Phenol anion, resonance structures

Two of the following nitrophenols are much more acidic than phenol itself. The third compound is only slightly more acidic than phenol. Use resonance structures of the appropriate phenox-ide ions to show why two of these anions should be unusually stable. 

An alternative explanation for the greater acidity of phenol relative to cyclohexanol can be based on similar resonance structures for the phenoxide ion. Unlike the structures for phenol, 2—4, resonance structures for the phenoxide ion do not involve charge separation. According to resonance theory, such structures should stabilize the phenoxide ion more than structures 2-4 stabilize phenol. (No resonance structures can be written for cyclohexanol or its anion, of course.) Greater stabilization of the phenoxide ion (the conjugate base) than of phenol (the acid) has an acid-strengthening effect. 

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. 

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. 

And what about an alternative product There are two lines of thought, and the most obvious is that the reaction is repeated, since we are using a dibromide as substrate. Alternatively, we could consider one of the other resonance forms of the phenolate anion as nucleophile. This would generate a C-alkylated phenol. In the majority of cases, C-alkylation is not observed, in that the preferred resonance structure has charge on the electronegative oxygen. 

When the reaction site comes into direct resonance with the substituent, the a constants of the substituents do not succeed in correlating equilibrium or rate constants. For example a />-nitro group increases the ionization constant of phenol much more than would be predicted from the ov ND2 constant obtained from the ionization of />-nitrobenzoic acid. The reason is readily understood when one realizes that the />-nitrophenoxide ion has a resonance structure (11) in which the nitro group participates in through-resonance7 with the O-. The extra stabilization of the anion provided by this structure is not included in the ap NOs constant... 

Many compounds involve combinations of the preceding resonance types. Figure 3.21 shows resonance structures for the anion that results from removing a proton (H+) from the oxygen of phenol. 

The anion derived from phenol by loss of a proton has Resonance structures and are equivalent Both five important resonance structures that contribute of these contribute equally to the resonance hybrid,... 

Resonance structures for the anion derived from phenol. 

The nitrogen is sp2 hybridized with its unshared pair of electrons in a p orbital. This is a combination of the resonance types involving a cycle of double bonds and a pair of electrons next to a double bond. In fact, the situation is veiy similar to that shown for the anion derived from phenol in Figure 3.21. The important resonance structures are as follows ... 

The acidity of phenols arises from the greater resonance stabilization of the phenoxide anion compared with phenol itself (Scheme 4.6). There is no energy-demanding separation of charge in the resonance structures... 

Figure 24.1. Molecular structure and position of equilibrium. Phenol yields resonance-stabilized anion is stronger acid than alcohol. (Plots aligned with each other for easy comparison.)...

Phenols, compounds that have an -OH group bonded to a benzene ring, are relatively acidic because their anions are stabilized by resonance. Draw as many resonance structures as you can for the phenoxide ion. 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

Phenol can be considered as the enol of cyclohexadienone. While the tautomeric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and / ara-quinonoid resonance structures. In the formation of the phenolate anion, the contribution of quinonoid resonance structures can stabilize the negative charge. 

The differences between the absorption spectra of un-ionized phenols (I) and their anions (II) were observed in very early studies of simple phenols by Baly and Ewbank (1905) and Ley (1920), but there are few systematic investigations of the effect in the more recent literature. An excellent example in the biochemical field is due to Callow (1936) who found that the phenolic hydroxyl groups in ring A of estrone, estradiol and estriol showed the same changes in alkali as simple phenols. The longwave shifts of both absorption bands and the increase in intensity can be explained by the increased number of low-energy resonance structures possible for the anion (III) compared with the neutral molecule and the consequent increase in stability of its first excited state (Pauling, 1942). 

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