Phenoxide resonance structures

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 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... 

Like acetate, phenoxide (CgHjO , the conjugate base of phenol) is also resonance stabilized. In the case of phenoxide, however, there are five resonance structures that disperse the negative charge over a total of four different atoms (three different carbons and the oxygen). 

Moreover, resonance structures 1 and 5 have intact aromatic rings, whereas structures 2-4 do not. This, too, makes structures 2-4 less stable than 1 and 5. Figure 19.6 summarizes this information about phenoxide by displaying the approximate relative energies of its five resonance structures and its hybrid. 

The relative energies of the five resonance structures for phenoxide and its hybrid... 

The two resonance structures that contain an intact aromatic ring and place a negative charge on an O atom are major contributors to the hybrid. Resonance stabilizes phenoxide but not as much as... 

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. 

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. 

As described above, formaldehyde exists as trimethylene glycol in aqueous solution. Phenol reacts quickly with the alkali-hydroxyl group and produces resonance structural phenoxide ion, and trimethylene glycol is added to the O and P positions in the phenoxide ion. This quinoid-transition-state is stabilized by the movement of proton. The monomethylene-derivative produced in this way reacts further with formaldehyde and produces two types of dimethylol derivative and one type of trimethyl derivative. These reactions are expressed as second-order reactions ... 

Note that the phenoxide radical has several resonance structures including those which contain the free radical inside the benzene ring. This radical may attack phenol to form a dimer which quickly tautomerizes to give 4,4 -di-hydroxybiphenyl, which in turn will repeat the previous sequence to form a more complex quinoid structure or it may react with the peroxide catalyst to form a two ringed quinone. Summarizing ... 

Thus a carboxylic acid is more acidic than a phenol. This can be explained by the different activation energies for ionization. The key difference is that the delocalization in the carboxylate anion stabilizes this species more relative to the reactant carboxylic acid than delocalization stabilizes the phenoxide anion in relation to the starting phenol. The delocalization in the carboxylate anion is significant as seen in the set of equivalent 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 phenoxide 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. 

PROBLEM 7.22 Write resonance structures that indicate how the unpaired electron in the phenoxy radical can be delocalized to the ortho and para positions (reason by analogy with the resonance structures for the phenoxide anion on page 215). 

The acidity of phenols, compared with aliphatic alcohols, can be accounted for by the combined — I effect of the benzene ring and the delocalization of the electrons of oxygen in the phenoxide ion, as shown by the resonance structures in equation (5) In general, the... 

Electron delocalization in phenoxide is represented by resonance among the structures... 

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

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