,on the basis of the relatively low boiling point and high chemical stability,PFAs have the advantages to be simply recovered after used as an organic solvent in organic synthesis.Therefore,the purpose of this dissertation is to study the electrophilic substitution of aromatics in a PFA to obtain the information of intermediates for understanding the mechanism,and also to develop the efficient electrophilic substitution reaction system of aromatics with the use of the equivalent of electrophilic reagents.In this research work,1,1,1,3,3,3-hexafluoroisopropanol(HFIP)is chosen as the solvent to study the formation of intermediates in the nitration of arenes and the transformation process of the intermediates.Firstly,the structural stability of HFIP was examined in the concentrated HNO3,which is a strong oxidant and strong acid,to confirm the feasibility and safety of HFIP as a solvent for the nitration of arenes.On the basis of the theoretical calculation of the bond dissociation energy and activation energy of nitration of 2,2,2-trifluoroethanol,HFIP,and alkyl alcohols such as ethanol and isopropanol,it has been disclosed that the major reasons for the different chemical stability of the alcohols in the presence of concentrated nitric acid.Secondly,the nitration of arenes with the use of the equivalent of nitric acid in HFIP was studied.It was found that at room temperature,both electron-rich arenes,benzene and electron-deficient arenes show good reactivity,and the nitrated products could be obtained in high yields when the equivalent of the concentrated nitric acid(>90%)was used,so as to develop an efficient nitration system for arenes.More importantly,in HFIP,it was found that the ?-complex intermediate of an arene with a nitronium ion can be directly observed by UV-Vis spectra.The UV-Vis spectra of the possible intermediates in the reaction process including the ?-complex and ?-complex intermediates are then calculated by using TD-DFT,and the predicted spectra support the experimentally observed spectra,which are resulting from the formation of the ?-complex intermediates of an arene with nitronium ion.These obtained results verify that the cationic intermediates in nitration process of arenes are greatly stabilized by HFIP.