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Unformatted text preview: 511 CARBOXYLIC ACIDS increase the acidity of carboxylic acids and alcohols relative to their nonﬂuorinated analogs,
but not enough to make ﬂuorinated alcohols as acidic as carboxylic acids. Ka
Ethanol (e) CF3SO2OH
CH3CH2OH The order of decreasing acidity is carboxylic acid pKa 106
10 16 5.9
4.2 -diketone 6
16 ketone Ka
O 1 O Cyclopentanone 10 5 5.0 10 9 9 10 20 20 45 45 O CH3CCH2CCH3 2,4-Pentanedione Cyclopentene (a) A triﬂuoromethyl group is strongly electron-withdrawing and acid-strengthening. Its ability to
attract electrons from the carboxylate ion decreases as its distance down the chain increases.
3,3,3-Triﬂuoropropanoic acid is a stronger acid than 4,4,4-triﬂuorobutanoic acid.
CF3CH2CO2H CF3CH2CH2CO2H 3,3,3-Triﬂuoropropanoic acid
Ka 9.6 10 4
(pKa 3.0) (b) 4,4,4-Triﬂuorobutanoic acid
Ka 6.9 10 5
(pKa 4.2) The carbon that bears the carboxyl group in 2-butynoic acid is sp-hybridized and is, therefore,
more electron-withdrawing than the sp3-hybridized carbon of butanoic acid. The anion of 2butynoic acid is therefore stabilized better than the anion of butanoic acid, and 2-butynoic acid
is a stronger acid.
CH3C CCO2H 2-Butynoic acid
Ka 2.5 10 3
(pKa 2.6) (c) Back Forward pKa 10 Cyclopentanecarboxylic acid 19.16 hydrocarbon. CH3CH2CH2CO2H
Ka 1.5 10 5
(pKa 4.8) Cyclohexanecarboxylic acid is a typical aliphatic carboxylic acid and is expected to be similar to acetic acid in acidity. The greater electronegativity of the sp2-hybridized carbon Main Menu TOC Study Guide TOC Student OLC MHHE Website __
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- Fall '01
- Organic chemistry