512 CARBOXYLIC ACIDS attached to the carboxyl group in benzoic acid stabilizes benzoate anion better than the cor responding sp-hybridized carbon stabilizes cyclohexanecarboxylate. Benzoic acid is stronger acid CO,H CO2H Benzoic acid K.1.2×10-5 (d) Its five fluorine substituents make the pentafluorophenyl group more electron-withdrawing than an unsubstituted phenyl group. Thus, pentafluorobenzoic acid is a stronger acid than benzoic acid COH CO,H Pentafluorobenzoic acid Benzoic acid 1×10-4 Ka6.7×10 (pK3.4) (e) The pentafluorophenyl substituent is electron-withdrawing and increases the acidity of a car- boxyl group to which it is attached. Its electron-withdrawing effect decreases with distance Pentafluorobenzoic acid is a stronger acid than p-(pentafluorophenyl)benzoic acid F COH COH Pentafluorobenzoic acid p-(Pentafluorophenyl)benzoic acid K241×10-4 (K, not measured in water; comparable (pk3.4) th benzoic acid in acidity) (f) The oxygen of the ring exercises an acidifying effect on the carboxyl group This effect is largest when the oxygen is attached directly to the carbon that bears the carboxyl group Furan-2-carboxylic acid is thus a stronger acid than furan-3-carboxylic acid COH Furan-2-carboxylic acid Furan-3-carboxylic acid K (g) Furan-2-carboxylic acid has an oxygen attached to the carbon that bears the carboxyl group, whereas pyrrole-2-carboxylic acid has a nitrogen in that position. Oxygen is more Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
attached to the carboxyl group in benzoic acid stabilizes benzoate anion better than the corresponding sp3 -hybridized carbon stabilizes cyclohexanecarboxylate. Benzoic acid is a stronger acid. (d) Its five fluorine substituents make the pentafluorophenyl group more electron-withdrawing than an unsubstituted phenyl group. Thus, pentafluorobenzoic acid is a stronger acid than benzoic acid. (e) The pentafluorophenyl substituent is electron-withdrawing and increases the acidity of a carboxyl group to which it is attached. Its electron-withdrawing effect decreases with distance. Pentafluorobenzoic acid is a stronger acid than p-(pentafluorophenyl)benzoic acid. ( f) The oxygen of the ring exercises an acidifying effect on the carboxyl group. This effect is largest when the oxygen is attached directly to the carbon that bears the carboxyl group. Furan-2-carboxylic acid is thus a stronger acid than furan-3-carboxylic acid. (g) Furan-2-carboxylic acid has an oxygen attached to the carbon that bears the carboxyl group, whereas pyrrole-2-carboxylic acid has a nitrogen in that position. Oxygen is more Furan-2-carboxylic acid (pKa 3.2) Ka 6.9 104 CO2H O Furan-3-carboxylic acid (pKa 3.9) Ka 1.1 104 CO2H O Pentafluorobenzoic acid Ka 4.1 104 (pKa 3.4) p-(Pentafluorophenyl)benzoic acid (Ka not measured in water; comparable with benzoic acid in acidity) CO2H F F F F F CO2H F F F F F Pentafluorobenzoic acid Ka 4.1 104 (pKa 3.4) Benzoic acid Ka 6.7 105 (pKa 4.2) CO2H CO2H F F F F F Benzoic acid Ka 6.7 105 (pKa 4.2) CO2H Cyclohexanecarboxylic acid Ka 1.2 105 (pKa 4.9) CO2H 512 CARBOXYLIC ACIDS __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOXYLIC ACIDS 513 electronegative than nitrogen and so stabilizes the carboxylate anion better. Furan-2 carboxylic acid is a stronger acid than pyrrole-2-carboxylic acid. COH COH Furan-2-carboxylic acid Pyrrole-3-carboxylic acid K.69×10-4 K.3.5×10-5 (pK23.2) (pK244) 19.17 (a) The conversion of 1-butanol to butanoic acid is simply the oxidation of a primary alcohol to a carboxylic acid. Chromic acid is a suitable oxidizing agent CH CHOH CH3 CH,CH,CO,H 1-Butanol Butanoic acid (b) Aldehydes may be oxidized to carboxylic acids by any of the oxidizing agents that convert primary alcohols to carboxylic acids K2Cr,O, CH3 CH CH_CH H.SO, Ho CH_ CH__COH Butanal Butanoic acid (c) The starting material has the same number of carbon atoms as does butanoic acid, and so all that is required is a series of functional group transformations. Carboxylic acids may be ob- tained by oxidation of the corresponding primary alcohol. The alcohol is available from the designated starting material, 1-butene CH3CH2CH,CO,H CH, CH, CH,CH,OH CHCH, CHECH, Hydroboration-oxidation of 1-butene yields 1-butanol, which can then be oxidized to butanoic acid as in part(a). CH_CH, CH=CH, 2H,02. Ho CH3 CH2, CH,_HCro, L. BAH CH CH, CH,CO,H 1-Butene 1-Butanol Butanoic acid (d) Converting 1-propanol to butanoic acid requires the carbon chain to be extended by one atom Both methods for achieving this conversion, carboxylation of a grignard reagent and form tion and hydrolysis of a nitrile, begin with alkyl halides. Alkyl halides in turn are prepared from alcohols CH, CHCHCO CH CHCHMgBr CH,.CN CH.CHCHOH CHCH, CH, Br Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
electronegative than nitrogen and so stabilizes the carboxylate anion better. Furan-2- carboxylic acid is a stronger acid than pyrrole-2-carboxylic acid. 19.17 (a) The conversion of 1-butanol to butanoic acid is simply the oxidation of a primary alcohol to a carboxylic acid. Chromic acid is a suitable oxidizing agent. (b) Aldehydes may be oxidized to carboxylic acids by any of the oxidizing agents that convert primary alcohols to carboxylic acids. (c) The starting material has the same number of carbon atoms as does butanoic acid, and so all that is required is a series of functional group transformations. Carboxylic acids may be obtained by oxidation of the corresponding primary alcohol. The alcohol is available from the designated starting material, 1-butene. Hydroboration–oxidation of 1-butene yields 1-butanol, which can then be oxidized to butanoic acid as in part (a). (d) Converting 1-propanol to butanoic acid requires the carbon chain to be extended by one atom. Both methods for achieving this conversion, carboxylation of a Grignard reagent and formation and hydrolysis of a nitrile, begin with alkyl halides. Alkyl halides in turn are prepared from alcohols. CH3CH2CH2CO2H CH3CH2CH2MgBr CH CH3CH2CH2Br 3CH2CH2OH or CH3CH2CH2CN CH3CH2CH CH3 CH3CH2CH2CH2OH CH3CH2CH2CO2H 1. B2H6 2. H2O2, HO H2CrO4 1-Butene 1-Butanol Butanoic acid CH3CH2CH2CO2H CH3CH2CH2CH2OH CH3CH2CH CH2 K2Cr2O7 H2SO4, H2O Butanal CH3CH2CH2CH O Butanoic acid CH3CH2CH2COH O H2CrO4 CH3CH2CH2CH2OH 1-Butanol CH3CH2CH2CO2H Butanoic acid Furan-2-carboxylic acid (pKa 3.2) Ka 6.9 104 CO2H O CO2H N H Pyrrole-3-carboxylic acid (pKa 4.4) Ka 3.5 105 CARBOXYLIC ACIDS 513 __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website