Acidity intrinsic

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

Tertiary amides 9-1, 9-2, and 9-3 are lithiated at the (3-carbon, rather than the a-carbon by s-butyllithium-TMEDA. It is estimated that the intrinsic acidity of the a-position exceeds that of the (3-position by about 9 pK units. What causes the (3-deprotonation to be kinetically preferred ... 

In order to arrive at values of the virtually intrinsic acidity, i.e., an acidity expression independent of the solvent used (Tremillon12 called it the absolute acidity), Schwarzenbach13 used the normal acidity potential as an expression for the potential of a standard Pt hydrogen electrode (1 atm H2), immersed in a solution of the acid and its conjugate base in equal activities analogously to eqn. 2.39 for a redox system and assuming n = 1 for the transfer of one proton, he wrote for the acidity potential... 

Catalytic activity of solid acids in hydrocarbon conversions is often correlated with their acidity. Problems arise from the difficulty to bridge the gap between the equilibrium thermodynamic concept of acidity and the composite kinetic concept of catalytic activity [1], The correlation is meaningful if connected parameters are related to each other, namely, intrinsic activities are correlated with intrinsic acidities or relationship is established between corresponding apparent parameters. 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

The intrinsic acidities of zeolite samples are correlated with the Av0h values induced by adsorption of N2 (Av0h,n2), which can be determined by subtracting the frequency of the shifted OH-band from the frequency of the unperturbed OH-band as shown in fig.l. One shifted OH-band was observed for the ZSM-5 and mordenite samples, while the... 

From alkalimetric-acidimetric titration curves on hydrous ferric oxide the following intrinsic acidity constants have been obtained (I = 0.1 M, 25° C)... 

The microscopic acidity constants as a function of ( AlOHg). Extrapolation to zero charge gives intrinsic acidity constants. 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

The intrinsic acidity constants Ka and Ka2 can then be determined from Q, and Qa2 as follows. Combination of Equations 19 and 20 with Equations 1 and 2 show that the acidity constants are related to the acidity quotients by... 

More recent studies in this field have revealed the importance of the intrinsic acidic sites on the aluminas in directing the course of the dehydration. Recently the consideration of stereochemical factors involved in the dehydration and the use of gas chromatography as an analytical tool has led to a better understanding of this reaction, with a resultant better appreciation of the reaction mechanism. 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

The dehydration of menthols over alumina, prepared from aluminum isopropoxide and having intrinsic acidic sites, was accompanied by double bond migration of the cycloalkenes produced. The isomerization was, however, suppressed by the preferential neutralization of the strong acid sites with bases. The neutralization of acidic sites thus preventing isomerization was confirmed by von Rudloff (61) who evacuated pyridine-treated alumina for 6 hours, when about 0.8% base was retained. 

The kinetic studies carried out in recent years on the dehydration of ethyl alcohol did not lead to identical conclusions. Much of the divergence is probably due to the fact that the various investigators paid no attention to the intrinsic acidities of the aluminas used in their studies. 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

Experimental evidence demonstrates that secondary and primary alcohols are dehydrated by a concerted mechanism, whereby both the intrinsic acid and base sites of the alumina participate. The steric course... 

By measuring the shifts of the various hydroxyl bands of the zeolite, a direct measure of the relative acid site strengths can be made without the need for thermal desorption. Table 4.6 lists the measured hydroxyl band shifts for a variety of hydroxyl groups on different zeolites using low temperature CO adsorption. This data indicates that there is indeed a difference in the intrinsic acid strength of the bridging hydroxyl groups in different zeolites as well as in the same zeolite structure with different framework aluminum content. 

MOR and MFl infrared spectra are not nearly as complex as those of Y zeolites. Single bands around 3620 and 3605 cm" , respectively, are typical for the acid forms of these framework types. The lower energy of these bands compared to those of zeolite Y is usually cited in explaining their higher intrinsic acidity and... 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

Intrinsic acidity constants (K im) represent the dissociation of surface hydroxyl groups at zero surface charge. These constants carmot be measured directly. They are obtained from the experimentally accessible conditional constants, Kcond, either by extrapolation to a situation of zero surface charge, or by fitting the experimental data to an appropriate double layer model (section 10.3) to compute the electrostatic component. 

Following Taft (83MI2), we define the intrinsic acidity of HAj (g) as the standard free energy change (AG ), corresponding to Eq. (1). The (AG ), value is also taken as a measure of the intrinsic basicity of Af" (g). 

Comparison of the intrinsic acidities and basicities of pyrrole (I), imidazole (4), and pyrazole (6), together with complementary information coming from the azine held, illustrate the main effects that control the acidity and the basicity of unsubstituted azoles (86JA3237). Particularly important are the role of electrostatic interactions between adjacent charged nitrogens (NH) and between adjacent lone pairs (N), as well as the aza electronegative effects. 

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