Carboxylic acid Acidity : Inductive Effect vs Resonance Effect

Organic Chemistry Reasoning Questions - 4

CSIR, GATE, JAM, IISc, and all other Chemistry Higher studies Exams has been changed their syllabus. If we analyze them quite precisely and it’s seems that many advanced are of research has been included. The previous year’s questions from those type exams are clearly indicated that, students must understand the fundamentals, and they must have reasoning ability to answer the question. In this view we wish to collect most important exam based reasoning questions and we discussed answer for them. It’s not only useful for competitive exams also for your interviews.


Arrange the following carboxylic acid according to their acid strength.

Answer:

Dear readers, This type questions often appeared in all chemistry competitive exams. In order to answer this question, we need to know, which are the factors affecting the acidity of the carboxylic acid.

1.      Equilibrium Acidity

2.      Solvent Effects

3.      Hybridization

4.      Stereoelectronic Control of Enolization

5.      Kinetic Acidity

6.      Substituent Effects on the Acidity

7.      Conjugation and Hybridization

8.      Ortho effect

9.      Hydrogen bonding

In the case of carboxylic acids, if the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease. Similarly, an increase in its electrophilicity will increase the acidity of the acid.

Electron donating substituent decreases the acidity. Electronegative substituents increase acidity by inductive electron withdrawal. Higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I), and the closer the substituent is to the carboxyl group the greater is its effect.

Oxygen has a much larger electronegativity than carbon, but it is an excellent p-π electron donor to sp² carbon functions. For the meta isomer, the inductive effect is somewhat stronger than the resonance donation, but the para-isomer is able to donate an oxygen electron pair directly into the electrophilic carboxyl function. Both the meta and para-nitro substituent withdraw electrons from the benzene ring by a combination of inductive and resonance action, and the corresponding acids are greatly strengthened.

Inductive Effect vs Resonance Effect

Substituents have both an inductive effect and a resonance effect. This creates a number of different behaviours when it comes to such things as the reactivity of substituted benzenes towards electrophilic substitution. For example:

Inductive = electron donating ; Resonance = electron donating:
Ortho/para directing. Meta positions have increased reactivity relative to benzene.
Examples of substituent: alkyl

Inductive = electron withdrawing ; Resonance = electron withdrawing:
Meta directing. Meta positions have decreased reactivity relative to benzene.
Examples of substituent: nitro, carboxyl, sulfonyl, quaternary ammonium.

Inductive = electron withdrawing ; Resonance = electron donating ; Resonance > Inductive:Ortho/para directing. Ortho/para positions have increased reactivity relative to benzene. Meta positions have decreased reactivity relative to benzene.
Examples of substituent: hydroxy, alkoxy, amino, acylamido.

Inductive = electron withdrawing ; Resonance = electron donating ; Resonance < Inductive:Ortho/para directing. Ortho/para positions have decreased reactivity relative to benzene. Meta positions have decreased reactivity relative to benzene.
Examples of substituent: fluoro, chloro.