Conjugated acid solvation

Ionisations 2, 3 and 5 are complete ionisations so that in water HCI and HNO3 are completely ionised and H2SO4 is completely ionised as a monobasic acid. Since this is so, all these acids in water really exist as the solvated proton known as the hydrogen ion, and as far as their acid properties are concerned they are the same conjugate acid species (with different conjugate bases). Such acids are termed strong acids or more correctly strong acids in water. (In ethanol as solvent, equilibria such as 1 would be the result for all the acids quoted above.) Ionisations 4 and 6 do not proceed to completion... 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. 

A comparison of the second-order rate coefficients for nitration of 2,4,6-tri-methylpyridine and 1,2,4,6-tetramethylpyridinium ion (both at the 3-position) shows similarity of profile in the common acidity region and a rapidly increasing rate with acidity for the trimethyl compound at acidities below 90 wt. % (where the usual maximum is obtained). These two pieces of evidence show reaction to occur on the conjugate acid as also indicated by the large negative entropy of activation. Surprisingly, the tetramethyl compound is less reactive than the trimethyl compound so maybe this is an example of steric hindrance to solvation. Calculation of the encounter rate also showed that reaction on the free base was unlikely. 

The reaction in Equation (6.12) illustrates the coexistence of two acids and two bases. We say the ethanoate ion and ethanoic acid represent a conjugate pair, and the solvated proton and the water form a second conjugate pair. Within the ethanoic-ethanoate pair, the ethanoic acid is the conjugate acid and the ethanoate anion is the conjugate base. Similarly, H30+ is a conjugate acid to the... 

Dimerization is the characteristic reaction of radical-anions from activated alkenes. The rate constants for dimerization are high and the conjugate acids from such alkene radical-anions in many cases have low pKa values and. The data in Table 3.4 were obtained by following the changes in uv-absorbance after pule-radiolysis of the substrate in an aqueous buffer. Attachment of a solvated electron leads to the radical-anion. Changes in the initial absorbance with pH lead to determination of the pKg value, while the dimerization rate can be determined from changes in absorbance over a longer time scale. Radical-anions from esters and amides are pro-... 

Arnett has presented an analysis of the solvation thermochemistry of the amines and their conjugate acids.96 Table 3.14 gives Arnett s data for the four... 

Correlation of nucleophilic rate data for phenyldimethylsulfonium ions with common nucleophiles, with pX e values shows that the slopes of the lines, jS[ e, correlate qualitatively with the Edwards hardness parameter for the nucleophile and not with the Swain-Scott n parameter.144 cw,cw-2,4,6-Trimethyl-l,3,5-triaminocyclohexane is weakly basic in aqueous solution, because of steric inhibition to solvation of the conjugate acid.145 The three NH2 groups are axial and the steric effect also results in reduced reactivity as a nucleophile in, S n2 reactions. Highly stereoselective syntheses of N-. and O-glycosides have been carried out by addition of anionic nucleophiles to glycosyl iodides.146 5 n2 reactions are involved, but some substrates are susceptible to E2 elimination when treated with highly basic anions. 

Alternatively, suppose you want to determine which heteroatom in a molecule is protonated first as pH is lowered. Or conversely, you may want to know which is the most acidic proton in a compound (even if it is a hydrocarbon, for example). In such cases, you can obtain optimized geometries for the parent molecule Z and its conjugate acid ZH+ (or conjugate base Z ) for each site of proton attachment or removal. Simply take the differences in total energy (obtained quantum mechanically), E(ZH+)-E(Z) [or E(Z ) E(Z)], and you have a theoretical assessment of the relative gas-phase acidity (basicity). (The electronic energy of a proton is zero because it has no electron.) Of course, these energy differences do not account for solvation, but if the two protonation (or deprotonation) sites are very similar, the vacuum results may suffice. Alternatively, you can turn on implicit (continuum) solvation in your calculation and obtain energies of the simulated solution species. 

Rate constants for superoxide ion (02 ) and its conjugate acid HOz as oxidant, reductant, and nucleophile have been measured in several solvents (Hendry and Schuetzle, 1976 Sawyer et al., 1978 Bielski et al., 1985), but few SARs have been developed. Moreover, the reactivity of superoxide ion generally is too low for the oxidant to be important in surface waters. Solvated electrons (e Aq) also form on insolation of DOM (Fischer et. al., 1985 Zepp et. al., 1988), but its concentration is very low, and target compounds are too few to make e (Aq) an important redox agent in surface waters (Buxton et al., 1988). One possible exception is nitroaromatics such as 2,4,6-trinitrotoluene (TNT), which exhibit strong acceleration of photolysis rates in the presence of DOM (Mabey et al., 1983). 

We have already discussed the acidity of NH-tetrazoles 61 to form tetrazolate anions 62 [75], In the same paper, we also studied the protonation. The substituent X strongly affects the tautomeric balance between 1,3-H,H+- (82) and 1,4-H,locations (84). In the case of electron-withdrawing substituents, the most preferred form of the conjugated acid is the 1,3-H,H+- form (82) structures 83 and 85 are much less stable. The acidity measures in solution and in the gas phase satisfactorily correlate with each other. In all cases, these relationships do not hold for 5-phenyltetrazole (56, X = Ph). This could be explained by the difference in solvation of this compound compared to other 5-X tetrazoles as well as by some peculiarity of its electronic structure, for example, the strong conjugation between the phenyl substituent and the tetrazole ring. 

Here we have less direct data on solvation energies and electron affinities, but there is no doubt that the pKa of the conjugate acid of the nucleophile increases rapidly from HF to R3CH (Table 2). 

The solvated proton is a hard electrophile, little affected by frontier orbital interactions. For this reason, the p/T, of the conjugate acid of a nucleophile is a good measure of the rate at which that nucleophile will attack other hard... 

---