Phenol resonance structures

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. 

Long before their theories were supported by computations, organic chemists found a way to use resonance structures to explain the product distribution in electrophilic substitution. Thus, the Lewis structure for phenol is regarded as a resonance hybrid of the following structures ... 

The first situation is illustrated in the following resonance structures in the case of phenol ... 

The spectra of niclosamide in methanol (Fig. 3a) and methanolic base (Fig. 3c), show four bands with the same 2max values but max values increase in base. In methanolic acid (Fig. 3b) only two bands appeared [19]. This could be explained in terms of resonance effects as well as the dissociation of the phenolic-OH group to phenolate in base [20,21]. The possible resonance structures of niclosamide are shown below ... 

A last example concerns the rotational barrier in phenoxyl radicals (Gilbert et ah, 1988). Compared to the parent phenols [37] and [39] the rotational barrier in [38] is increased by a factor of seven, whereas, with a captor substituent [40], the barrier increases only by a factor of 1.2. This could be interpreted in terms of a captodative stabilization in [38]. The captodative character of the radical [38] is represented by a resonance structure [41]. 

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions (Unit 13, Class XI). The -OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become eiectron rich due to the resonance effect caused by -OH group. The resonance structures are shown under acidity of phenols. 

Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of the corresponding phenoxlde ions. 

Although phenoxlde ion has more number of resonating structures than carbojQ late Ion, carboxylic acid is a stronger acid than phenol. Why ... 

Note that m-nitrophenol has pATa 8.4, and is a lot less acidic than o-nitrophenol or p-nitrophenol. We can draw no additional resonance structures here, and the nitro group cannot participate in further electron delocalization. The increased acidity compared with phenol can be ascribed to stabilization of resonance structures with the charge on a ring carbon through the nitro group s inductive effect. 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

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. 

Many times, you can look at the hybrid of the starting material in order to predict where the electrophile will attack. For example, start by looking at Figure 7-18, which shows the resonance structures for the phenolate ion. The resonance hybrid of the phenolate ion is shown in Figure 7-19. 

Figure 9 shows plots of Hammett fr+ values versus E j2 for the 8-p-X-Ph-dG adducts. In Fig. 9A, the OH (—0.92 ) fr+ value was used and the regression deviated from linearity. However, Fig. 9B shows that the regression is improved to almost unity when the O (2.30 ) fr+ value is used. These results suggested that the oxidation of 8-p-PhOH-dG may be coupled with phenol deprotonation. As shown in Scheme 12, resonance structures for the radical cation of 8-p-PhOH-dG create a p-substituted phenol radical cation, which possess negative pAa values (pifa for phenol radical cation ). Phenolic radical cations undergo deprotonation rapidly in the presence of water (0.6-6 x to yield neutral phenolic radicals. In the anhydrous DMF solvent used for electrochemical measurements, an N-7 adduct atom or adventitious water in the solvent could serve as base to facilitate phenolic radical production. 

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... 

Silicon bound to a phenyl group can also influence the bond system by additional (p- -d) back donation from carbon to silicon. In agreement with this model, p-trimethylsilyl-substituted benzoic acid shows a greater acidity than expected from inductive effects. Furthermore, p-trimethylsilyl phenol exhibits a greater acidity than phenol itself, and p-trimethylsilyl aniline shows a decreased basicity as compared with that of the nonsubstituted compound. This behaviour can be described by the following resonance structures [Eqs. (4) and (5)] ... 

The phenolic enone 169 was recovered unchanged from treatment with sodium methoxide however, treatment of furanone 70 under the same basic conditions gave smoothly the enone isomer 169. Therefore, the lack of closure of 169 is the result of an unfavorable equilibrium. The process 170 -169 can be looked at as a 5-Exo-Trigonal process due to the resonance structure 171B. 

Naphtholsulfonic acids mainly couple at the 2-position. The 4-coupling products obtained as byproducts must be carefully removed from the azo dyes, because unlike the 2-substitution products, their shade changes as a function of the pH value (shade intensification with rising pH due to formation of phenolate or naphtholate resonance structures). 

The phenolic initially gives up its labile hydrogen, which in turn reacts with the various radicals produced in chain reactions then the phenoxy radical becomes stabilized owing to its ability to form resonance structures. The resonance-stabilized forms of the phenoxy radical will not attack tertiary carbon—hydrogen bonds in the polypropylene chain but will react with other radicals such as a peroxide, resulting in the elimination of a second free radical. 

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 ... 

Show the resonance structures for the conjugate base of phenol. 

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. 

Fig. 7-30. Examples of proposed leucochromophoric and chromophoric structures. Aryl-coumarones (1) and stilbene quinones (2) are thought to be formed from stilbenes after oxidation. Butadiene quinones (3) could arise from oxidation of hydroxyarylbutadienes being formed from phenolic pinoresinol structures during kraft or neutral sulfite pulping. Cyclization may yield intermediates which are further oxidized to cyclic diones (4). A resonance-stabilized structure (5) results from the corresponding condensation product formed during pulping. o-Quinoid structures (7) are oxidation products of catechols (6) formed during alkaline or neutral pulping processes.

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