Resonance phenols

Hammett a Ionization of phenol Resonance in addition to induction Same as above... 

Phenol condenses with phthahc anhydride in the presence of concentrated sulphuric acid or anhydrous zinc chloride to yield the colourless phenolphthalein as the main product. When dilute caustic alkah is added to an alcoholic solution of phenolphthalein, an intense red colouration is produced. The alkali opens the lactone ring in phenolphthalein and forms a salt at one phenolic group. The reaction may be represented in steps, with the formation of a h3q)othetical unstable Intermediate that changes to a coloured ion. The colour is probably due to resonance which places the negative charge on either of the two equivalent oxygen atoms. With excess of concentrated caustic alkali, the first red colour disappears this is due to the production of the carbinol and attendant salt formation, rendering resonance impossible. The various reactions may be represented as follows ... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

By analogy to phenols we would expect the isomers with —OH groups on benzene like rings to be more stable This turns out not to be true because the keto forms are also aromatic owing to amide resonance... 

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

A Hammett plot of the pK values of p-substituted phenols against the Op values shows serious deviations for the members of the series at the extremes of the o scale, that is, for substituents that are strongly electron donating or electron withdrawing. It was recognized very early that such deviations could be rectified by choosing an appropriate o value for such substituents in effect, this means a different model reaction was adopted. The chemical basis of the procedure can be illustrated with the p-nitro substituent. The p-nitrophenolate ion is stabilized by through resonance as shown in 2. 

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

The hydroxylation of a phenol 1 upon treatment with a peroxodisulfate in alkaline solution, to yield a 1,2- or 1,4-dihydroxybenzene 3, is called the Elbs reaction The phenol is deprotonated by base to give a phenolate anion 4, that is stabilized by resonance, and which is activated at the ortho or the para position towards reaction with an electrophilic agent ... 

Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O-H bond. Draw the structures of the anions resulting from loss of H+ from phenol and methanol, and use resonance structures to explain the difference in acidity. 

Figure 16.14 Carbocation intermediates in the nitration of phenol. The ortho and para intermediates are more stable than the meta intermediate because of resonance donation of electrons from oxygen.

Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The pKa of unsubstituted phenol, for example, is 9.89, while that of p-nitrophenol is 7.15, Draw resonance structures of the corresponding phenoxide anions and explain the data. 

It is generally believed that the absorption (and fluorescence excitation) peak at about 400 nm is caused by the neutral form of the chro-mophore, 5-(p-hydroxybenzylidene)imidazolin-4-one, and the one in the 450-500 nm region by the phenol anion of the chromophore that can resonate with the quinoid form, as shown below (R1 and R2 represent peptide chains). However, the emission of light takes place always from the excited anionic form, even if the excitation is done with the neutral form chromophore. This must be due to the protein environment that facilitates the ionization of the phenol group of the chromophore. This is also consistent with the fact that the pACa values of phenols in excited state are in an acidic range, between 3 and 5 (Becker, 1969), thus favoring anionic forms at neutral pH. 

SOMe the enhancement in the meta-position is almost as large as in the para-position. The authors go on to show the applicability of op (g) values to certain solution processes, particularly those in non-aqueous solvents, but including the dissociation of thiophenols in 48% ethanol, the results of Bordwell and Andersen80 to which reference has been made earlier (Section III.A.1). A separation of field/inductive and resonance effects is also essayed for the gas-phase acidities of the phenols, and SOMe and S02Me feature in the discussion. There is reference to a oR° value of + 0.07 for SOMe as an unpublished result of Adcock, Bromilow and Taft (cf. 0.00 from Ehrenson and coworkers65 and — 0.07 from Katritzky, Topsom and colleagues128.)... 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

The reaction is generally believed to proceed via the formation of ionic acylam-monium intermediate compounds (Reaction 1, Scheme 2.27). The equilibrium constant of the acylammonium formation depends mostly on steric and resonance factors, while the basicity of the tertiary amine seems to play a secondary role.297 In die case of the less basic compounds, such as acidic phenols, and of strong tertiary amines, such as Uialkylamines, the reaction has been reported to proceed through a general base mechanism via the formation of hydroxy-amine H-bonded complexes (Reaction 2, Scheme 2.27).297... 

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