Heat of protonation

Fig. 3. Variation of heats of protonation in the gas phase of bases with N, 0, P, and S donor atoms as hydrogens are replaced by methyl groups as substituents on the donor atoms. Data from Ref. (12).

Another measure of the homoaromatic stabilization of the homotropylium systems can be gained from the heats of protonation of the ketones [17]—[20] (Childs et al., 1983). The difference in heats of protonation between [18] and [19] (A//I8 i9) is significantly larger than the corresponding difference between [17] and [18] (AHl7 ls). This increase in stabilization is associated with the homoaromaticity of [21]. Similarly, there is a large discontinuity between A//17 18 and A//18 2o which is associated with the aromaticity of the tropylium ion [22]. 

R. J. Angelici. Basicities of Transition Metal Complexes from Studies of their Heats of Protonation A Guide to Complex Reactivity. Acc. Chem. Res. 1995, 28, 51-60. 

A more direct approach to measurement of homoaromatic stabilization was taken by Childs and colleagues in which the heats of protonation of the series of ketones 26-31 in FS03H were measured102. Of particular interest are the differences in heats of protonation for the various ketones and these data are summarized in Scheme 7. 

On the basis of the measurement of the heats of protonation and isomerization and estimates of the heats of formation of the starting ketones, it has been shown that the additional delocalization energy in 33 as compared to 36 is 2.8 kcal mol 169. 

Arnett and his collaborators have extended the heat of protonation concept (Section 3.2) to weak acids by measuring heats of deprotonation, AHD, of weak acids in dimethylsulfoxide containing the dimethylsulfoxide conjugate base, H3CSOCH2-.60 The results correlate well with pA0 values of the amines used... 

E. M. Arnett, Accts. Chem. Res., 6, 404 (1973). Gas-phase data are reported for (CH3)3P and PH3 only, but heat of protonation results indicate that secondary and primary phosphines will fall in that order between these two. 

The oxygen and sulfur bases are weaker than the nitrogen bases, and accurate solution basicities are not available. Arnett s heat of protonation studies indicate that the order of decreasing basicity is ROR > ROH > HaO,98 a result that is in agreement with gas-phase measurements.99 Hydrogen sulfide in the gas phase has basicity comparable to that of water (Table 3.18), and substitution of H by alkyl produces stronger gas-phase bases just as does similar substitution on oxygen. 

These distonic superelectrophiles (145-147) have been characterized by low-temperature H NMR (and 13C NMR in the case of 147) from FSO3H-SbFs solution. Dication 146 was also studied by calorimetric studies to determine the heat of diprotonation of 2,5-hexanedione.48 It was found that the heat of diprotonation for the y-diketones (like 2,5-hexanedione) is about 5 kcal/mol less than expected, when compared to twice the heat of protonation of acetone or other monoketones. The destabilization of dication 146 by 5 kcal/mol can be the result of electrostatic effects, and it can be considered evidence for the superelectrophilic character of such dications. When 2,6-admantanedione is reacted in FSO3H—SbFj solution, the dication 148 is formed as a persistent species, observable by and 13C NMR.12 The carboxonium carbons of 148 are observed at Z I3C 247.7, while the mono-cationic species (149) has a carboxonium carbon at 513C 267.1. These 13C NMR data were interpreted as evidence for the increasing importance of the carboxonium-type resonance structure (148a) due to electrostatic repulsive effects. Some examples of aromatic diketones (i.e., diacetylbenzenes) have also been reported to produce bis-carboxonium dications in their protonation reactions in superacids.47... 

Several methods are available for estimating acidity in solution for the homogeneous superacids spectroscopy (UV, NMR), electrochemical methods, chemical kinetics, and heats of protonation of weak bases (9). Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. The most simple and useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions we usually compare the activity with that of Si02-Al20v... 

Alkyl aryl ethers have a long history in lithiation reactions, and there are detailed reports on the mechanism (see section 2.3.1) and relative efficiency135 of the lithiation of anisole and its derivatives. Methyl aryl ethers are often lithiated in Et20, but THF can be just as effective. The stabilisation afforded to an ortholithiated anisole relative to a /rara-lithiated anisole can be judged from their relative heats of protonation by s-BuOH.233... 

Proton transfer could sometimes be nothing more than a case of conventional hydrogen bonding, but in many cases the concurrent transfer of an electron and a proton produces a new type of adduct, and complex formation may indeed be dramatic [32]. Arnett and Mitchell [32] point out that no correlation existed between the heats of protonation and of hy-... 

As can be seen, the introduction of a conjugated double bond into the seven-membered ring of 26 increases the heat of protonation by 3.7 kcalmoF As excepted, the effect of introducing a second double bond is much smaller (27 28 or 27 30). There is, however, a major effect on introduction of the third double bond, 28 29, where a very large... 

Table 2. hydrides 3.3 Absolute Bond-dissociation Energies 687 Heats of protonation, oxidation potentials, and bond-dissociation energies of metal in DCE [26], ... 

The heats of protonation of the ketones 131-135 (Table 17) indicate that the vinyl and cyclopropyl groups both give a nearly 5 kcal mol more favorable heat of reaction, and consideration of ground state energies probably would make the difference even greater... 

Conjugative and substituent properties of the cyclopropyl group TABLE 17. Heats of protonation of ketones... 

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