Electron acceptor strength

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. 

The precursor dithiocarbonates were also used to prepare mesomorphic derivatives of TTF (tetrathiafulvalene), a strong electron donor compound used in the formation of highly conductive charge transfer complexes. Attempts to prepare CT complexes between the mesomorphic dithiolenes as acceptors and these mesomorphic TTF donors showed that the electron acceptor strength of this type of dithiolene does not suffice to form strong donor-acceptor complexes mixtures of the two components form mixed crystals without any visible degree of charge transfer.208... 

The theoretical models discussed above indicate that the sulfonyl group, although slightly weaker in electron acceptor strength, is indeed a viable alternative to the nitro group. In particular, sulfonyl derivatives of stilbene and azobenzene display large molecular hyperpolarizabilities and can be used as bifunctional chromophores for the construction of materials with nonlinear optical properties. 

The rapid hydrolysis by water could indicate that these halides are stronger Lewis acids than BF3. In fact, the molar heats of solution of the trihalides in nitrobenzene and the heats of reaction with pyridine in nitrobenzene show that under these conditions the electron-acceptor strength decreases in the order BBr3 > BC13 > BF3. 

X 10 esuand66 x 10 esu for the formyl and dicyanoethynyl species, respectively. The larger value for dicyanoethynyl compound is attributed to its higher relative electronic acceptor strength. 

Recently, a new concept in fixed film reac tors that uses an expanded or fluidized bed of particdes as the biomass support medium has been introduced. This reactor type can easily handle both low- and high-strength wastes with most electron acceptors. It will be discussed in detail in a later section. 

TABLE 25-41 Favorable (F) and Unfavorable (U) Combinations of Electron Acceptor, Waste Strength, and Reactor Type... 

Increasing catalyst surface work function causes an increase in the heat of adsorption (thus chemisorptive bond strength) of electropositive (electron donor) adsorbates and a decrease in the heat of adsorption (thus chemisorptive bond strength) of electronegative (electron acceptor) adsorbates. 

That the synergistic action of an electron donor (Na8+) and electron acceptor (O5") promoter can cause dramatic enhancement in rate and selectivity. This is very likely due to the increase in the field strength, E, of the effective double layer discussed in Chapters 5 and 6 and to the concomitant enhanced interaction with the adsorbate dipoles, leading to more pronounced promotional behaviour (Chapter 6). 

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