62nd Annual Report on Research 2017

The goal of the project is to identify and characterize the “true” intermediates in Friedel-Crafts alkylation reactions, as well as the associated ionization processes. The proposed intermediates in these reactions are alkyl halide – Lewis acid Complexes (RX’–MX₃) and in the initial phases of this work, we explored the structural and energetic properties of several such complexes (M = B, Al, Ga; X/X’ = F, Cl; R = CH₃) using density functional theory. During the award period, we conducted an extensive computational study, and not only obtained structures and binding energies, but also M-X’ potential curves in the gas-phase and bulk dielectric media.

Further computational efforts were focused on exploring the relative energies of two distinct ionization pathways that lead to the key electrophiles in these processes. The first involves a single alkyl halide, e.g.,

RX + MX₃ —> R⁺ + MX₄^– (1)

and could presumably proceed through the 1:1 complex intermediates noted above. The other pathway involves a second alkyl halide intermediate, and leads to the formation of a dialkyl halonium ion, e.g.,

2 RX + MX₃ —> R₂X⁺ + MX₄^– (2)

We have found that (2) is a much lower energy process, both in the gas-phase and in bulk dielectric media, for every permutation of M, X and X’ we have studied.

For example, for the most common reactions involving an alkyl chloride and AlCl₃, e.g.

CH₃Cl + AlCl₃ —> CH₃⁺ + AlCl₄^– ΔE₁ (3)

2 CH₃Cl + AlCl₃ —> (CH₃)Cl⁺ + AlCl₄^– ΔE₂ (4)

we find that ΔE₁ is +154.7 kcal/mol while ΔE₂ is +86.1 kcal/mol, roughly 70 kcal/mol lower in energy. (These particular results are from M06/aug-cc-pVTZ calculations.) In a bulk dielectric medium with ε=10 (PCM/M06/aug-cc-pVTZ), the reaction energies are significantly lower, and ΔE₂ becomes slightly exothermic; ΔE₁ is +51.2 kcal/mol while ΔE₂ is -1.0 kcal/mol.

For the analogous CH₃F/BF₃ processes, e.g.,

CH₃F + BF₃ —> CH₃⁺ + BF₄^– DE₁ (5)

2 CH₃F + BF₃ —> (CH₃)F⁺ + BF₄^– DE₂ (6)

which are more closely related to our previous work in these systems, we find that ΔE₁ is +182.7 kcal/mol while ΔE₂ is +134.5 kcal/mol, roughly 50 kcal/mol less (via M06/aug-cc-pVTZ). Again, in a bulk dielectric medium with ε=10 (PCM/M06/aug-cc-pVTZ), the reaction energies are significantly lower; ΔE₁ is +67.3 kcal/mol while ΔE₂ is +32.4 kcal/mol. We are currently in the process of validating these results with high-level post Hartree-Fock methods, as well as exploring the free energy changes associated with these reactions, which will provide direct insight into their relative thermodynamic feasibility.

Meanwhile, our experimental efforts were focused on obtaining low-temperature IR spectra of RX/MX₃ mixtures, in hopes of spectroscopically identifying and characterizing the key intermediates in situ, but in the absence of the aromatic substrate that would be present in an actual reaction mixture. To this end, we designed and construct a short-path (0.01 – to 0.1 mm), liquid IR cell with will be housed inside the vacuum chamber of our optical cryostat apparatus, such that it could be filled by condensing RX’ and/or MX₃ vapor from ambient temperature bulbs located outside the main vacuum chamber. We were able to successfully build a cell of this type, and record spectra of both (CH₃)₂CHF (l) and BF₃ (l) at ~140 K, but we were not able to produce a mixture of these components inside the cell that exhibited any distinct spectral features that were not present in the single component spectra.

In terms of scholarly products and contributions to student development, we note that 12 undergraduate students and one high school student worked on these projects during the award period, and of the five UWEC student who recently graduated, 4 are in graduate programs (2 MS, 2 PhD). Eight of these 13 students were female, three were from underrepresented ethnic/racial groups, and two were international students. IN addition, we note that these students have been the most successful in group history in terms of presentations. There were a total of 26 off-campus student presentations (both poster and oral) during the award period – over half of the PI’s career total – including one by a student who was selected for the “Undergraduate Physical Chemistry Workshop” at the 2015 Fall ACS meeting. Moreover, there are five research publications, in print, and two in review.