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Unformatted text preview: Practice Problem 6‐25 – Answer Account for the fact that water‐insoluble carboxylic acids (pKa 4–5) dissolve in 10% aqueous sodium bicarbonate (pH 8.5) with the evolution of a gas but that water insoluble phenols (pKa 9.5–10.5) do not dissolve in 10% sodium bicarbonate. When the pH of a solution is greater than the pKa of an acid, the acid exists primarily in its deprotonated form (i.e. as its conjugate base). At a pH of 8.5, a carboxylic acid will be deprotonated, and the carboxylate salt will dissolve in the aqueous solvent. Gas is evolved when carbonic acid, the conjugate acid of the bicarbonate anion, decomposes to yield CO2 and H2O. When the pH of a solution is less than the pKa of an acid, the acid exists primarily in its protonated form. At a pH of 8.5, then, a phenol will be protonated, and will not dissolve in an aqueous solvent. Practice Problem 6‐26 – Answer Match each compound with its appropriate pKa value. (a) 4‐Nitrobenzoic acid, benzoic acid, 4‐chlorobenzoic acid pKa = 4.19 (benzoic acid), 3.98 (4‐chlorobenzoic acid), and 3.41 (4‐Nitrobenzoic acid) (b) Benzoic acid, cyclohexanol, phenol pKa = 18.0 (cyclohexanol), 9.95 (phenol), and 4.19 (benzoic acid) (c) 4‐Nitrobenzoic acid, 4‐nitrophenol, 4‐nitrophenylacetic acid pKa = 7.15 (4‐Nitrophenol), 3.85 (4‐Nitrophenylacetic acid), and 3.41 (4‐Nitrobenzoic acid) Practice Problem 6‐27 – Answer Explain the trends in acidity of phenol and the monofluoro derivatives of phenol. Fluorine has an electron‐withdrawing inductive effect. All of the monofluoro deriviates of phenol are therefore more acidic than phenol itself. The closer F is to the oxygen atom, the greater its ability to stabilize the negative charge of the phenoxide anion as a result of this inductive effect. This is why the pKa increases (acid strength decreases) as F moves from ortho to meta to para with respect to the hydroxyl group. Practice Problem 6‐28 – Answer Arrange the molecules and ions in each set in order of increasing acidity (from least acidic to most acidic). (1 = least acidic; 3 = most acidic) Practice Problem 6‐29 – Answer From each pair, select the stronger base. Practice Problem 6‐30 – Answer Predict the products resulting from vigorous oxidation of each compound by H2CrO4. Practice Problem 6‐31 – Answer Show how to convert 1‐phenylpropane into the following compounds. In addition to the starting material, use any necessary inorganic reagents. Any compound synthesized in one part of this problem may be used to make any other compound in the problem. Practice Problem 6‐32 – Answer Using ethylbenzene as the only aromatic starting material, show how to synthesize the following compounds. In addition to ethylbenzene, use any other necessary organic or inorganic chemicals. Any compound already synthesized in one part of this problem may them be used to make any other compound in the problem. ...
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- Summer '08