Phenol Substituent effects

As Table 24 2 shows most phenols have ionization constants similar to that of phenol Itself Substituent effects m general are small... 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

The pattern K j < K P < K° and X > X° > X is also clearly shown in the correlation of the proton nmr shifts of substituted phenols in DMSO solution. The o and p- substituent effects are best fitted by the a (A) parameters (cf. para set no. 24 of Table VI), whereas the m- substituent effects are best fitted by parameters. The following fitting parameters are... 

Remarkable substituent effects on the absorption bands in the colored form are observed on substitution in positions 3, 6, and 8 of the spiroben-zopyran (Table 1). A nitro group at the 8-position yields a higher Xmax ( 40nm) compared with a nitro group at the 6-position due to interaction of phenolate anion and oxygen atom of the nitro group. In many cases, it... 

The of 33 shows a bathochromic shift, compared to that of the corresponding spironaphthopyran [ max 531, 558(s) nm in toluene].78 The substituent effect in 2, 5, 6 - and 5-position of 33 on the absorption band of the colored form has been examined.72,77,7s The donor substituent group in 6 -position, such as piperidino group, gives a hypsochromic shift by 35 nm, but 5 -carbomethoxy substitution results in a bathochromic shift by 20 nm. This may be due to interaction between oxygen atom of the phenolate and methoxy group. 

M. M. Bizarro, B. J. Costa Cabral, R. M. Borges dos Santos, J. A. Martinho Simoes. Substituent Effects on the O-H Bond Dissociation Enthalpies in Phenolic Compounds Agreements and Controversies. Pure Appl. Chem. 1999, 71, 1249-1256. 

In conclusion, therefore, a judicious use of CV methodology may lead to absolute thermodynamic data that are accurate to ca. 15 kJ mol-1. Relative values (i.e., differences between bond dissociation enthalpies in similar compounds) can be more reliable, but the approximations described suggest that some caution be exercised when using the results to draw conclusions that rely on small differences between bond dissociation enthalpies. This is the case, for example, for ring substituent effects on the O-H bond dissociation enthalpies in substituted phenols [346,349],... 

A study39 of substituent effects on the 15N chemical shift (515N) (Table 10) for 4-substituted anilines in DMSO was interpreted in terms of substituent solvation-assisted resonance (SSAR) effects. Solvation of certain conjugated jr-electron-acceplor (+R) substituents has been found to give significant enhancements in the acidities of anilines, phenols and other acids40,41, and the magnitudes of these enhancements increase with... 

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

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