Halogenation Thermodynamics of Pyrrolidinium-Based Ionic Liquids

TL;DR

This study investigates the thermodynamics of halogenation reactions in pyrrolidinium-based ionic liquids, revealing preferences for fluorination over chlorination and bromination, and establishing correlations with electronic density for predicting reaction pathways.

Contribution

It provides the first detailed gas-phase thermodynamic data for halogenation of pyrrolidinium cations, enabling better design of tunable ionic liquids.

Findings

Fluorination is more thermodynamically favorable than chlorination and bromination.

Reaction site differences are modest but often exceed thermal energy at simulated temperatures.

Electronic density correlates with reaction thermodynamics, aiding pathway prediction.

Abstract

Room-temperature ionic liquids (RTILs) exhibit large difference between melting and boiling points. They are highly tunable thanks to numerous accessible combinations of the cation and the anion. On top of that, cations can be functionalized using methods of organic chemistry. This paper reports gas-phase thermodynamics (enthalpy, entropy, Gibbs free energy) of the halogenation reactions (fluorination, chlorination, bromination) involving protonated pyrrolidine C4H10N+, protic N-ethylpyrrolidinium C4H9N(C2H5)+, and aprotic N-ethyl-N-methylpyrrolidinium C4H8N(CH3)(C2H5)+ cations. Substitution of all symmetrically non-equivalent hydrogen atoms was compared based of the thermodynamic favorability. Fluorination of all sites is much more favorable than chlorination, whereas chlorination is somewhat more favorable than bromination. This is not trivial, since electronegative fluorine and chlorine have to compete for the already insufficient number of electrons with other atoms belonging to the pyrrolidinium-based cations. The difference between different reaction sites within the cations is modest, although it often exceeds kT at simulated temperatures. The correlation between thermodynamics and electronic density distribution has been established, which allows new simple prediction of the reaction pathways. The reported results inspire further chemical modifications of the pyrrolidinium-based RTILs to achieve ever finer tunability of physical chemical properties.