Lewis basicity

The Underlying Ligand Typology A Basic Lewis Analysis... 

The trends anticipated from basic Lewis and resonance theories are, of course, qualitative in nature. The three ligand types A, B, and C are now briefly described in decreasing order of their anticipated CT-donating vs 7i-accepting ability. 

The first perfluoroalkykadmium compounds were prepared via metathesis between bis(trifluaromethyl)mercury and dialkylcadmiums [12d] An equilibrium mixture of all possible cadmium and mercury compounds was formed, the equilibrium could be shifted toward the bis(tnfluoromethyl)cadmium by using excess bis(tnflu-oromethyl)mercury When the reaction was earned out in glyme, a stable bis(tnflu-oromethyl)cadmium gly ine complex was isolated [124, 125 126] (equation 96) Other analogues were synthesized and isolated in essentially quantitative yield via direct reaction of primary and secondary perfluoroalkyl iodides with di-alkylcadmiums in basic Lewis solvents [127 128] (equaUon 97)... 

It must also be noticed that the rehydrated Mg/Al-LDH, in spite of their moderate basic strength/1081 due to the Brpnsted nature of the active sites have been able to catalyse Michael additions normally requiring basic Lewis sites of high strength/301... 

If the definitions of nucleophiles and electrophiles sound similar to those given in Section 2.11 for Lewis acids and Lewis bases, that s because there is indeed a correlation between electrophilicity/nucleophilicity and Lewis acidity/basicity. Lewis bases are electron donors and behave as nucleophiles, whereas Lewis acids are electron acceptors and behave as electrophiles. Therefore, much of organic chemistry is explainable in terms of acid-base reactions. The main difference is that the terms nucleophile and electrophile are used when bonds to carbon are involved. We ll explore these ideas in more detail in Chapter 10. 

Adsorption at acidic sites M causes the solution to become acidic and adsorption of H on Lewis basic sites causes the solution to become basic. Lewis sites are important in two ways they contribute to the Helmholtz double layer, and they result in chemical adsorption and passivation of the intrinsic active surface sites. The surface of sihcon is dominated by basic Lewis sites as manifested by the strong hydrogen adsorption. But the associated surface states are not active because they are located energetically in the valence band. ... 

Pak has shown, by a correlation between O—H stretching frequency and the strength of the M—OH bond, that the Bronsted acidity of ZnO is similar to that of MgO and NiO and is much less than that of y-Al20a. A similar conclusion was reached from studies of benzene adsorption and of its effect on hydroxyl stretching frequencies, a for ZnO of 10.5 being reported. In general then we may expect ZnO to display basic/Lewis acid properties in catalytic reactions, as shown by MgO, on which carbanions are readily produced from both alkenes... 

P-22 - A DFT study of the cracking reaction of thiophene activated by zeotype catalysts role of the basic Lewis site... 

A DFT study of the cracking reaction of thiophene catalyzed by zeolite is reported. The use of different zeotype catalysts (viz. acidic, metal-exchanged and methoxy) has been shown to successfully induce this reaction. It appears that the thiophene cracking reaction is catalyzed by a basic Lewis site and that acid Bronsted site has only a limited influence on the reaction. 

Acidic/Basic Lewis acidity/basicity determines the solvent s abihty to demate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this acid/base pre ierty was proposed by Gutman (DN and AN -donor and acceptor number, reflectively) based on caleuimetric determination. The complete proton transfer reaction with femnatiem of protonated ions is determined by proton affinity, gas phase acidity, acid or base dissociation constants. Both concepts differ in terms of net chemicsd reactiem. 

Besides doping, co-doping in where two or more elements are present on the G sheet is also interesting from the point of view of using Gs as catalysts. One case that could be of interest is the presence of electron deficient atoms such as B and heteroatoms with excess of electrons such as N. In a certain way the simultaneous presence of B and N could be similar to the introduction of acid and basic Lewis sites. The concept of bifunctional acid-base solid catalyst has found wide application in heterogeneous catalysis of solids such as aluminophosphates containing NH groups (ALPONs) due to the cooperative activity of both sites in the reaction mechanism. 

It is now necessary to modify the basic Lewis formula to accommodate for operating conditions and temperatures when plastics gears are to be used. Figures 4.7-4.9 are used for this, and the Lewis formula is modified to become... 

Instead of direct proton transfer to a basic Lewis oxygen atom, coadsorbed H2O will more readily accept the proton and then transfer one of its other hydrogen atoms as a proton to the accepting oxygen atom. This two-step path can also be carried out simultaneously, thus leading to more direct hydrogen shuttling path. 

In the H spectrum, the peak from trace amounts of DMSO-ds is seen at about 2.6 ppm. The peak at 3.4 ppm is probably due to water or another impurity in the sample. Note the two small peaks located equidistant to the taU singlet near 2.0 ppm. The small "satellite" peaks are the result of the 1.1% of the methyl groups that have instead of and thus here coupling between the carbon and the protons is observed. The two doublets for the aromatic protons are observed near 7.5 ppm. The amide NH proton, which is probably hydrogen bonded to the basic (Lewis) sulfoxide functional group in DMSO-de/... 

For instance, two series of alkali-metal ion-exchanged zeoUtes have been investigated in order to analyze the possible correlations between the acidity and basicity of the X and Y zeolite structures and their catalytic properties [137]. The catalytic results for the 4-methylpentan-2-ol conversion show that activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The activity was found to increase from Cs to li for both X and Y zeoUtes. The dehydrogenation reaction occurred only on Cs-X+CS2O, which presents very strong basicity. The product selectivity of the reaction was found to depend on both Lewis acidity and basicity Lewis basic or acidic sites of zeoUtes can be considered as acid-base pairs, in which both basic framework oxygens and neighboring cations are important [ 137]. 

Adsorption equilibrium constant in the Langmuir equation Ligand with basic Lewis character as in Scheme 4a Mesoporous aluminosilicate... 

Aside from the Group I and If elements and Co, Co, Ni, Ni, Ag, " " Ag, B, Mn and S, all other elements indicated full abatement (95 5 %) as would be expected in the absence of the magnesium matrix. For example, the slight abatement of boron is consistent with the chemistry of boric acid. Boric acid B(OH)3 or H3BO3 is a weak mono-basic Lewis... 

These organic additives are basically Lewis bases such as esters, silyl ether, etc. A few electron donors commonly used in industry are shown. Depending on the process parameters, reactors, etc., a specific combination of two electron donors is usually employed. 

X. Rozanska, R. A. van Santen and F. Hutschka. A DFT study of the cracking reaction of thiophene activated on zeolite catalysts The role of the basic Lewis site. Stud. Surf. Sci. Catal. 135, 2001, 2611-2617. 

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