Resonance protonated benzoic acid

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

A combination of steric and electrostatic factors is presumably decisive with regard to the form of the acid most stable in sulfuric acid solution. The simple protonated form XX of benzoic acid is stabilized by resonance structures sterically prohibited in mesitoic acids. The ortho methyl groups of mesitoic acid would interfere with a coplanar dihydroxymethylene group. On the other hand, the inductive and resonance effects of the methyl groups help stabilize the acylium ion form of mesitoic acid as in the formulae XXI. In the case of 2,4,6-tribromobenzoic acid the steric effect and its abetting electronic effects are not sufficient, and this acid behaves like benzoic acid.17 >177... 

Skinner, J. L. and Trommsdorf, H. P. Proton transfer in benzoic acid crystals A chemical spin-boson problem. Theoretical analysis of nuclear magnetic resonance, neutron scattering, and optical experiments, J.Chem.Phys., 89 (1988), 897-907... 

A para-substituent may stabilize mesomerically either the conjugate acid of an acid-base pair rather more than it stabilizes benzoic acid, or it may stabilize the conjugate base rather more than it stabilizes the benzoate anion. The first situation is found in car-boxonium ions [13], where the delocalization of the positive charge on to a mesomerically electron-donating substituent stabilizes the cation. A similar resonance in the benzoic acid molecule [14] involves a separation of charge and affects the binding of the proton... 

Further evidence for nitrogen as the site of protonation of amides in largely aqueous acid comes from studies of substituent effects on the p/Sfa Value of benzamide (Edward et al., 1960 Yates and Stevens, 1965). It has been observed that the pi g-values are correlated with a-constants of the substituents, rather than with o. This means that the structure of the dominant form of the cation is N-protonated as in [112], because resonance interactions of para-substituents in this kind of cation are similar to those in benzoic acid. The p-value (0 92)... 

The DSP approach nicely answers the controversial question about which substituent parameters should be employed to correlate pKa data for 4-substituted pyridinium ions. Statistically, the best correlation is given by Eq. (9), which has values to measure the resonance contribution of a substituent, a result in keeping with chemical intuition. This correlation is statistically superior to a Hammett treatment, where both resonance and inductive effects of a group are combined into a single parameter, p or ap.53,54 Moreover, now it is possible to rationalize why a simple Hammett treatment using ap works so well. Equation (9) reveals that the protonation equilibrium is much more sensitive to an inductive effect (p, — 5.15) than to a resonance effect (p = 2.69). Hence, substituent parameters, such as erp, which are derived from a consideration of the dissociation constants for benzoic acids where resonance contributions are small serve as a useful approximation. The inductive effect is said to have a larger influence on pKa values for pyridinium ions than for benzoic acids because the distance between the substituent and the reactive site is shorter in the pyridine series.53... 

Because it is easier to protonate an aldehyde than a carboxylic acid (compare benzoic acid and benzaldehyde in Appendix C), the ring closure would be written best as protonation of the aldehyde oxygen followed by nucleophilic reaction of the carboxylic acid carbonyl oxygen. The carbonyl oxygen acts as the nucelophile because a resonance-stabilized cation is produced. If the hydroxyl oxygen acts as a nucleophile, the cation is not resonance-stabilized. 

Formic acid, pK 3.75, serves as the reference carboxylic acid. Benzoic acid is weaker because the phenyl group is electron-donating by resonance, stabilizing the protonated form. Methyl substituents are known to be electron-donating by induction, strengthening the electron-donating effect, making the meta- and para-substituted acids weaker than benzoic acid. 

When a proton is lost from the / ara-nitroanilinium ion, the electrons that are left behind are shared by five atoms. (Draw resonance contributors if you want to see which atoms share the electrons.) When a proton is lost from /jara-nitrobenzoic acid, the electrons that are left behind are shared by two atoms. In other words, loss of a proton leads to greater electron delocalization in one base than in the other base. Because electron delocalization stabilizes a compound, we now know why the addition of a nitro substituent has a greater effect on the acidity of an anilinium ion than on benzoic acid. Now continue on to Problem 14. 

An excellent example of the use of the a parameter is in the distinction between ether or carbonyl protonation of substituted benzoic acids [41]. The excellent correlation with (Fig. 11) indicates that resonance interactions are very important and that carbonyl protonation is therefore the major contributor (Scheme 1). 

For veral classes of nitrogen bases, the position of protonation is a controversial matter and it appears that the pjnroles, indoles, and possibly amides undergo protonation on all possible basic sites depending on the circumstances. These same compounds are some of the womt offenders for non-adherence to the activity coefficient postulate, and furthermore have in common a resonance-stabilized system in which two of the basic sites are in a position 1, 3 to each other. Nitrobenzene and benzoic acid which exhibit abnormal activity be-... 

This is indeed what might reasonably be expected in view of the different resonance and solvation factors that must obtain from one series to the other. For example, resonance stabilizes the phenoxy anion aiding acidic ionization of phenols and will also work to favor ionization of the conjugate acids of the phenols. On the other hand, in the benzoic acid series there is not as much difference in resonance stabihzation between the molecular acid and its anion. However, protonation of the carboxyl group makes an urgent demand for resonance stabilization of the carbonium-like conjugate acid. 

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