Carboxylic acids are typically weak acids, meaning that they only partially dissociate into cations and anions in neutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.001% of the acid are dissociated (i.e. 10−5 moles out of 1 mol). Electron-withdrawing substituents, such as -CF3 group, give stronger acids (the pKa of acetic acid is 4.76 whereas trifluoroacetic acid, with a trifluoromethyl substituent, has a pKa of 0.23). Electron-donating substituents give weaker acids (the pKa of formic acid is 3.75 whereas acetic acid, with a methyl substituent, has a pKa of 4.76) Carboxylic acid pKaFormic acid () 3.75Chloroformic acid () 0.27Acetic acid () 4.76Glycine ()2.34Fluoroacetic acid () 2.586Difluoroacetic acid () 1.33Trifluoroacetic acid () 0.23Chloroacetic acid () 2.86Dichloroacetic acid () 1.29Trichloroacetic acid () 0.65Benzoic acid ()4.22-Nitrobenzoic acid (ortho-)2.16Oxalic acid () (first dissociation) 1.27Hydrogen oxalate () (second dissociation of oxalic acid)4.14 Deprotonation of carboxylic acids gives carboxylate anions; these are resonance stabilized, because the negative charge is delocalized over the two oxygen atoms, increasing the stability of the anion. Each of the carbon–oxygen bonds in the carboxylate anion has a partial double-bond character. The carbonyl carbon's partial positive charge is also weakened by the -1/2 negative charges on the 2 oxygen atoms.
carboXylic aCidS arE tyPIcalLy weak acIds, MeanIng that they onLy PaRtiaLlY DIsSOCiaTE iNto CatiOns aNd anIons In neutraL aqUeouS SolUTion. foR eXample, aT rOOm tEMPerature, IN a 1-moLaR solutioN of acetiC aCID, Only 0.001% of The ACid aRe DiSsociated (I.e. 10−5 Moles out of 1 mol). eLEctron-WithdraWiNG subsTituEnts, such aS -cF3 grOup, GivE stRongeR acidS (tHe pka Of aCEtic aciD Is 4.76 whEReAs trifluoroaCEtic AcId, witH A TrIfluORometHyl subsTItuent, HaS a pKa of 0.23). EleCtRon-doNaTINg subsTItuEnts give wEaker aCidS (tHe pka of formIC Acid is 3.75 wHEREAS ACetiC Acid, wiTH A metHYl SubstituenT, has a Pka oF 4.76) caRboxylIC acID pkaformiC acId () 3.75ChloroFormic aCid () 0.27AceTic aCiD () 4.76glYcine ()2.34fluoRoaceTic acid () 2.586DiFluOROAcEtic acid () 1.33TRIfluoroacETiC acid () 0.23chlOroAcetic acid () 2.86dicHLorOAcEtiC Acid () 1.29triChlOrOaCetic aCid () 0.65beNZoic acid ()4.22-nItrObenzoic aCid (oRTho-)2.16OXaliC acid () (fIrst DIssOciatiOn) 1.27hydrogen oxaLatE () (secOnd diSsocIaTion of OXaliC acID)4.14 depROtoNAtIon of caRboxyliC aCidS gIvEs caRBoxylatE anionS; thesE are resonancE Stabilized, BecauSe ThE Negative chaRge is DeLocAlizEd oveR The two OXygEn AtOms, IncreaSInG the staBIlity Of the anion. eacH of THE cARBoN–oXygen BoNds in tHE carboxylate aNiOn haS A parTIaL DouBle-BoNd chaRacter. THe carbonYL cARbon'S PaRtiAl posiTivE CHaRGe IS AlsO weakeNeD by the -1/2 NegAtIve cHarges oN the 2 oxygEn atoMs.