Basis and Limitations of the Reactivity‐Selectivity Principle

Abstract

The reactivity-selectivity principle (RSP), which describes a linear relationship between the decrease in selectivity and increase in reactivity of molecules X , in their reactions with Y1 and Y2, has been much criticized during the last few years. The present paper shows the kind of reactions where the RSP does not hold and reveals how the individual types of reaction can be distinguished by varying the solvent. The dependence of molecule selectivity on temperature produces a further effect. An isoselective relationship (ISR) can be deduced when this dependence is taken into account. This relationship is not concerned with the connection between the temperature-dependent rate and competition constants, but with the connection existing between activation enthalpies and activation entropies, which are temperature-independent over wide ranges. The application of the ISR to different types of reactions makes it possible to characterize and distinguish between short-lived intermediates and underlines the significance of the isoselective temperature ( Ts) in interpreting the reaction parameters of linear free energy relationships and in planning syntheses.

Ligand and Substrate Effects in Gas‐Phase Reactions of NiX+/RH Couples (X=F, Cl, Br, I; R=CH3, C2H5, nC3H7, nC4H9)

“…While for homolytic bond cleavages of primary versus secondary C À H bonds varying preferences have been noted previously, for example, in iron-and copper-mediated gas-phase oxygenation of propane, [31,32] the trend mentioned above for NiX + /propane is opposite to what is usually observed for radical-based processes and also what would be expected taking into consideration only the reactivity-selectivity principle. [33] 2) Dehydrogenation: While for all NiX + /RH systems studied, a combination of oxidative addition/reductive elimination steps occurs, though with varying importance, this bond-activation mode dominates the ion-molecule reactions of the couples NiBr + /RH (R = C 2 H 5 , nC 3 H 7 ) and NiI + /RH (R = C 2 H 5 , nC 3 H 7 , nC 4 H 9 ), Table 1. The investigation of the labeled substrates 2, 3, 4 a, and 4 b reveals several interesting mechanistic details.…”

Brønsted βNuValues and Leaving Group Effects in SN2 Reactions. Tests of the Reactivity—Selectivity Postulate

“…The radicals (or carbenes) will exhibit (+) RSP behavior below this temperature and (-) RSP behavior at higher temperatures [27]. Similar problems can be expected for RSP competitions for free radicals and carbenes.…”

The SN2 Transition State. 6: Breakdown of the Reactivity–Selectivity Principle in the SN2 Reaction of α‐Halocarbonyl Compounds. A Molecular Orbital Analysis [1]

“…(l)) of o-carbonyl substrates with four different nucleophile-Ieaving group pairs (X·-rr, F-, BH 4 ,NH 3 ) [11 ] . The geometries have been extensively optimized with respect to minimum energy, with the following constraints: (a) T8 structures were located by imposing equal C-X bond lengths as well as equal X-C-C angles, (b) The X-C-C-Odihedral angle was held fixed at 0°a nd 90°, respectively, corresponding to conformations of minimum ("off', la4a) and maximum ("on", overlap between the carbonyl 1r system and tpe lI'·type three-center orbital associated with the X-C-X (reaction coordinate) axis (Fig.…”

“…The reactivity-selectivity principle (RSP) has long been a cornerstone in intuitive mechanistic chemical thinking [2,3 J. Despite its wide scope and application, however, many failures of the RSP have been recorded [2][3][4] .up to the point where its very existence has been cast in doubt [4].…”