The Transmission of Polar Effects Polar Effects. Part II

Abstract

SummaryThe pK: values for nine acyclic, mono-and bicyclic a-, p-and y-(trimethy1)ammonio acids, 1 (a-y), 2 (a-y) and 3 (u-y), respectively, have been measured in 50% aqueous ethanol. The comparison of these values with those for the nine homomorphous a-, p-and y-acids, 4(a-y), 5(a-y) and 6(a-y), respectively, permits an evaluation of the steric effect of the (trimethy1)ammonio groups which is superimposed on their polar effect in the former acids. The pKl values, corrected for the steric effect, correlate very well with the reciprocal distance between the positive nitrogen atom and the dissociable proton. This confirms our earlier conclusion that the strength of these acids is determined by the field effect of the (trimethy1)ammonio groups only, the inductive model failing to account for the results. The distinction between two different transmission mechanisms for the polar effect therefore becomes unnecessary, the inductive model appearing as an atomistic description of the field effect.One of the prominent effects which determine the reactivity of organic molecules can be traced to the presence of charged or polar substituents. It is, therefore, of electrostatic origin and referred to as the polar effect. Two modes of transmission of this effect are usually distinguished, namely through the intervening bonds and atoms and directly through space or solvent [ 11. According to the former inductive model the substituent effect is propagated by successive polarization of bonds, the magnitude of the effect depending on the length and number of chains. The direct or field effect, however, is dependent on the distance and orientation of the substituent with respect to the reaction center. This dichotomy of transmission mechanisms for the polar effect appears to stem from the notion, clearly stated by Ingold [ 11, that the inductive effect is transmitted internally, i. e. through the molecule and that the field effect is transmitted externally, i.e. through the space immediately enveloping the molecule. In practice it is extremely difficult to distinguish between these two modes of transmission, the relative importance of which is still under dispute').') For reviews see [2].

Switching radical stability by pH-induced orbital conversion

“…Interestingly, in these latter species the BDEswitch at 5 Å separation between the radical and the carboxylate group exceeds 20 kJ mol -1 , and linearly decays with 1/r, as could indeed be expected for a polar effect 18 . However, the energy breakdown reveals significant contributions of correlation and Hartree components to the magnitude of the BDE-switch, which also depend linearly on 1/r, non-surprisingly considering that the molecular Hamiltonian operator contains 1/r two-electron terms 20 .…”

“…Therefore, to determine whether this standard polar effect could account for the observed BDFEswitching, we employed the elegant procedure of Grob et al, who studied the effect of amino group protonation on the COOH pK a in the homologue series + NR 3 (CH 2 ) n COOH via comparison to a reference system CR 3 (CH 2 ) n COOH that is structurally very similar but lacks a polar contribution 18 . In our case we chose reference compounds that were structurally similar but lacked orbital conversion.…”

In‐Situ Electronegativity and the Bridging of Chemical Bonding Concepts

“…Figure 6 exemplifies our approach on four complex carboxylic acids 1 – 4 , whose p K a values have been determined in 50 wt% ethanol−water solution at room temperature. [102] One of the compounds ( 4 ) has two different p K a values reported in the literature, 6.6 [103] and 6.9, [102] but which one is more accurate? To find out, we can perform a linear regression between and p K a for compounds 1 – 3 and predict value of 7.0 for the p K a of 4 .…”

“…[107] The correct identification of HÀ N3 as the most acidic proton in 4) can be used to predict the experimentally uncertain pK a of 4 in ethanol/water solution 50 % w/w (○). Experimental data for 4 are indicated as a red square [102] (pK a = 6.9) and a blue triangle [103] (pK a = 6.6). The linear regression of compounds 1-3 is: pK a = À 2.9024 � c val H + 68.109. vacuum does not appear possible using atomic charge estimates (Table S7).…”

Conformational Preferences of 3-(Dimethylazinoyl)propanoic Acid as a Function of pH and Solvent; Intermolecular versus Intramolecular Hydrogen Bonding

“…The average p K a from the two groups were p K 1 = 3.11 ± 0.09 and p K 2 = 5.22 ± 0.11. The value of p K 2 is interesting when compared to the p K 1 of 4,4-dimethylpentanoic acid, 4.83, and 3- N , N , N -trimethylaminopropanoic acid, 3.27, and will be discussed further.…”