Protonation of phenol

In contrast to these gas-phase findings, the oxygen protonation was found to be favoured in various solutions . The influence of the solvent is known to be a crucial factor determining the strength of bases. In some cases, the relative basicity ordering is even reversed by external effects. 

At the B3LYP/6-31 l-b- -G(d,p) -b ZPE level of theory, the local PAs of phenol at different sites are evaluated in kJmoL as follows 820 for para-C, 809 for ortho-C2, 757 for meta-Cs, 699 for ipso-C and 743 for oxygen . The coupled-cluster CCSD(T) approach in conjunction with the 6-311-1— -G(d,p) basis set yields a PA(C4) of 819 kJmol . When using an appropriate basis set, the calculated PAs thus compare reasonably well with the experimental value quoted above. 

The regioselectivity of the gas-phase protonation of phenol can be understood in simple terms of its resonance structures. Drawing them, we may figure out that a positive r-charge of the protonated form is mainly localized in the para- and ortfio-positions with respect to the protonation site. If the OH group is attached to one of these positions, the relevant molecule is then described by four resonance structures, resulting in the positive r-charge 

FIGURE 21. Portion of the potential energy surface of the protonated phenol showing the proton migration between the adjacent heavy atoms. Values given in kJrnol were obtained from B3LYP/6-31- -G(d,p)- -ZPE computations . Adapted from Reference 299 with permission 

TABLE 26. The B3LYP/6-31- -G(d,p) proton affinities (kJ mol ) of halogenated phenols  

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NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

The protonation of phenol and anisole in pure fluorosulphuric acid occurs on the para-carbon (Birchall et al., 1964), but the introduction of methyl substituents in the para-position leads to competing ortho-caihon protonation (Hartshorn et al., 1971). So, for example, in the solutions of 5-methoxyhemimellitene, cations [226] and [227] occur in the ratio 2 5 at low temperature. Similar results... 

The shifts of protons ortho, meta, or para to a substituent on an aromatic ring are correlated with electron densities and with the effects of electrophilic reagents (Appendix Chart D.l). For example, the ortho and para protons of phenol are shielded because of the higher electron density that also accounts for the predominance of ortho and para substitution by electrophilic reagents. Conversely, the ortho and para protons of nitrobenzene are deshielded, the ortho protons more so (see Figure 3.23). 

Equation 4.13 suggests that there ought to be some sort of correlation between the shielding factor and the electron density around the hydrogen. For example, a more acidic proton, such as the OH proton in phenol, should be less shielded than the corresponding less acidic proton in an alcohol. This is indeed found to be the case, the chemical shift for the OH proton of phenol occurring... 

Nmr. The outstanding feature of the nmr spectrum of a carboxylic acid is the absorption far downfield (S 10.5-12) by the proton of —COOH. (Compare the absorption by the acidic proton of phenols, ArOH, in Sec. 24.14.)... 

Hydrogen-1. Bases such as phosphites deshield the acidic proton of phenols, alcohols, and chloroform. Exceptions are bases like pyridine, which have a large magnetic anisotropy. One possible explanation is that the shift is due primarily to the distortion of charge around the proton produced by the donor s lone pair of electrons. The reactivity of orthophosphate monoesters to hydrolysis was related to 5qh of the protonated leaving group. Further work has been completed on the stereochemical assignments of substituted... 

Brust and coworkers studied chemical reactivity of thiol-capped gold colloids. A colloid modified with p-mercaptothiophenol is soluble in ethyl acetate. It can, however, be transferred into alkaline aqueous phase due to deprotonation of the phenolic groups. Acidification of the system effects protonation of phenolates and transfer of the colloid back to the aqueous phase483. Exposure of the above colloid to propionic anhydride leads to partial esterification. That phenolic groups, however, remain intact was attributed to steric hindrance. 

Variable acidic protons of phenols and carboxylic acids, confirmed by D O exchange... 

Temperature-dependent changes in the UV spectra of 1,3,5-triaminobenzene in an acidifiol mixture of alcohol and ether at a pH corresponding to monoprotonation made it possible to elucidate the monoprotonation. It both affects the C and the N atom, the beorenium ion (68) predominating in an equilibrium mixture of ions at room temperature and the ammonium cation (67) at about —60 °C (cf. C- and O-protonation of phenols and their ethers). 

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