Acidity of Carboxylic acids
Carboxylic acids undergo ionisation to produce H+ and carboxylate ions in aqueous
solution. The carboxylate anion is stabilised by resonance which make the Carboxylic acid to donate the proton easily.
R C OH R C O + H+
O
Carboxylic acid Carboxylate ion
O
The resonance structure of carboxylate ion are given below.
The strength of carboxylic acid can be expressed in terms of the dissociation constant(Ka):
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R COOH + H2O RCOO - + H3O+
Ka [RCOO - ] [H3O+]
[RCOOH] =
The dissociation constant is generally called acidity constant because it measures the relative strength of an acid. The stronger the acid, the higher will be its Ka value.
The dissociation constant of an acid can also be expressed in terms of pKa value.
pKa = - log Ka A stronger acid will have higher Ka value but smaller pKa value.
Ka and pKa values of some Carboxylic acids of 298 K
Carboxylic acid pKa Value
Name of acid Molecular formula
Trichloroacetic acid Cl3CCOOH 0.64
Dichloroacetic acid Cl2CHCOOH 1.26
Fluoroacetic acid FCH2COOH 2.59
Chloroacetic acid ClCH2COOH 2.87
Bromoacetic acid BrCH2COOH 2.90
Iodoacetic acid ICH2COOH 3.17
Formic acid HCOOH 3.75
Benzoic acid C6H5COOH 4.20
Acetic acid CH3COOH 4.76
Propionic acid CH3CH2COOH 4.88
o – nitrobenzoic acid o-NO2C6H4COOH 2.17
m-nitrobenzoic acid m-NO2C6H4COOH 3.49
p- nitrobenzoic acid p-NO2C6H4COOH 3.44
Effect of substituents on the acidity of carboxylic acid.
i) Electron releasing alkyl group decreases the acidity
The electron releasing groups (+I groups) increase the negative charge on the carboxylate ion and destabilise it and hence the loss of proton becomes difficult. For example, formic acid is more stronger than acetic acid.
H C OH >CH3 C OH
O
> CH3 CH2 OHC
OO
Formic acid Acetic acid Propionic acid
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| Carboxylic acid | pKa Value | |
|---|---|---|
| Name of acid | Molecular formula | |
| Trichloroacetic acid | Cl CCO OH3 | 0.64 |
| Dichloroacetic acid | Cl CHCO OH2 | 1.26 |
| Fluoroacetic acid | FCH CO OH2 | 2.59 |
| Chloroacetic acid | ClCH CO OH2 | 2.87 |
| Bromoacetic acid | BrCH CO OH2 | 2.90 |
| Iodoacetic acid | ICH CO OH2 | 3.17 |
| Formic acid | HCO OH | 3.75 |
| B enzoic acid | C H CO OH6 5 | 4.20 |
| Acetic acid | CH CO OH3 | 4.76 |
| Propionic acid | CH CH CO OH3 2 | 4.88 |
| o – nitrobenzoic acid | o-NO C H CO OH2 6 4 | 2.17 |
| m-nitrobenzoic acid | m-NO C H CO OH2 6 4 | 3.49 |
| p- nitrobenzoic acid | p-NO C H CO OH2 6 4 | 3.44 |
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ii) Electron withdrawing substituents increases the acidity
The electron – withdrawing substituents decrease the negative charge on the carboxylate ion and stabilize it. In such cases, the loss of proton becomes relatively easy.
Acidity increases with increasing electronegativity of the substituents. For example, the acidity of various halo acetic acids follows the order
F CH2 COOH > Cl CH2 COOH > Br CH2 COOH > I CH2 COOH
Acidity increases with increasing number of electron – withdrawing substituents on the α - carbon. For example
Cl3C COOH > Cl2CH COOH > ClCH2COOH > CH3COOH
The effect of various, electron withdrawing groups on the acidity of a carboxylic acid follows the order,
- NO2 > - CN > - F > - Cl > - Br > - I > Ph
The relative acidities of various organic compounds are
RCOOH > ArOH > H2O > ROH > RC CH