Question

Question: The pKa values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:

The relative pKa values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least

acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic. Explain these relative acidities.

a. Cl: ortho > meta > para

b. NO₂: ortho > para > meta

c. NH₂: meta > para > ortho

Short answer

The compound for graph a is 2,2-dimethyl-1-phenylpropane.

The compound for graph b is 1-tert-butyl-4-methylbenzene..

Step-by-step solution

Activator or deactivator

The groups which are electron donating are called activators and the groups which are electron withdrawing are called deactivators.

Activators increase reactivity and deactivators decrease reactivity.

Effect of substituents on orientation

All activating substituents and the weakly deactivating halogens present on ring are ortho–para directors, and all substituents that are more deactivating than the halogens are meta directors for electrophile.

Relation of pKa and acidic strength NO2

All activating substituents and the weakly deactivating halogens present on ring are ortho–para directors, and all substituents that are more deactivating than the halogens are meta directors for electrophile.

Factors affecting acidic strength

• An electron withdrawing group will increase the acidity of carboxylic acid. It disperses the negative charge by inductive effect or by resonance effect and hence stabilizes the carboxylate ion.
• The ortho substituted benzoic acids are stronger acids no matter whether the substituent is electron releasing or electron withdrawing. This effect is known as ortho effect.
• Ortho effect doesn`t apply whenOH, F and amine group present due to intermolecular hydrogen bonding with H atom of COOH group.
• The conjugate base of benzoic acid is destabilized by electron-donating groups making it less acidic by pushing more electron density toward the negative charge in the carboxylate ion.

Part ‘a’: Cl: ortho > meta >  para

• As mentioned earlier ortho substituents in benzoic acid are most acidic due to ortho effect and –I effect also affected by distance.
• Cl withdraws electron by –I effect and when benzoic acid form carboxylate ion then its negative charge is more stable when chlorine is at closer position i.e meta than para as it withdraws electrons more efficiently making it more acidic than para.

Part ‘b’: ortho > para > meta

• Due to ortho effect, caused by ortho substituents in benzoic acid it is most acidic.
• Para position with nitro group is more acidic than nitro group at meta position because of dominant -M effect shown by para-nitro group and it also shows -I effect while nitro group present at the meta position of benzene ring shows only -I effect.
• Carboxylate ion formed frombenzoic acid is more stabilized by the -M effect which is shown by the nitro group than -I effect and due to presence of this nitro group, this is strong deactivator.
• Thus m-nitrobenzoic acid is less acidic than p-nitrobenzoic acid.

Part ‘c’: NH2: meta > para > ortho

• As mentioned earlier ortho effect is not applicable for amine.
• Amine is a strong activator group and hence will increase electron density at ortho and para position making carboxylate ion more unstable and hence meta is more acidic than ortho and para- amine.
• As two electron rich groups repel each other or are more unstable if closer so para is more acidic than ortho as para is farthest position and ortho is closest position from carboxyl group.