Phenols

Chapter 24

Phenols

Structure and nomenclature

• phenols are compounds of the general formula Aaroh, where AR is phenyl, substituted phenyl, or one of the offer aryls groups we shall study later (e.g., " naphthyl, Chap. 30). Phenols differ from alcohols in having the OH group attached directly to an aromatic ring. Phenols are generally named as derivatives of the simplest member of the family, phenol. The methyl phenols are given the special name of cresols. Occasionally phenols are named as hydroxy- compounds.,

• Both phenols and alcohols contain the OH group, and as a result the two families resemble each other to a limited extent. We have already seen, for example, that both alcohols and phenols can be converted into ethers and esters. In most of; their properties, however, and in their preparations, the two kinds of compound differ so greatly that they well deserve to be classified as different families.

Physical properties

• The simplest phenols are liquids or low-melting solids; because of hydrogen bonding, they have quite high boiling points. Phenol itself is somewhat soluble in water (9 g per 100 g of water), presumably because of hydrogen bonding with the water; most other phenols are essentially insoluble in water. Unless some group capable of producing color is present, phenols themselves are colorless. However, like aromatic amines, they are easily oxidized; unless carefully purified, many phenols are colored by oxidation products.

•  Let us consider first the m- and /?-isomers. They have very high boiling points because of intermolecular hydrogen bonding: 

Salts of phenols

• Phenols are fairly acidic compounds, and in this respect differ markedly from alcohols, which are even more weakly acidic than water. Aqueous hydroxides convert phenols into their salts; aqueous mineral acids convert the salts back into the free phenols. As we might expect, phenols and their salts have opposite solubility properties, the salts being soluble in water and insoluble in organic solvents.

• Most phenols have #a 's in the neighborhood of 10~ lo , and are thus considerably weaker acids than the carboxylic acids (AVs about 10 ~ 5 ). Most phenols are weaker than carbonic acid, and hence, unlike carboxylic acids, do not dissolve in aqueous bicarbonate solutions. Indeed, phenols are conveniently liberated from their salts by the action of carbonic acid.

• The acid strength of phenols and the solubility of their salts in water are useful both in analysis and in separations. A water-insoluble substance that dissolves in aqueous hydroxide but not in aqueous bicarbonate must be more acidic than water, but less acidic than a carboxylic acid; most compounds in this range of acidity are phenols. A phenol can be separated from non-acidic compounds by means of its solubility in base; it can be separated from carboxylic acids by means of its insolubility in bicarbonate.

Industrial source 

• Most phenols are made industrially by the same methods that are used in the laboratory; these are described in Sec. 24.5. There are, however, special ways of obtaining certain of these compounds on a commercial scale, including the most important one, phenol. In quantity produced, phenol ranks near the top of the list of synthetic aromatic compounds. Its principal use is in the manufacture of the phenol-formaldehyde polymers. A certain amount of phenol, as well as the cresols, is obtained from coal tar. Most of it (probably over 90%) is synthesized. One of the synthetic processes used is the fusion of sodium benzenesulfonates with alkali; another is the Dow process, in which chlorobenzene is allowed to react with aqueous sodium hydroxide at a temperature of about 360. Like the synthesis of aniline from chlorobenzene this second reaction involves nucleophilic substitution under conditions that are not generally employed in the laboratory.

Preparation

• In the laboratory, phenols are generally prepared by one of the two methods outlined below.

• Much simpler and more direct is a recently developed route via thallation. An aryl thallium compound is oxidized by lead tetraacetate (in the presence of triphenylphosphine, Ph3 P) to the phenolic ester of trifluoroacetic acid, which on hydrolysis yields the phenol. The entire sequence, including thallation, can be carried out without isolation of intermediates. Although the full scope of the method has not yet been reported, it has two advantages over the diazonium route.

Reactions

• Aside from acidity, the most striking chemical property of a phenol is the extremely high reactivity of its ring toward electrophilic substitution. Even in ring substitution, acidity plays an important part; ionization of a phenol yields the O~ group, which, because of its full-fledged negative charge, is even more strongly electron-releasing than the OH group. Phenols undergo not only those electrophilic substitution reactions that are typical of most aromatic compounds, but also many others that are possible only because of the unusual reactivity of the ring. We shall have time to take up only a few of these reactions.

Acidity of phenols

• Phenols are converted into their salts by aqueous hydroxides, but not by aqueous bicarbonates. The salts are converted into the free phenols by aqueous mineral acids, carboxylic acids, or carbonic acid.

• Phenols must therefore be considerably stronger acids than water, but considerably weaker acids than the carboxylic acids. Table 24.1 (p. 788) shows that this is indeed so: most phenols have Ka 's of about 10" 10, whereas carboxylic acids have Ka's of about 10" 5. Although weaker than carboxylic acids, phenols are tremendously more acidic than alcohols, which have Ka's in the neighborhood of 10~ 16 {o 10~ 18 How does it happen that an OH attached to an aromatic ring is so Munmorah acidic than an -~OH attached to an alkyl group? The answer is to be found in an examination of the structures involved. As usual we shall assume that differences phaticity are due to differences in stabilities of reactants and products \ Let us examine the structures of reactants and products in the ionization of an alcohol and of phenol. We see that the alcohol and the alkoxide ion are Who represented satisfactorily by a single structure. Phenol and the phenoxide ion.

• contain a benzene ring and therefore must be hybrids of the Kekul6 structures I and II, and III and IV. This resonance presumably stabilizes both molecule and ion to the same extent. It lowers the energy content of each by the same number of kcal/moles, and hence does not affect the difference in their energy contents. If there were no other factors involved, then, we might expect the acidity of a phenol to be about the same as the acidity of an.

• Now, are these two sets of structures equally important? Structures V-VII for phenol carry both positive and negative charges; structures VIII-X for phenoxide ion carry only a negative charge. Since energy must be supplied to separate opposite charges, the structures for the phenol should contain more energy and hence be less stable than the structures for phenoxide ion. (We have already encountered the effect of separation of charge on stability in. The net effect of resonance is therefore to stabilize the phenoxide ion to a greater extent than the phenol, and thus to shift the equilibrium toward ionization and make Ka larger than for an alcohol.

 Formation of ethers. Williamson synthesis

• As already discussed, phenols are converted into ethers by reaction in alkaline solution with alkyl halides; methyl ethers can also be prepared by reaction with methyl sulfate. In alkaline solutions a phenol exists as the phenoxide ion which, acting as a nucleophilic reagent, attacks the halide (or the sulfate) and displaces halide ion (or sulfate ion.)

• While alkoxy groups are activating and o/7/K?,/>0rtf-directing in electrophilic aromatic substitution, they are considerably less so than the OH group. As a Resu* ethers do not generally undergo those reactions (Sees. 24.10-24.12) which require the especially high reactivity of phenols: coupling, Kolbe reaction, Reimer. Tiemann reaction, etc. This difference in reactivity is probably due to the fact that, unlike a phenol, an ether cannot ionize to form the extremely reactive phenoxide ion.

Ester formation. Fries' rearrangement

• Phenols are usually converted into their esters by the action of acids, acid chlorides, or anhydrides as discussed in Sees. 18.16, 20.8, and 20.15. Problem 24.9 Predict the products of the reaction between phenyl benzoate and one mole of bromine in the presence of iron. When esters of phenols are heated with aluminum chloride, the acyl group migrates from the phenolic oxygen to an ortho or para position of the ring, thus yielding a ketone. This reaction, called the Fries rearrangement, is often instead of direct acylation for the synthesis of phenolic ketones. For example:

 

Ring substitution

• Like the amino group, the phenolic group powerfully activates aromatic rings toward electrophilic substitution, and in essentially the same way. The intermediates are hardly carbonium ions at all, but rather oxonium ions (like I and II), in which every atom (except hydrogen) has a complete octet of electrons.

•  they are formed tremendously faster than the carbonium ions derived from benzene itself. Attack on a phenoxide ion yields an even more stable and even more rapidly formed intermediate, an unsaturated ketone (like HI and IV). With phenols, as with amines, special precautions must often be taken /o prevent Poly substitution and oxidation.

• Treatment of phenols with aqueous solutions of bromine results in replacement of every hydrogen ortho or para to the OH group and may even cause displacement of certain other groups. For example: 

Kolbe reaction. Synthesis of phenolic acids

• Treatment of the salt of a phenol with carbon dioxide brings about substitution of the carboxyl group, COOH, for hydrogen of the ring. This reaction is known as the Kolbe reaction; its most important application is in the conversion of phenol itself into 0-hydroxybenzoic acid, known as salicylic acid. Although some p-hydroxybenzoic acid is formed as well, the separation of the two isomers.

Reimer-Tiemann reaction. Synthesis of phenolic aldehydes. Dichlorocarbene

• Treatment of a phenol with chloroform and aqueous hydroxide introduces an aldehyde group, CHO, into the aromatic ring, generally ortho to the OH. This reaction is known as the Reimer-Tiemann reaction. For example:

• The Reimer-Tiemann reaction involves electrophilic substitution on the highly reactive phenoxide ring. The electrophilic reagent is dichlorocarbene, CCl2, generated from chloroform by the action of base. Although electrically neutral, dichlorocarbene contains a carbon atom with only a sextet of electrons and hence is strongly electrophilic.

Analysis of phenols

• The most characteristic property of phenols is their particular degree of acidity. Most of them (Sees. 24.3 and 24.7) are stronger acids than water but weaker acids than carbonic acids. Thus, a water-insoluble compound that dissolves in aqueous sodium hydroxide but not in aqueous sodium bicarbonate is most likely a phenol. Many (but not all) phenols form colored complexes (ranging from green through blue and violet to red) with ferric chloride. (This test is also given by enols.) Phenols are often identified through bromination products and certain esters and ether.

Spectroscopic analysis of phenols

• Infrared. As can be seen in Fig. 24.2 (p. 806), phenols show a strong, broad band due to O H stretching in the same region, 3200-3600 cm* 1 , as alcohols. 

• Namr. Absorption by the O H proton of a phenol, like that of an alcohol (Sec. 16.13), is affected by the degree of hydrogen bonding, and hence by the temperature, concentration, and nature of the solvent. The signal may appear anywhere in the range 8 4-7, or, if there is intramolecular hydrogen bonding, still lower.