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 WebsiteTo 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 ethox￾ide 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 elec￾tron 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 solu￾tion 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 bicar￾bonate, 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 intro￾duced it as an antiseptic in 1865 to prevent postopera￾tive bacterial infections that were then a life-threatening hazard in even minor surgi￾cal procedures. The electrostatic poten￾tial 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 phe￾nol? What are their respec￾tive pKa values? How do we know that car￾bonic 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