Properties
Acidity of carboxylic acids in water
acid formula pKa
ethanoic acid CH 3 -COOH 4.75
propanoic acid CH 3 CH 2 -COOH 4.87
propenoic acid 4.25
benzoic acid C 6 H 5 -COOH 4.19
trifluoroethanoic acid CF 3 -COOH 0.3
phenol C 6 H 5 -OH 10.0
ethanol CH 3 CH 2 -OH 15.9
water H 2 O 15.7Data from CRC Handbook of Chemistry & Physics, 64th edition, 1984 D-167-8 Except
http://en.wikipedia.org/wiki/Trifluoroacetic_acid
Clearly, the carboxylic acids are remarkably acidic for organic molecules. Somehow, the re-
lease of the H+ion is favoured by the structure. Two arguments: The O-H bond is polarised
by the removal of electrons to the carbonyl oxygen. The ion is stabilised by resonance: the
carbonyl oxygen can accept the charge from the other oxygen. The acid strength of car-
boxylic acid are strongly modulated by the moiety attached to the carboxyl. Electron-donor
moiety decrease the acid strength, whereas strong electron-withdrawing groups increase it.