Resonance phenoxide anions

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. 

The o scale was developed to incorporate the effect of through-resonance. Both types of correlations included ortho- as well as meta- and para-substituted phenoxides, and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding because this effect is not incorporated in most molecular descriptors. The rate constants for oxidation of phenoxide anions give good Hammett correlations to cr constants, as shown in the following equation ... 

Notice that upon removal of the hydroxy hydrogen by a base, the phenoxide anion results. This anion is resonance stabilized by delocalization of an electron pair throughout the molecule, as shown by the contributing structures. 

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... 

The hydroxyl group of a phenol activates the aromatic ring towards electrophilic attack, whilst the aromatic ring increases the acidity of the hydroxyl group compared to an aliphatic alcohol. Thus many phenols are soluble in sodium hydroxide solution to form the phenoxide anion. Electron-withdrawing nitro groups in the ortho and para positions provide additional resonance stabilization for the phenoxide anion. 2,4,6-Trinitrophenol (picric add) is quite a strong acid. 

Use SpartanView to compare electrostatic potential maps of benzoate anion 1 4-nitrobenzoate anion. Is the nitro group electron-donating or clcctron-withdr ing Next, compare electrostatic potential maps of phenoxidc anion and 4-nit phenoxide anion. Which is more strongly affected by the nitro group, benzoate ai or phenoxide anion Explain, using resonance structures. 

Another commonly encountered acidic functional group found in drug molecules is phenol, or hydroxybenzene. Phenols are weak acids that liberate protons to give the phenoxide anion. This anion is resonance-stabilised and four canonical forms may be drawn (see Figure 3.5). 

Resonance forms of phenoxide anion show the negative charge delocalized onto the ring only at carbons 2, 4, and 6 ... 

Resonance stabilization of the anion of umbelliferone gives not only the same three forms as the phenoxide anion, but in addition, gives an extra resonance form with (-) charge on a carbon, and the most significant resonance contributor, another form with the (-) charge on the other carbonyl oxygen. This anion is much more stable than phenoxide which we interpret as enhanced acidity of the starting material, umbelliferone. In fact, the pKa of umbelliferone is 7.7 while phenol is about 10. 

It was noted that nucleophiles are less nucleophilic if they that can delocalize electrons by resonance. An example is the phenoxide anion, where electron density is delocalized away from oxygen by the adjacent phenyl ring, meaning that oxygen cannot donate electrons to carbon as effectively and is less nucleophilic. It should also be noted that nucleophilic reactions are usually under kinetic control, whereas acid-base reactions are under thermodynamic control. Under kinetic control, the most nucleophilic species will react faster and dominate the substitution reaction. 

The hydroxyl groups also lose their protons to form resonance stabilized phenoxide anions in which the hydroxyl group and the carbonyl group both stabilize the negative charge within the ring by an inductive effect. For example... 

---