Carbonyls protonation

Electron delocalization in carbonyl protonated benzoic acid... 

In order to predict the structure of the product, you must identify the factors that will tend to favor selective ketal formation. Consider selective carbonyl protonation first. Obtain energies and atomic charges, and display electrostatic potential maps of the alternative protonated ketones (protonated ketone A, protonated ketone B). Identify the more stable isomer. Compare geometries and draw whatever Lewis structures are needed to account for your data. Why is one isomer more stable than the other Is the more stable isomer also that in which the positive charge is better delocalized Will the more stable isomer undergo nucleophilic attack more or less easily than the other Explain. 

A question that may justifiably be raised here is whether these proofs of exclusive carbonyl protonation of carboxylic acids in concentrated and anhydrous acids necessarily imply the dominance of this form in dilute acid. Evidence that this is not so for amides has been discussed on pages 328 ff. It is possible that the alternative protonated form of carboxylic acids [201] is dominant in aqueous acid, but as the overall extents of protonation are small, it is not detectable by any spectroscopic method. Unlike amides, carboxylic acids become measurably protonated in quite concentrated acid (>60% sulphuric acid), which would tend to favour the formation of the protonated form with a delocalized charge. The form [201]... 

Ultraviolet spectra of benzoic acid in sulphuric acid solutions, published by Hosoya and Nagakura (1961), show a considerable medium effect on the spectrum of the unprotonated acid, but a much smaller one in concentrated acid. The former is probably connected with a hydrogen-bonding interaction of benzoic acid with sulphuric acid which is believed to be responsible for a peculiarity in the activity coefficient behaviour of unprotonated benzoic acid in these solutions (see Liler, 1971, pp. 62 and 129). The absence of a pronounced medium effect on the spectra in >85% acid is consistent with dominant carbonyl oxygen protonation. In accordance with this, Raman spectra show the disappearance in concentrated sulphuric acid of the carbonyl stretching vibration at 1650 cm (Hosoya and Nagakura, 1961). Molecular orbital calculations on the structure of the carbonyl protonated benzoic acid have also been carried out (Hosoya and Nagakura, 1964). 

An extensive nmr study of the protonation of acyclic carboxylic acid anhydrides in HSO3F—SbFs—SO2 solution at —70°C has proved the formation of carbonyl-protonated anhydrides [206],... 

Carbonic acid and dialkyl carbonates are also carbonyl-protonated in HSO3 F—SbFj —SO2 low temperature (Olah and White, 1968). 

C-protonated cation has also been reported for 2,4,6-triniethyl-benzoic acid in 99-103% sulphuric acid, in which it exists in equilibrium with the carbonyl-protonated cation and the corresponding benzoyl cation (Beistel and Atkinson, 1969). The latter appears in 100% sulphuric acid, increases in concentration with increasing acid concentration at the expense of the other forms and is the only form observed at concentrations >105% sulphuric acid. 

Some substrates like benzoic acid, benzaldehyde etc. are so deactivated that direct nitration to their trinitro derivatives is not possible. Direct nitration of benzoic acid with excess fuming nitric and concentrated sulfuric acids at a temperature of 145 °C for several hours results in the formation of 3,5-dinitrobenzoic acid (54-58 %). The use of oleum in such reactions can significantly reduce the rate of nitration due to carbonyl protonation (see Section 4.3.3). Consequently, indirect routes are used for the synthesis of polynitroarylenes like 2,4,6-trinitrobenzoic acid and 2,4,6-trinitrobenzaldehyde (Section 4.9). 

These calculations show that (i) the methyl affinities (MeAs) of C=S compounds are consistently much higher (for X = H, 15 to 25 kcalmol-1 over the range Y = F to Y = NH2) than those of the C=0 homologs, and (ii) the sensitivity to substituent effects of the thiocarbonyls is ca 72% that of the carbonyls. Protonation and methylation therefore display the same pattern of structural effects (there is also a nearly perfect correlation between the PAs and MeAs for each family, although in all cases the PA exceeds the MeA by some 100 kcalmol-1). 

Esters behave in an analogous fashion, with carbonyl protonation being predominant [Eq. (3.66)]. Thus, protonated methyl formate 276 is present in HSC F-SbFs-S02 solution as two isomers in a ratio of 90% to 10%.571,572... 

If Ke signifies the concentrations of enol and keto tautomers in a carboxylic acid, then pfCE is the difference between the pKa values of the a-protons of the keto tautomer and the hydroxyl group of the enol tautomer. pfCE, however, is also the difference in pfCa values between the a-protons and the proton of the carbonyl group of the carbonyl-protonated acid, that is deemed to be decisive (Gerlt, 1991) for the kinetics of abstraction of a proton, rather than the pfQ value of the substrate in solution. The pki of mandelic acid (15.4) links the pka of the a-proton of the keto tautomer (22.0) with the pK.d value of the enol tautomer (6.6). The pk, value also links the pka value of the a-proton with that of the carbonyl-bound proton of the protonated mandelic acid. If the pka value of the carbonyl-bound proton of the protonated mandelic acid is assumed to be about -8.0 then the pfQ value of the a-proton is about 7.4. This value matches well with the pka -values of Lys and His residues which have been assigned recently in the active center of mandelate race-mase, so electrophilic catalysis alone is able to explain the catalytic power of mandelate racemase. 

According to Appendix 1, the pKA value for an acetylene proton is approximately 25, and the pKA value for a carbonyl-protonated amide is approximately 0. Therefore, the order of protonation is as follows ... 

Carbonic acid and dialkyl carbonates are also carbonyl-protonated in HS03 F—SbFs —S02 at low temperature (Olah and White, 1968). The trihydroxycarbonium ion [207] is stable up to 0°C in the absence of S02. This and analogous ions formed by the protonation... 

Formation of 648 from 647 was experimentally discarded. Since the most basic site of 646 is the carbonyl group, both Bronsted and Lewis acids should first coordinate to this position. In the case of Bronsted acid, protonation could occur at the electron-rich 9-position, and this was followed by deprotonation at the 1 la-position, which is promoted by the carbonyl protonation. In contrast, the Lewis acid cannot add to the 9-position and thus the skeletal rearrangement to give 648 took place. This kind of rearrangement through norcaradiene tautomers, shown in Scheme 128, is called walk rearrangement in thermal reaction of cycloheptatrienes <2002CL260>. 

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