3。8 CHAPTER 24 PHENOLS henols are compounds that have a hydroxyl group bonded directly to a benzene or benzenoid ring. The parent compound of this group, C6HsOH, called simply phe- nol, is an important industrial chemical. Many of the properties of phenols are anal ogous to those of alcohols, but this similarity is something of an oversimplification. Like arylamines, phenols are difunctional compounds; the hydroxyl group and the aromatic ring interact strongly, affecting each other's reactivity. This interaction leads to some novel and useful properties of phenols. A key step in the synthesis of aspirin, for exam- ple, is without parallel in the reactions of either alcohols or arenes. With periodic reminders of the ways in which phenols resemble alcohols and arenes, this chapter emphasizes the ways in which phenols are unique 24.1 NOMENCLATURE An old name for benzene was phene, and its hydroxyl derivative came to be called ph nol.* This, like many other entrenched common names, is an acceptable IUPAC name Likewise. 0- and p-cresol e acceptable names for the various ring-substituted hydroxyl derivatives of toluene. More highly substituted compounds are named as deriv- atives of phenol Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction that gives the lower number to the next substituted carbon Sub stituents are cited in alphabetical order H3 Phenol -Cresol 5-Chloro-2-methylpheno The systematic na 93 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
939 CHAPTER 24 PHENOLS Phenols are compounds that have a hydroxyl group bonded directly to a benzene or benzenoid ring. The parent compound of this group, C6H5OH, called simply phenol, is an important industrial chemical. Many of the properties of phenols are analogous to those of alcohols, but this similarity is something of an oversimplification. Like arylamines, phenols are difunctional compounds; the hydroxyl group and the aromatic ring interact strongly, affecting each other’s reactivity. This interaction leads to some novel and useful properties of phenols. A key step in the synthesis of aspirin, for example, is without parallel in the reactions of either alcohols or arenes. With periodic reminders of the ways in which phenols resemble alcohols and arenes, this chapter emphasizes the ways in which phenols are unique. 24.1 NOMENCLATURE An old name for benzene was phene, and its hydroxyl derivative came to be called phenol.* This, like many other entrenched common names, is an acceptable IUPAC name. Likewise, o-, m-, and p-cresol are acceptable names for the various ring-substituted hydroxyl derivatives of toluene. More highly substituted compounds are named as derivatives of phenol. Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction that gives the lower number to the next substituted carbon. Substituents are cited in alphabetical order. OH Phenol OH CH3 m-Cresol OH CH3 Cl 1 2 3 4 5 6 5-Chloro-2-methylphenol *The systematic name for phenol is benzenol. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-FoUR Phenols The three dihydroxy derivatives of benzene may be named as 1, 2-,1,3-, and 1, 4- benzenediol, respectively, but each is more familiarly known by the common name indi cated in parentheses below the structures shown here. These common names are per missible Iupac names OH OH OH rocatechol is often called 1.2-Benzenediol 3-Benzenediol 1. 4-Benzenediol The common names for the two hydroxy derivatives of naphthalene are 1-naph thol and 2-naphthol. These are also acceptable IUPAC names PROBLEM 24.1 Write structural formulas for each of the following compounds (a)Pyrogallol (1, 2, 3-benzenetriol) (c)3-Nitro-1-naphthol (d)4-Chlororesorcinol SAMPLE SOLUTION (a) Like the dihydroxybenzenes, the isomeric trihydroxy benzenes have unique names. Pyrogallol, used as a developer of photographi film, is 1, 2, 3-benzenetriol. The three hydroxyl groups occupy adjacent positions a benzene ring (1, 2, 3-benzenetriol) Carboxyl and acyl groups take precedence over the phenolic hydroxyl in deter- mining the base name. The hydroxyl is treated as a substituent in these cases. HO -COH CH p-Hydroxybenzoic acid 2-Hydroxy-4-methy lacet 24.2 STRUCTURE AND BONDING Phenol is planar, with a C-O-H angle of 109%, almost the same as the tetrahedral angle and not much different from the 108.5C-0-H angle of methanol 142pm this chapter is a molecular mo structure and electrostatic po- Methanol Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The three dihydroxy derivatives of benzene may be named as 1,2-, 1,3-, and 1,4- benzenediol, respectively, but each is more familiarly known by the common name indicated in parentheses below the structures shown here. These common names are permissible IUPAC names. The common names for the two hydroxy derivatives of naphthalene are 1-naphthol and 2-naphthol. These are also acceptable IUPAC names. PROBLEM 24.1 Write structural formulas for each of the following compounds: (a) Pyrogallol (1,2,3-benzenetriol) (c) 3-Nitro-1-naphthol (b) o-Benzylphenol (d) 4-Chlororesorcinol SAMPLE SOLUTION (a) Like the dihydroxybenzenes, the isomeric trihydroxybenzenes have unique names. Pyrogallol, used as a developer of photographic film, is 1,2,3-benzenetriol. The three hydroxyl groups occupy adjacent positions on a benzene ring. Carboxyl and acyl groups take precedence over the phenolic hydroxyl in determining the base name. The hydroxyl is treated as a substituent in these cases. 24.2 STRUCTURE AND BONDING Phenol is planar, with a C±O±H angle of 109°, almost the same as the tetrahedral angle and not much different from the 108.5° C±O±H angle of methanol: O H 136 pm 109° Phenol O H 142 pm 108.5° CH3 Methanol HO COH O p-Hydroxybenzoic acid CH3 CCH3 O OH 5 6 4 1 3 2 2-Hydroxy-4-methylacetophenone OH OH OH Pyrogallol (1,2,3-benzenetriol) OH OH 1 2 3 4 5 6 1,2-Benzenediol (pyrocatechol) OH OH 1 2 3 4 5 6 1,4-Benzenediol (hydroquinone) OH OH 1 2 3 4 5 6 1,3-Benzenediol (resorcinol) 940 CHAPTER TWENTY-FOUR Phenols Pyrocatechol is often called catechol. The graphic that opened this chapter is a molecular model of phenol that shows its planar structure and electrostatic potential. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
24.3 Physical Properties As weve seen on a number of occasions, bonds to sp-hybridized carbon are shorter than those to sp-hybridized carbon, and the case of phenols is no exception. The carbon-oxygen bond distance in phenol is slightly less than that in methanol In resonance terms, the shorter carbon-oxygen bond distance in phenol is attrib- uted to the partial double-bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring +Oh +OH H Most stable lewis Dipolar resonance forms of phenol tructure for Many of the properties of phenols reflect the polarization implied by the resonance description. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophiles attack the aromatic ring of phenols much faster than they attack benzene, indicating that the ring, especially at the positions ortho and 24,3 PHYSICAL PROPERTIES The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules(Figure 24.1a) and cal properties of with water(Figure 241b). Thus, phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular ected in Appendix 1 weight. Table 24.1 compares phenol, toluene, and fluorobenzene with regard to these Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower boiling points than those of the meta and para isomers. This is because the intramolec- ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. TABLE 24.1 Comparison of Physical Properties of an Arene, a Phenol, and an Aryl Halide Toluene, Phenol Fluorobenzene Physical property C6H5CH3 C6HsOH Molecular weight Melting point 95°c 43°C -41°C Boiling point (1 atm) 111°C 32°C Solubility in water(25.C) 0.05g/100mL 8.2g/100mL 0.2g/100ml Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
As we’ve seen on a number of occasions, bonds to sp2 -hybridized carbon are shorter than those to sp3 -hybridized carbon, and the case of phenols is no exception. The carbon–oxygen bond distance in phenol is slightly less than that in methanol. In resonance terms, the shorter carbon–oxygen bond distance in phenol is attributed to the partial double-bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring. Many of the properties of phenols reflect the polarization implied by the resonance description. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophiles attack the aromatic ring of phenols much faster than they attack benzene, indicating that the ring, especially at the positions ortho and para to the hydroxyl group, is relatively “electron-rich.” 24.3 PHYSICAL PROPERTIES The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules (Figure 24.1a) and with water (Figure 24.1b). Thus, phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular weight. Table 24.1 compares phenol, toluene, and fluorobenzene with regard to these physical properties. Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower boiling points than those of the meta and para isomers. This is because the intramolecular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. Dipolar resonance forms of phenol H H H H H OH Most stable Lewis structure for phenol H H H H H OH H H H H H OH H H H H H OH 24.3 Physical Properties 941 The physical properties of some representative phenols are collected in Appendix 1. TABLE 24.1 Comparison of Physical Properties of an Arene, a Phenol, and an Aryl Halide Physical property Molecular weight Melting point Boiling point (1 atm) Solubility in water (25°C) Toluene, C6H5CH3 92 95°C 111°C 0.05 g/100 mL Phenol, C6H5OH 94 43°C 132°C 8.2 g/100 mL Fluorobenzene, C6H5F 96 41°C 85°C 0.2 g/100 mL Compound Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-FoUR Phenols FIGURE 24.1(a)A hy drogen bond between two phenol molecules; (b) hydr gen bonds between water and phenol molecules Intramolecular hydrogen bond H PROBLEM 24.2 One of the hydroxybenzoic acids is known by the common name salicylic acid. Its methyl ester, methyl salicylate, occurs in oil of wintergr Methyl salicylate boils over 50oC lower than either of the other two methyl hydroxybenzoates. What is the structure of methyl salicylate? Why is its boiling point so much lower than that of either of its regioisomers? 24. 4 ACIDITY OF PHENOLS The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols but less acidic than carboxylic acids. Recall that carboxylic acids have ioniza tion constants Ka of approximately 10(pKa 5), whereas the Kas of alcohols are in the 10-16 to 10-20 range(pka 16-20). The Ka for most phenols is about 10-0(pka 10 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 24.2 One of the hydroxybenzoic acids is known by the common name salicylic acid. Its methyl ester, methyl salicylate, occurs in oil of wintergreen. Methyl salicylate boils over 50°C lower than either of the other two methyl hydroxybenzoates. What is the structure of methyl salicylate? Why is its boiling point so much lower than that of either of its regioisomers? 24.4 ACIDITY OF PHENOLS The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols but less acidic than carboxylic acids. Recall that carboxylic acids have ionization constants Ka of approximately 105 (pKa 5), whereas the Ka’s of alcohols are in the 1016 to 1020 range (pKa 16–20). The Ka for most phenols is about 1010 (pKa 10). N O O H O Intramolecular hydrogen bond in o-nitrophenol 942 CHAPTER TWENTY-FOUR Phenols (a) (b) -------------------- --------------- --------------- FIGURE 24.1 (a) A hydrogen bond between two phenol molecules; (b) hydrogen bonds between water and phenol molecules. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
24.4 Acidity of Phenols To help us understand why phenols are more acidic than alcohols, let's compare Because of its acidity, phenol the ionization equilibria for phenol and ethanol. In particular, consider the differences in was known as carbolic acid charge delocalization in ethoxide ion and in phenoxide ion. The negative charge in ethox- when Joseph Lister intro. ide ion is localized on oxygen and is stabilized only by solvation forces CH3CH20-H H+ CH3CH20: Ka=10 (pKa=16) were then a life-threatening hazard in even minor surge- Ethanol Proton Ethoxide ion he negative charge in phenoxide ion is stabilized both by solvatic elec tron delocalization into the ring. Ka=10-10(pKa=10) Proton Phenoxide ion Electron delocalization in phenoxide is represented by resonance among the nto the ring H H The negative charge in phenoxide ion is shared by the oxygen and the carbons that are ortho and para to it. Delocalization of its negative charge strongly stabilizes phenoxide ion To place the acidity of phenol in perspective, note that although phenol is more than a million times more acidic than ethanol. it is over a hundred thousand times weaker than acetic acid. Thus, phenols can be separated from alcohols because they are more acidic, and from carboxylic acids because they are less acidic On shaking an ether solu- tion containing both an alcohol and a phenol with dilute sodium hydroxide, the phenol salt, which is extracted into the aqueous phase How do we know that water OH+ H,O tive pk. values? Phenol Hydroxide ion Phenoxide ion Water (stronger acid (weaker base) weaker acid) On shaking an ether solution of a phenol and a carboxylic acid with dilute sodium bonate, the carboxylic acid is converted quantitatively to its sodium salt and extr into the aqueous phase. The phenol remains in the ether phase OH+ HCO3 O+ H,CO3 ger acid than phenol? What are their Bicarbonate ion noxide ion Carbonic acid espective pKa values? Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
To help us understand why phenols are more acidic than alcohols, let’s compare the ionization equilibria for phenol and ethanol. In particular, consider the differences in charge delocalization in ethoxide ion and in phenoxide ion. The negative charge in ethoxide ion is localized on oxygen and is stabilized only by solvation forces. The negative charge in phenoxide ion is stabilized both by solvation and by electron delocalization into the ring. Electron delocalization in phenoxide is represented by resonance among the structures: The negative charge in phenoxide ion is shared by the oxygen and the carbons that are ortho and para to it. Delocalization of its negative charge strongly stabilizes phenoxide ion. To place the acidity of phenol in perspective, note that although phenol is more than a million times more acidic than ethanol, it is over a hundred thousand times weaker than acetic acid. Thus, phenols can be separated from alcohols because they are more acidic, and from carboxylic acids because they are less acidic. On shaking an ether solution containing both an alcohol and a phenol with dilute sodium hydroxide, the phenol is converted quantitatively to its sodium salt, which is extracted into the aqueous phase. The alcohol remains in the ether phase. On shaking an ether solution of a phenol and a carboxylic acid with dilute sodium bicarbonate, the carboxylic acid is converted quantitatively to its sodium salt and extracted into the aqueous phase. The phenol remains in the ether phase. K 1 OH Phenol (weaker acid) HCO3 Bicarbonate ion (weaker base) O Phenoxide ion (stronger base) H2CO3 Carbonic acid (stronger acid) K 1 OH Phenol (stronger acid) HO Hydroxide ion (stronger base) O Phenoxide ion (weaker base) H2O Water (weaker acid) H H H H H O H H H H H O H H H H H O H H H H H O Ka 1010 (pKa 10) Proton H Phenol O H Phenoxide ion O Ka 1016 CH H (pKa 16) 3CH2O Ethanol Proton H CH3CH2O Ethoxide ion 24.4 Acidity of Phenols 943 Because of its acidity, phenol was known as carbolic acid when Joseph Lister introduced it as an antiseptic in 1865 to prevent postoperative bacterial infections that were then a life-threatening hazard in even minor surgical procedures. The electrostatic potential map of phenoxide ion on Learning By Modeling displays the delocalization of electrons into the ring. How do we know that water is a weaker acid than phenol? What are their respective pKa values? How do we know that carbonic acid is a stronger acid than phenol? What are their respective pKa values? Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website