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  Reactions of Phenols Homework Help - K-12 Grade Level, College Level Chemistry

Introduction of Reactions of Phenols

Phenols are the compounds in which a hydroxyl group is bonded to an aromatic ring. The chemical behaviour of phenols is distinct in some respects from that of the alcohols, so it is reasonable to treat them as an identical but characteristically different group. A subsequent variation in reactivity was observed in comparing aryl halides, like bromobenzene, with alkyl halides, like butyl bromide and tert-butyl chloride. So, nucleophilic substitution and elimination reactions were general for alkyl halides, but uncommon with aryl halides. This distinction carries over when comparing phenols and alcohols, thus for all practical purposes elimination and/or substitution of the phenolic hydroxyl group does not take place.

Acidity of Phenols

Alternatively, the hydroxyl hydrogen's atom substitution is even more facile with phenols, that are approximately a million times more acidic than equal alcohols. This phenolic acidity is additionally improved by electron-withdrawing substituents ortho and para to the hydroxyl group, as shown in the diagram. The alcohol cyclohexanol is displayed for reference at the top left. It is notable that the affect of a nitro substituent is over ten times stronger in the para-location than it is meta, in spite of the fact that the latter position is closer to the hydroxyl group. In addition additional nitro groups have an additive affect if they are positioned in para or ortho locations. The trinitro compound displayed at the lower right is a very strong acid called picric acid.

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As noticed in our previous treatment of electrophilic aromatic substitution reactions, an oxygen substituent improves the reactivity of the favors and ring electrophile attack at para and ortho sites. It was planned that resonance delocalization of an oxygen non-bonded electron pair into the pi-electron system of the aromatic ring was in charge for this substituent effect. An identical set of resonance structures for the phenolate anion conjugate base appears below the phenol structures.

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The resonance stabilization in these two cases is very distinct. A significant principle of resonance is that charge separation diminishes the importance of canonical contributors to the resonance hybrid and reduces the entire stabilization. The contributing structures to phenol hybrid all suffer charge separation, resultant in very modest stabilization of this compound. Alternatively, the phenolate anion is already charged, and the canonical contributors perform to disperse the charge, resultant in a substantial stabilization of this species. By charge delocalization, the conjugate bases of simple alcohols are not stabilized so the acidity of these compounds is identical to that of water. An energy picture displaying the effect of resonance on cyclohexanol and phenol acidities is displayed on the right. Because the resonance stabilization of the phenolate conjugate base is much larger than the stabilization of phenol itself, the acidity of the phenol relative to cyclohexanol is get increased. Supporting proof that the phenolate negative charge is delocalized on the para and ortho carbons of the benzene ring comes from the affect of electron-withdrawing substituents at those sites.

Ring Substitution of Phenols

The facility by which the aromatic ring of phenols and phenol ethers goes through electrophilic substitution has been noted. Two illustrations are displayed in the diagram. The first depicts the Friedel-Crafts synthesis of the food preservative BHT from para-cresol. The second reaction is interesting in that it further illustrates the delocalization of charge that comes into existence in the phenolate anion. Carbon dioxide is weak electrophile and generally does not react with aromatic compounds; though, the negative charge concentration on the phenolate ring allows the carboxylation reaction displayed in the 2nd step. The sodium salt of salicylic acid is the important product, and the preference for ortho substitution may reflect the effect of the sodium cation. This is termed the Kolbe-Schmidt reaction and it has served in the preparation of aspirin, as the last step demonstrates.

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Oxidation to Quinones

Phenols are rather easily oxidized in spite of the nonexistence of a hydrogen atom on the hydroxyl bearing carbon. Between the colored products from the oxidation of phenol by chromic acid is the dicarbonyl compound para-benzoquinone ; an ortho isomer is also identified. These compounds are simply reduced to their dihydroxybenzene analogs and it is from these compounds which quinones are best prepared. Note: meta-quinones having identical structures do not present. The redox equilibria among the dihydroxybenzenes hydroquinone and catechol and their quinone oxidation states are so facile that milder oxidants than chromate (Jones reagent) are usually preferred. One such type of oxidant is Fremy's salt, displayed on the right. Reducing agents other than stannous chloride (e.g. NaBH4) may be used for the reverse reaction.
The position of the quinone-hydroquinone redox equilibrium is proportional to the square of the hydrogen ion concentration, as displayed by the following half-reactions (electrons are colored blue in the diagram). The electrode potential for this interconversion may therefore be employed to measure the pH of solutions.

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Even though chromic acid oxidation of phenols having an unsubstituted para-position gives some p-quinone product, the reaction is difficult and is not synthetically helpful. It has been found that salcomine, a cobalt complex binds the oxygen reversibly in solution, and catalyzes the oxidation of several substituted phenols to the corresponding p-quinones. The structure of salcomine and an instance of this reaction are displayed in the equation below. The solvent of choice for these oxidations is generally dimethylformamide (DMF) or methanol.

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