CHAPTER TWENTY-FoUR Phenols It is necessary to keep the acidity of phenols in mind when we discuss prepara- tion and reactions. Reactions that produce phenols, when carried out in basic solution, require an acidification step in order to convert the phenoxide ion to the neutral form of do we know that hy HO oH+ H. respective pk, values? Phenoxide ion onium ion Phenol (weaker acid)(weaker base) Many synthetic reactions involving phenols as nucleophiles are carried out in the ence of sodium or potassium hydroxide. Under these conditions the phenol is con rted d to the corresponding phenoxide ion, which is a far better nucleophile 24.5 SUBSTITUENT EFFECTS ON THE ACIDITY OF PHENOLS As Table 24. 2 shows, most phenols have ionization constants similar to that of phenol itself. Substituent effects, in general, are small TOn. Alkyl substitution produces negligible changes in acidities, as do weakly elec- ative groups attached to the ring TABLE 24.2 Acidities of Some Phenols lonization Compound constant k monosubstituted phenols 1.0×10 4.7×10-11 Recall from Section 24.1 o-Cresol hat cresols are methyl m-Cresol 80×10-11 10.1 ubstituted derivatives of phenol. o-Chlorophenol 2.7×10-9 p-Chlorophenol 7.6×10-9 chlorom 3.9×10 10×10-10 10.0 p-Methoxyphenol 6.3×10 10.2 5.9×10 7.2 6.9 2, 4-Dinitrophenol 1.1×10-4 4.0 3, 5-Dinitrophenol 20×10-7 4.2×10 Naphthols 1-Naphthol 5.9×10-1 3.5×10 9.5 Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteIt is necessary to keep the acidity of phenols in mind when we discuss prepara￾tion and reactions. Reactions that produce phenols, when carried out in basic solution, require an acidification step in order to convert the phenoxide ion to the neutral form of the phenol. Many synthetic reactions involving phenols as nucleophiles are carried out in the presence of sodium or potassium hydroxide. Under these conditions the phenol is con￾verted to the corresponding phenoxide ion, which is a far better nucleophile. 24.5 SUBSTITUENT EFFECTS ON THE ACIDITY OF PHENOLS As Table 24.2 shows, most phenols have ionization constants similar to that of phenol itself. Substituent effects, in general, are small. Alkyl substitution produces negligible changes in acidities, as do weakly elec￾tronegative groups attached to the ring. K  1 OH Phenol (weaker acid) H3O Hydronium ion (stronger acid) O Phenoxide ion (stronger base) H2O Water (weaker base) 944 CHAPTER TWENTY-FOUR Phenols How do we know that hy￾dronium ion is a stronger acid than phenol? What are their respective pKa values? Recall from Section 24.1 that cresols are methyl￾substituted derivatives of phenol. TABLE 24.2 Acidities of Some Phenols Ionization constant Ka 1.0  1010 4.7  1011 8.0  1011 5.2  1011 2.7  109 7.6  109 3.9  109 1.0  1010 2.2  1010 6.3  1011 5.9  108 4.4  109 6.9  108 1.1  104 2.0  107 4.2  101 5.9  1010 3.5  1010 pKa 10.0 10.3 10.1 10.3 8.6 9.1 9.4 10.0 9.6 10.2 7.2 8.4 7.2 4.0 6.7 0.4 9.2 9.5 Compound name Monosubstituted phenols Phenol o-Cresol m-Cresol p-Cresol o-Chlorophenol m-Chlorophenol p-Chlorophenol o-Methoxyphenol m-Methoxyphenol p-Methoxyphenol o-Nitrophenol m-Nitrophenol p-Nitrophenol Di- and trinitrophenols 2,4-Dinitrophenol 3,5-Dinitrophenol 2,4,6-Trinitrophenol 1-Naphthol 2-Naphthol Naphthols Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website