Effect of Substituents on Acidity

If electron-withdrawing groups are present on the benzene ring of benzoic acid, the benzoate ion stabilises and acidity thus increases. This is attributed to inductive and resonance effects. In case of p-nitrobenzoic acid and its m-isomer, based on the inductive effect alone the m-isomer would be assumed to be stronger than the p-isomer but the reverse is also true. No direct resonance of the carboxylate ion with the ring exists; but the resonance structures for each isomer shown below can be considered:

Effect of Substituents on Acidity

Apparently the resonance in p-isomer places the positive charge on the carbon atom carrying the −COOion. Therefore, the ion stabilises due to strong inductive effect. In m-isomer, the positive charge is on the carbon atom adjacent to the carbon carrying the −COOion. The inductive stabilisation of this ion is less. Therefore, acidity of p-nitrobenzoic acid is more than that of the m-isomer.

Substituents on Acidity

For p-chlorobenzoic acid isomers, similar arguments can be used. The resonance structures shown depict that the p-isomer is a stronger acid than the m-isomer because the negative charge is positioned ortho to the −COO ion in the former.

Substituent on AcidityOn the other hand, the effect of electron-donating groups is just the opposite as they destabilise the ion and place a negative charge on the −COOion. This increases the ion’s ability to regain the proton. Table lists the pKa values of some substituted benzoic acids. Lower the pKa value, stronger is the acid.

Substituent (X) pKa
Ortho Meta Para
−NO 2.21 3.49 3.42
−OCH3 4.09 4.09 4.47
−OH 2.98 4.08 4.57
−CN 3.14 3.64 3.55
−Br 2.85 3.81 3.97
−CI 2.92 3.83 3.97
−CH3 3.91 4.27 4.38
−H 4.20 4.20 4.20

The values in the table state that an alkyl group produces a little effect on the strength of benzoic acids. But the electron-withdrawing groups increase the acidity. These groups when present in o and ppositions produce a more prominent effect. The electron withdrawing effect of oNO2 group is significant due to the short distance over which the powerful inductive effect along with the resonance effect of this group is operating.

Presence of groups like −OCH3,−OH, or halogens (electron-withdrawing groups inductively but electron-donating mesomerically) in o and ppositions weakens the p substituted acids than the m-substituted acids.

Sometimes it is also observed that some o substituted benzoic acids behave strangely and their acidity is higher than expected. This is because the group interacts with the adjacent carboxyl group, forming intermolecular hydrogen bond.

Read More Topics
Effect of substituents on basicity
Miscellaneous compounds
Aromatic character aromaticity

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Santhakumar Raja

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