The carboxyl group (COOH) is made up of (i) carbonyl group (>C=O) and (ii) hydroxyl group (−OH). Carboxylic acids easily
release protons, and hence they are acidic in nature.
Acidity of carboxylic acids
Carboxylic acids are weaker acids than the mineral acids (HCl, H2SO4, HNO3) and sulphonic acids.
They are, however, stronger acids than alcohols and phenols.
Why are carboxylic acids stronger acids than alcohols?
Carboxylic acids are stronger acids than alcohols because the conjugate base (product left after the removal of hydrogen) of carboxylic
acid is stabilised by resonance.
Carboxylate ion (conjugate base of carboxylic acids)
Since the carboxylate ion is stabilised by resonance, it is easier for carboxylic acids to release a proton to form carboxylate ion.
Alkoxide ion, on the other hand, does not exhibit resonance; therefore, it is less stable.
Alkoxide ion (conjugate base of alcohols)
In other words, alcohols do not release protons easily to form less stable alkoxide ion; therefore, alcohols are less acidic.
Why are carboxylic acids more acidic than phenols?
Let us take the case of phenols first. Phenoxide ion (C6H5O−) is the conjugate base of phenol which has the
following resonance structures :
In structures III-V, the negative charge is present on less electronegative carbon atom which does not contribute much in the resonance
stabilisation of phenoxide ion. Therefore, there contribution can be ignored.
Let us compare the first two structures of phenoxide ion with the resonance structures of carboxylate ions :
In phenoxide ion, the negative charge on oxygen atom is localized (i.e., sticks to the same oxygen atom) whereas in carboxylate ion,
the negative charge is delocalized; therefore, carboxylate ion is more resonance stabilized. Thus, the release of a proton from
carboxylic acids is much easier than phenols. In other
words, carboxylic acids are stronger acids than phenols.
Effect of substitution on acidity of carboxylic acids :
Electron attracting groups (−I groups) increase the acidity of carboxylic acids. Some electron attracting groups in order of their increasing
−I-effect are given below :
Ph < I < Br < Cl < F < CN < NO2 < CF3
Electron releasing groups (+I groups) decrease acidity.
Increasing order of +I-effect
CH3− < CH3CH2− < (CH3)2CH− (CH3)3C−
Arrange 'CCl3COOH, O2NCH2COOH, CF3COOH, NC−CH2COOH, CHCl2COOH'
in order of decreasing aciditiy.
All of them contain electron attracting groups, the order is :
Ortho substituted benzoic acids are more acidic than benzoic acid irrespective of the nature of the substituent (whether electron
donating or electron withdrawing). For example, o-toluic acid is more acidic than benzoic acid.
Ortho-effect in aminobenzoic acid :
Amino group (NH2) has a strong +R effect and weak −I effect. Due to strong +R effect, all aminoacids are weaker
acids than benzoic acids. Even o-aminobenzoic acid is weaker acid than benzoic acid. The reason is given below :
Since −COOH group is acidic while −NH2 group is basic, the nitrogen atom of amino group and the hydrogen atom
of carboxyl group form H-bond leading to the formation of zwitterion (A neutral molecule with both positive and negative charges).
As a result, ortho-effect is reduced to such an extent that it becomes weaker acid than benzoic acid.
In o-aminobenzoic acid, the H-bond is formed between N and H atoms. Do you know if the bond is intermolecular
Since the H-bond is formed within the same molecule, the bond is intramolecular hydrogen bond ( see
Hydrogen bonding ).
We will study the reactions of carboxylic acids from next page onwards.
Question : Which acid of each pair shown here would you expect to be stronger?
CH3COOH or CH2FCOOH
CH2FCOOH or CH2ClCOOH
CH2FCH2CH2COOH or CH3CHFCH2COOH
p-CF3−C6H4−COOH or p-CH3−C6H4−COOH
CH2FCOOH is stronger than CH3COOH because it contains electron withdrawing group F which
CH2FCOOH is stronger because -I-effect of F is greater than that of Cl.
Since I-effect decreases as the distance increases, CH3CHFCH2COOH is stronger acid than
p-CF3−C6H4−COOH is stronger acid than
Question : Arrange the following carboxylic acids in increasing order of their acid strength: CH3CH2CH(Br)COOH, CH3CH(Br)CH2COOH,
Answer : Br is an electron attracting group. Electron attracting groups (−I groups) increase
the acidic strength of carboxylic acids. Moreover, −I-effect decreases with distance; therefore,
CH3CH2CH(Br)COOH is a stronger acid than CH3CH(Br)CH2COOH.
Since +I groups decrease the acidiy of carboxylic acids, the presence of alkyl groups decreases the acidity.
Also, the +I-effect of (CH3)2CH− (isopropyl group) is greater than that of
CH3CH2CH2− (n-propyl group). Hence, the acidic strength of
(CH3)2CHCOOH is greater than that of CH3CH2CH2COOH.
Combining all concepts together, the order of acidic strength should be: CH3CH2CH2COOH < (CH3)2CHCOOH <
CH3CH(Br)CH2COOH < CH3CH2CH(Br)COOH