Acceptor strength

Regularities are more apparent when the acceptor atom remains constant and the attached groups are varied e.g., for all ligands so far studied the acceptor strength diminishes in the sequence... 

There is a similar trend relating t/(C—O) to the 7r-acceptor strength of X (Figure 2.74). 

The data indicate that the formation of cyclic intermediates creates a stabilization of the cationic chain ends (AH° < 0 and AH s < 0), also expressed by a decrease of both the acceptor strength (Ae(LUMO) > 0) and the donor strength (Ae(HOMO) < 0) of the cations. The positive charge of the cationic centre is distinctly decreased (Aqc+ < 0) as a consequence of the interaction of this centre with the oxygen of the methoxy group. A partially covalent C + —O-bond is formed (pt Q(f) > 0.6 rc+ 0if) 146 pm). 

HYBOT will compute hydrogen bond donor (SCa) and acceptor (2Ca) factors that describe the donor and acceptor strengths, respectively, of a compound. Using these two parameters, the authors developed models with significant statistical quality (r2 = 0.91, q2 = 0.87) of intestinal absorption. 

H-bond acceptor strength Drug MW Formula EU Reference... 

Substituents modulate the electron-donor or acceptor strengths depending upon their electron-releasing or electron-withdrawing properties as evaluated by the Hammett a+ (or a) constant(s).62 For example, the multiple substitution of methyl groups on ethylene leads to the strong re-donor 2,3-dimethyl-2-... 

The acceptor strength is quantitatively evaluated by the energetics of the reductive conversion of an organic acceptor (A) to its anion radical (A- ) and it is most readily evaluated in solution (Table 4) by the reversible potential E for one-electron reduction,63 i.e.,... 

Figure 16a shows the progressive bathochromic shift in the CT absorption bands (hvct) obtained from PyN02+ with aromatic donors with increasing donor strength (or decreasing ionization potential). A similar red shift is observed in the CT absorption bands (hvCj) of hexamethylbenzene complexes with various para-substituted JV-nitropyridinium cations (X-PyNO ) as shown in Fig. 16b. Such a trend in the hvct is in accord with the increasing acceptor strength of X-PyNO in the order X = OMe < Me < H < Cl < C02Me < CN.

It is particularly noteworthy that the acceptor strengths of nitropyridinium cations as measured by the oxidation potential (Ep) parallel the acidities of the corresponding hydropyridinium cations (pXa) as shown in Fig. 17.235a Such a remarkable correlation derives directly from the interconvertibility of oxidants and acids (Chart 6)... 

Moreover, the thermal nitration of various aromatic substrates with different X-PyNO cations shows the strong rate dependence on the acceptor strength of X-PyNO and the aromatic donor strength. This identifies the influence of the HOMO-LUMO gap in the EDA complexes (see Chart 3), and thus provides electron-transfer activation as the viable mechanistic basis for the aromatic nitration. Indeed, the graphic summary in Fig. 18 for toluene nitration depicts the isomeric composition of o-, m- and p-nitrotoluene to be singularly invariant over a wide range of substrate selectivities (k/kQ based on the benzene... 

Aromatic nitrosation with nitrosonium (NO + ) cation - unlike electrophilic nitration with nitronium (NO ) cation - is restricted to very reactive (electron-rich) substrates such as phenols and anilines.241 Electrophilic nitrosation with NO+ is estimated to be about 14 orders of magnitude less effective than nitration with N02+. 242 Such an unusually low reactivity of NO+ toward aromatic donors (as compared to that of NO ) is not a result of the different electron-acceptor strengths of these cationic acceptors since their (reversible) electrochemical reduction potentials are comparable. In order to pinpoint the origin of such a reactivity difference, let us examine the nitrosation reaction in the light of the donor-acceptor association and the electron-transfer paradigm as follows. 

A simple model, known as the Internal equivalent field model (9) (see "Figure 1") accounts for the different p values of mono-substltuted benzene derivatives R-X, and relates them to the respective donor or acceptor strength of X. It Is assumed In this model that the substituent action on the it electrons of the ring Is equivalent to that of a D-C field EQ with direction and Intensity related to the substituent electronegativity. Identifying dipole expansions of R-X In presence of E and of R In presence of E0 + E yields ... 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

Thus, among possible binary complexes that can be formed from these monomers, the H3N- HF (best donor, best acceptor) complex is expected to be strongest, whereas the inverted HF- -HNH2 (worst donor, worst acceptor) complex should be weakest. These extreme differences in donor-acceptor strength are consistent with the wide disparity in H-bond energies (5.35a) and (5.35b). 

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