Proton acidities calculation

Both cis and tran -cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer cyclohexene protonated cyclohexene, trans-cyclohexene protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problem 5.)... 

Dissociation constant of silicic acid calculated according to the a + = [(Kacx Kw)/c]1/2fbrmula for dissociation of salts formed from weak acid and strong base a+ is the activity of protons (from pH), K w is the ionization constant of water, and c is the concentration of silicate solution. 

Interestingly, Arduengo observed that Villa was stable with respect to dimerization in the absence of a Brpnsted or Lewis acid catalyst." Similarly, in the absence of an acid catalyst, dimerization of IXb is extremely slow and is first order in carbene. Therefore, the observed formal dimerization of Villa and IXb,c does not involve the coupling of two carbenes, but the nucleophilic attack of one carbene upon its conjugate acid, followed by proton elimination, as already suggested by Chen and Jordan (Scheme 8.13). It is important to keep in mind that even A,A-dialkylimidazolium ions have pXa values of 24 in DMSO, and based on the calculated proton acidity, Alder estimated the pXa values for acyclic diamino-carbenes to be from 2 to 6 pXa units higher than for imidazol-2-ylidenes. " ... 

I cite three papers to show that standard continuum calculations can give satisfactory first-principles pKa values Shields and coworkers used a thermodynamic cycle with gas phase and continuum calculations to obtain satisfactory results for six simple carboxylic acids [46]. These were absolute calculations in the sense that no acid was used as a reference point, although the experimental gas phase free energy and aqueous solvation energy of the proton were resorted to. Not quite as esthetically satisfying perhaps, were relative calculations in which acetic acid was used as a reference compound [47]. Similar to the absolute acid calculations was work with phenols that was said to be among the most accurate of any such calculations for any group of compounds [48]. 

All of these free energy values can be calculated using quantum chemistry, except AGsoi(H+) and Ggas(H+), which must be determined experimentally or using thermodyamic theory. These values will be discussed in the corresponding sections. A similar thermodynamic cycle that is often used is based on the acid dissociation equation of a protonated acid [5] ... 

EDTA is a polyprotic acid with four protons (H4Y). Calculate the hydrogen ion concentration of a 0.0100 M solution of Na2EDTA (Na2H2Y). 

It is worth noting that no exothermic peak was obtained on Cs salts compared to their acid analogues, showing their greater thermal stability. Note also that the content of protons, as calculated from the constitutional water evolved between 200 and 450°C, is reliable for the acid samples (close to 4) whereas it varies between 1.6 and 2.8 instead of 1.5 as expected for Cs salts. The TGA curves shown in Figure 1 illustrate this phenomenon, in particular for Cs salts for which water loss occurred up to 600°C, with a small step between 450 and 500°C. This water loss between 200 and 600°C corresponds both to water adsorbed on Cs cations and to dehydroxylation of constitutional water, i.e. to the elution of protons. This is why the number of protons given in Table 2 is more than 1.5 as expected... 

The acid strength of proton acid-base pairs (Bronsted acids) is given by the pk value (Table 1.4.2). At the pH that corresponds to the pk, half of the molecules are in the acid form, the other half in the form of the corresponding base. Each pH unit then shifts the ratio by a factor of 10. Phenol, for example, is 50% phenolate and 50% phenol at pH 10, corresponding to a 1 1 ratio at its pk. At pH 7 the ratio of phenohphenolate is then 1000 1. The free energy of acidbase reactions can be directly calculated from the pk values. The pk of water (14 at a concentration of 10 M) is also drastically changed by coordination to metal ions, e.g., to 10 in the coordination sphere of... 

The proton acidities of oxazoles have been theoretically calculated and determined experimentally. The reactivity of oxazoles indicates that the acidity of a hydrogen atom decreases in the order C(2) > C(5) > C(4). This is true in most cases however, exceptions are well documented and depend on the substitution. The acidity of the hydrogen at C(2) was estimated to be pKa 20 while for oxazole itself the pKb is reported to be pKb... 

When a protonic acid HX is placed in water, it reacts partially (or completely if the acid is strong) to form some ions. The degree of hydronium hydration will not affect the calculations when water activity 1 is assumed for dilute solutions. 

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