Acidity Trends in α,β‐Unsaturated Sulfur, Selenium, and Tellurium Derivatives: Comparison with C‐, Si‐, Ge‐, Sn‐, N‐, P‐, As‐, and Sb‐Containing Analogues

Abstract

The gas-phase acidity of CH3-CH2XH (X=S, Se, Te), CH2=CHXH (X=S, Se, Te) and PhXH (X=S, Se) compounds was measured by means of Fourier transform ion cyclotron resonance mass spectrometry. To analyze the role that unsaturation plays on the intrinsic acidity of these systems, a parallel theoretical study, in the framework of the G2 and the G2(MP2) theories, was carried out for all ethyl, ethenyl (vinyl), ethynyl, and phenyl O-, S-, Se-, and Te-containing derivatives. Unsaturated compounds are stronger acids than their saturated analogues, because of the strong pi-electron donor ability of the heteroatoms that contributes to a large stabilization of the unsaturated anions. Ethynyl derivatives are stronger acids than vinyl compounds, while phenyl derivatives have an intrinsic acidity intermediate between that of the corresponding vinyl and ethynyl analogues. The CH2=CHXH vinyl compounds (enol-like) behave systematically as slightly stronger acids than their CH3-C(H)X (keto-like) tautomers. Vinyl derivatives are stronger acids than ethyl compounds, because the anion stabilization attributable to unsaturation is greater than that undergone in the neutral compounds. Conversely, the enhanced acidity of the ethynyl derivatives with respect to the vinyl compounds is due to two concomitant effects, the stabilization of the anion and the destabilization of the neutral compound. The acidities of ethyl, vinyl, and ethynyl derivatives containing heteroatoms of Groups 14, 15, and 16 of the periodic table are closely related, and reflect the differences in electronegativity of the CH3CH2-, CH2=CH-, and CH[triple chemical bond]C- groups.

Unexpected Gas-Phase Ion Chemistry Results Unraveled by Computational Chemistry

“…This seems to be however, a quite general phenomenon and similar gas-phase acidity enhancements have been reported for a large set of , -unsaturated amines, phosphines [73], arsines [74], silanes, germanes, stannanes [75], thiols, selenols and tellurols [76], both from the experimental Fourier transform-ICR mass spectrometry and the theoretical viewpoints. Conversely, the enhanced acidity of the ethynyl derivatives with respect to the vinyl compounds is due to two concomitant effects, the stabilization of the anion and the destabilization of the neutral [76]. The origin of the acidity increase on going from saturated to unsaturated compounds is associated with the enhancement of the -delocalization in the anions with respect to neutrals when the heteroatoms belong to groups 14, 15 and 16.…”

“…The first evidence of this interesting phenomenon was the dramatic acidity enhancement of ethynol with respect to its saturated counterpart ethanol, theoretically predicted by Radom and coworkers in 1989 [72]. Besides, theoretical calculations also predicted unsaturated stibines to exhibit the same behavior [76]. Besides, theoretical calculations also predicted unsaturated stibines to exhibit the same behavior [76].…”

The Ever‐Surprising Chemistry of Boron: Enhanced Acidity of Phosphine⋅Boranes

“…As a consequence, a great deal of effort was concentrated on determining intrinsic reactivities, in particular intrinsic basicities and acidities. [2][3][4][5] The gas-phase acidity of phosphines has been reported [6,7] and the role played by the substituent on the acidity of the molecule clearly evidenced. In particular the presence of an a,b-unsaturated substituent [6] with or without an electronwithdrawing group [5,7] led to a huge increase in acidity.…”

α,β‐Unsaturated and Saturated Derivatives of Be, Mg, and Ca: Are They Carbon or Metal Acids in the Gas Phase?

“…In addition, for derivatives that contain heteroatoms from groups 14-16, this acidity enhancement depends mainly on the nature of the unsaturated moiety and on the heteroatom, which is mirrored in the existence of good linear correlations between the gas-phase acidities of homologous compounds of these three groups. [32] However, primary saturated and a,b-unsaturated boranes, alanes, and galanes, in which the heteroatoms belong to a group to the left of carbon, were predicted to have quite peAbstract: The gas-phase acidity of R À XH (R = H, CH 3 , CH 2 CH 3 , CH= CH 2 , C CH; X = Be, Mg, Ca) alkalineearth-metal derivatives has been investigated through the use of high-level CCSD(T) calculations by using a 6-311 + GA C H T U N G T R E N N U N G (3df,2p) basis set. BeH 2 is a stronger acid than BH 3 and CH 4 for two concomitant reasons: 1) the dissociation energy of the Be À H bond is smaller than the dissociation energies of the B À H and C À H bonds, and 2) the electron affinity of BeHC is larger in absolute value than those of BH 2 C and CH 3 C. The acidity also increases on going from BeH 2 to MgH 2 due to these two same factors.…”

“…In our group, we have devoted some effort to establishing and analyzing the acidity trends of heterocompounds. The main conclusions of these studies were that a,b-unsaturated amines, phosphines, [30] arsines, [31] stibines, [32] silanes, germanes, stannanes, [33] thiols, selenols, and tellurols [32] exhibit enhanced acidity compared with the corresponding saturated derivatives. In addition, for derivatives that contain heteroatoms from groups 14-16, this acidity enhancement depends mainly on the nature of the unsaturated moiety and on the heteroatom, which is mirrored in the existence of good linear correlations between the gas-phase acidities of homologous compounds of these three groups.…”