Acids, acid strength substitution

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

Experimental observations indicate that acid strength significantly affects the reaction rate. For example, sulfuric acid promotes nucleophilic substitution of alcohols by bromide, but acetic acid does not. How would a change in acid strength affect your calculated reaction energies ... 

Display and compare electrostatic potential maps for methanol, ethanol, 2-propanol and trifluoroethanol. Identify the acidic sites as those where the potential is most positive and, assuming that the more positive the potential the more acidic the site, rank the acidities of the compounds. Does increased alkyl substitution have a significant effect on acid strength What is the effect of replacing the methyl group in ethanol by a trifluoromethyl group Why Do you find a correlation between the most positive value of the potential and the experimental pKa ... 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

Fig. 5. Correlation of rate data (log A ) for the methoxy-dechlorination of substituted 4-chloroquinolmes with the acid strengths (logiC ) of 1-naphthoic acids.

Indolmycin, biosynthesis of, 864 Inductive effect. 37, 562 alcohol acidity and. 604 carboxylic acid strength and. 758 electronegativity and, 37 electrophilic aromatic substitution and, 562... 

The log rate versus acid strength curve for the latter compound is of the exact form expected for reactions of the free base, whilst that of the former compound is intermediate between this form and that obtained for the nitration of aniline and phenyltrimethylammonium ion, i.e. compounds which react as positive species. That these compounds react mainly or entirely via the free base is also indicated by the comparison of the rate coefficients in Table 8 with those in Table 5, from which it can be seen that the nitro substituent here only deactivates weakly, whilst the chloro substitutent appears to activate. In addition, both compounds show a solvent isotope effect (Table 9), the rate coefficients being lower for the deuterium-containing media, as expected since the free base concentration will be lower in these. 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

Further substitution of alkyl groups in ethanoic acid has much less effect than this first introduction and, being now essentially a second-order effect, the influence on acid strength is not always regular, steric... 

The effect of introducing electron-withdrawing substituents into simple aliphatic acids is more marked. Thus halogen, with an inductive effect acting in the opposite direction to alkyl, might be expected to increase the strength of an acid so substituted, and this is indeed observed as pKa values show ... 

Another of the important concepts in dealing with acid strength is illustrated by the dissociation constants for the chloro-substituted acetic acids. The dissociation constants are as follows CH3COOH,... 

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered. 

Water is the ever-present proton acceptor in the subsurface. During the dissocia-hon of an acid in subsurface water, H3O is one of the dissociation products and the acid strength is a measurable parameter. In a dilute solution the activity of the hydrated protons equals that of H3O and the pH value characterizes the H-ion achvity. Substituting for pH in Eq. 6.1, we obtain... 

After allowing for a statistical correction for the number of protons that may be lost from the cation, the order of acid strengths (per NH-proton) of these cations remains unchanged. The initial trend of decreasing acidity with methyl substitution is reversed in the trimethylammonium ion, because with a decreasing number of protons in the cation, its hydration stabilization by hydrogen... 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

The variation of cracking selectivity in the conversion of alkanes over substituted H-Ga-MFI and H-Al-MFI zeolites has been correlated with the basicity of the C-C bond of the alkane, while the selectivity toward dehydrogenation was found to be related to the attenuation of the acid strength of the zeolite [251]. 

Microcalorimetric experiments of NH3 adsorption have shown that the isomor-phous substitution of A1 with Ga in various zeolite frameworks (offretite, faujasite, beta) leads to reduced acid site strength, density, and distribution [250,252,253], To a lesser extent, a similar behavior has also been observed in the case of a MFI framework [51,254]. A drastic reduction in the acid site density of H,Ga-offretites has been reported, while the initial acid site strength remained high [248,250]. 

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