Acid strength solvent effects

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Clearly, complete understanding of solvent effects on the enantioselectivity of Lewis-acid catalysed Diels-Alder reactions has to await future studies. For a more detailed mechanistic understanding of the origins of enantioselectivity, extension of the set of solvents as well as quantitative assessment of the strength of arene - arene interactions in these solvent will be of great help. 

Another example is the acidities of a series of carboxylic acids. It is known that the substitution effect on these compounds also depends on the environment. The behavior of the halo-substituted acetic acids is one of the prototype problems for the solvent effect on acidity The order in strength of the haloacetic acids in the gas phase is... 

The equilibrium in this reversible reaction will be greatly influenced by the nature of the acid and that of the solvent. Weak acids are normally used in the presence of strongly protophilic solvents as their acidic strengths are then enhanced and then become comparable to those of strong acids — this is referred to as the levelling effect . 

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. 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

Solvent effects on the magnitude of spin-spin coupling constants in phosphoryl compounds have been reported, as have the solvent effects on the relative strengths of phosphoric acid and some of its partially esterified derivatives.The variation of i.r. frequencies of (122) and (123)... 

Attempts were early made to explain the relative acidities of ammonium and methyl-substituted ammonium ions (Brown, 1946 Hammett, 1940 Palit, 1947). The order of decreasing acid strength in water at 25°C is NH4+ > Me3NH+ > MeN H3+ % Me2N H2+, an order which obviously cannot be rationalized on the basis of inductive effects. There were two difficulties with the explanations given for this order. First, there is nothing magic about the temperature 25° and comparisons at other temperatures lead to inversions of the order at 25°. Second, the specific effect of the solvent was not taken into account. Since there is... 

The rate of reaction of DCC with aliphatic acids in THF depends on the acid strength, i.e., chloroacetic acid reacts considerably faster than acetic acid." It was also found that the higher the acid strength, the higher the yields of anhydride and urea. The reaction exhibits first order kinetics both in acid and in carbodiimide, and the rate determining step is the formation of the O-acylurea." Solvent effects are also observed in the reaction of DCC with acetic acid. The reaction rates are highest in THF, and the proportion of N-acylurea is also the greatest in THF." ... 

Determination of the acid strength of solid catalysts using Hammett indicators, however, has been criticized frequently because of the heterogeneity of the solid surface [81, 104, 110, 114—116]. The principle of the Hammett acidity function is based on the equilibrium equation in a homogeneous solution, and its application to the heterogeneous condition is subject to severe criticism. In addition, the color change of the adsorbed indicators on solids as determined by the naked eye is subjective. The effects of interactions between the solvent and the solid surface has also been raised [9]. 

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