tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Abstract

Despite the seeming similarity of the infrared (IR) spectra between tert-butyl cations (t-Bu(+)) in gaseous and condensed phases, there are important but so far unrecognized differences. The IR spectroscopic investigation of the hydrogen (H)-bonding of t-Bu(+) with the immediate environment together with the X-ray crystallographic data shows that one CH3 group of t-Bu(+) differs from the other two. In the Ar-tagged t-Bu(+) in vacuum, this group is predominantly polarized, showing three C-H stretch vibrations at 2913, 2965, and 3036 cm(-1) whereas the other two methyls are predominantly involved in strong hyperconjugation, yielding an intense triple IR band with a maximum at 2839 cm(-1). In a condensed phase, the bulk solvent effect promoted participation of the polarized CH3 group in additional hyperconjugation, decreasing its νCH3 frequencies by approximately 120 cm(-1), whereas frequencies of the other CH3 groups decreased by only ca. 4-10 cm(-1). This observation indicates that the influence of the condensed phase on t-Bu(+) stabilization is substantial. Thus, enhancement of H-bonding between t-Bu(+) and Anion(-) strengthens hyperconjugation and promotes further cation stabilization.

Stabilization of carbocations CH3+, C2H5+, i-C3H7+, tert-Bu+, and cyclo-pentyl+in solid phases: experimental data versus calculations

“…The spectrometer was installed inside Scheme 2 Schematic representation of electron density redistribution in t-Bu + according to empirical data. 28 the dry box. The spectra were manipulated using GRAMMS/A1 (7.00) software from Thermo Scientific.…”

“…Our recent experimental study of the t-Bu + cation in condensed phases, together with interpreted IR spectra of gaseous t-Bu + , showed 28 that empirical findings contradict the generally accepted hyperconjugation mechanism (Scheme 1). It follows from the experiments that all C-H bonds of the two CH 3 groups of t-Bu + donate s-electrons to the carbon's 2p z orbital (Scheme 2), and the third CH 3 group is mostly affected by polarization.…”

“…It leads to the strengthening of hyperconjugation and contributes to the scattering of the positive charge to the environment. 17,28 In the present work we expanded the comparison of the computational results explaining stabilization of carbocations (structurally optimized and having the lowest energy) with empirical findings in IR spectra of carbocations in the series CH 3 + , C 2 H 5 + , i-C 3 H 7 + , t-Bu + , and cyclo-C 5 H 9 + , with the carborane counterions (CHB 11 Hal 11 À , Hal = F, Cl; hereinafter abbreviated as {Hal 11 À }, see Fig. 1), whose conjugated acids are the strongest pure Brønsted superacids available today.…”

“…Despite the importance of intermolecular hydrogen bonding for carbocation stabilization in the condensed phase, this effect will not be examined here in detail because it does not change the hyperconjugation mechanism when going from vacuum to a condensed phase as shown previously. 17,28…”

In Situ Separation of Chemical Reaction Systems Based on a Special Wettable PTFE Membrane

“…Due to its importance in synthetic industry, the reaction for producing 2‐bromo‐2‐methylpropane ( tert ‐C 4 H 9 Br) from tert ‐butanol ( tert ‐C 4 H 10 O) and hydrobromic acid (HBr) is chosen as a typical example ( Figure a). It is one of the most classic nucleophilic substitution reactions to synthesize alkyl halides and the mechanism can be divided into three steps (Figure S5, Supporting Information) . Hereinto, Step 2 is the rate‐determining step but sulfuric acid (H 2 SO 4 ) can be induced to strongly accelerate the dehydration to generate tert ‐butyl carbocation, which enables the reaction to complete within 1 min.…”