Phenols dissociation

The (Tj value for SOMe is slightly different from that given by Taft and Lewis . The continuous increase in a, produced by attached electronegative atoms at S or C is, of course, to be expected. This also encourages octet expansion as measured by The zero value for SOMe presumably means that any tendency to octet expansion [conventionally n (pd) conjugation] is essentially cancelled by the n (pp) conjugation of the sulfur lone pair. For these -f R substituents no exaltation of (Tj,-type values in the direction - (Tj would be expected. The table shows two (T -type constants [Pg.510]

The hypsochromic shift of the longest absorption band on complexation in ethanol solution and the bathochromic shift in chloroform solution are explained as follows. The hypsochromic shift in ethanol is affected by the phenol dissociation equilibrium (Eq. (1) in Scheme 2), the ion-exchange equilibrium, and the interaction of phenolate anion-metal cation [Eq. (2)]. In facts, the observed shifts depend upon the cation species, the amount of added salts, the reaction time, and, in some case, the counter anion species. 

Alcohols and phenols dissociate to a slight extent to form alkoxide ions and phenoxide ions. 

As weak ocidA alcohols and phenols dissociate to a slight extent in dilute aqueous solution by donating a proton to water, generating HiO and im alkoxide ion, RO, or a phcnojtido ion, ArO ... 

For example, phenol dissociates to form phenate ions and a proton ... 

Substituting in equation 11 the known experimental parameters for phenol dissociation (AG, = 13.8 kcalmol" calculated from the ground-state equilibrium constant, pX, = 10.0), AGt((PhO ) — (PhOH)) of the phenolate/phenol system is about —76 kcalmoH, which is about 10% less than the accepted value for the electrostatic solvation energy of the chloride anion in water, AGe(Cr) = —85 kcalmol". These simple considerations imply that the AGt((PhO ) — (PhOH)) contribution to the overall free energy of solvation is largely electrostatic, and that relatively small differences in the gas-phase proton affinity of the base and in specific solvent-solute interactions of the photoacid and the base determine the relatively narrow (in free-energy units) acidity scale in aqueous solution. It... 

Charges on atoms in transition structure, reactant and product are independent of the distance of the polar substituent from the reaction centre as illustrated in Equation (5a), (see Chapter 4, Section 4.1). The greater sensitivity of the phenol dissociation compared with that of benzoic acid standard (Equation 5b) is interpreted on the basis of a smaller distance (d) between substituent and dissociation centre (compared with in the benzoate standard) and to the delocalisation of charge in the benzoate ion (Equation 6, where s, and 8,. are the effective charges on product and reactant respectively). 

R. J. Washkuhn, V. K. Patel, and J. R. Robinson, Linear free energy models forester solvolysis with a critical examination of the alcohol and phenol dissociation model, J. Pharm. Sci. 60, 736-744 (1971). 

It has been stated that phenols are weak acids, and that their acidic property probably results from the fact that the aryl group which they contain is negative. The negative nature of the phenyl group is not highly developed, and, as a consequence, phenol is a very weak acid its salts are decomposed by carbonic acid. The extent to which acetic acid, carbonic acid, and phenol dissociate in one-tenth normal solution is 1.3, 0.174, and 0.0037 per cent, respectively, whereas at this concentration hydrochloric acid is dissociated to the extent of over 90 per cent. Notwithstanding the fact that phenols are such weak acids, they dissolve readily in alkalies. As they are weaker than carbonic acid, they are not soluble in carbonates. This fact is made use of in the separation of acids from phenols. 

From studies of the high-spin monophenolate complexes of Fe using the temperature-jump method, it is suggested that pathways involving [Fe(H20)e] + and [Fe(H20)5(OH)] + ions i.e. proton ambiguity) can be distinguished by measuring values of for the phenolate dissociation reactions. The pertinent data are collected in Table 16. Comparison with related systems shows that is rather different for the two reactions [Fe(H20)8] ++L and [Fe(H20)5(0H)]2+ + HL. 

Phenols, dissociation constants of, in aqueous solution.XXVIII... 

It appears that another mechanism may be involved. If we examine the initial ring opening step by protonation arising from the partial phenol dissociation thus ... 

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