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Phenol, C6H5OH, used as a disinfectant and in the manufacture of plastics, dyes, and indicators, ionizes as an acid.
C6H5OH(aq) + H2O(l) H3O+(aq) + C6H5O(aq)
Ka = 1.0×1010
Arrange the following substituted phenols in the order in which you would expect their Ka values to increase. Where would you expect phenol itself to fit into this ranking?
(a) 3-chlorophenol (c) 2,4,6-trichlorophenol
(b) 2,4-dichlorophenol (d) 4-chlorophenol
The structures of the acids are:
The acid strength will change depending upon the relative electronegativity experienced by the ionizable hydrogen atom. All of these examples are substituted by Cl atoms, which have a high electronegativity. The general trend will be determined by first, the number of Cl atoms (more Cl atoms will give more total electroneagtivity), and second, by the location of the Cl atom relative to the ionizable H atom (a closer Cl atom withdraws more electron density from the H atom than a more remote Cl atom). Using these principals:
2,4,6-Trichlorophenol (c) has three electronegative chlorine atoms so is the most acidic. 2,4-Dichlorophenol (b) has two Cl atoms, so is next. 3-Chlorophenol (a) and 4-Chlorophenol (d) both have only a single Cl atom but in 4-chlorophenol the Cl atom is farther from the H atom, so is the least acidic. Phenol is less acidic than any of these because it has no electronegative Cl atoms.
Net: (least acidic) phenol < 4-chlorophenol < 3-chlorophenol < 2,4-dichlorophenol < 2,4,5-trichlorophenol