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Acid-Base Properties

Carboxylic acids are the acids par excellence of Organic Chemistry. It is difficult to find organic functions more acidic than a carboxylic acid. In the table, the lowest pKa value (highest acidity) belongs to the CO2H group.

Acid-base properties of carboxylic acids▶ Watch the video on YouTube
pKa table of several organic functions: RCH₃ 45, R–NH₂ 35, terminal alkyne 25, ketone α-position 20, alcohol 18, water 15, phenol 10 and carboxylic acid 4

Click here to recall the general property of acidity

The more stable the conjugate base A⁻, the stronger the corresponding acid AH.

The carboxylate ion, the conjugate base of the carboxylic acid function, is highly stable because its negative charge is largely delocalized.

Dissociation of a carboxylic acid in water: the carboxylate anion has two equivalent resonance forms, with the negative charge on one oxygen or the other

The resonance forms of the carboxylate anion are completely equivalent, and the negative charge is located on the oxygen atoms, which are very electronegative.

For instance, the formate ion is completely symmetrical. The C–O bond lengths are identical and the negative charge is equally shared between the two oxygens.

Electrostatic potential of the formate ion: the negative charge (red) is equally shared between both oxygens
Electrostatic potential of the formate ion: the negative charge (red) is equally shared between both oxygens.
Overlapping p orbitals of the O–C–O fragment of the formate ion
Overlap of the p orbitals of the O–C–O fragment of the formate ion, showing the extensive electron delocalization.

The acidity of a carboxylic acid is modulated by the structure of the R group (table).

An electron-releasing R group will destabilize the carboxylate anion and make the acid less acidic. This is the case of alkyl R groups.

In contrast, an electron-withdrawing R group will stabilize the carboxylate anion even further, thus making the acid more acidic. This is the case of halomethyl, ammonium and nitro groups.

R-CO2HStructurepKaExplanation
⁻OOC-CH2-COOH5.69See note
(CH3)3C-COOH5.05See note
CH3-CH2-COOH4.88See note
(CH3)2CH-COOH4.86See note
CH3-COOH4.74See note
H2C=CH-CH2-COOH4.35See note
PhCH2-COOH4.31See note
PhCOOH4.19See note
H-COOH3.77See note
CH3-O-CH2-COOH3.54See note
I-CH2-COOH3.18See note
Br-CH2-COOH2.90See note
Cl-CH2-COOH2.86See note
HOOC-CH2-COOH2.83See note
F-CH2-COOH2.59See note
(CH3)3N+-CH2-COOH1.83See note
O2N-CH2-COOH1.68See note
pKa of acetic and haloacetic acids: CH₃ 4.74, ClCH₂ 2.86, Cl₂CH 1.26, Cl₃C 0.64 and F₃C 0.23

Increasing the number of halogen atoms increasingly stabilizes the carboxylate anion, and the acidity goes up (pKa goes down). Replacing chlorine by the more electronegative fluorine increases the acidity.

pKa of butanoic and chlorobutanoic acids: no chlorine 4.90, 2-chloro 2.84, 3-chloro 4.06, 4-chloro 4.52

A chlorine atom next to the carboxylate group delocalizes the negative charge and increases the acidity. But this effect is very sensitive to distance.