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.

Organic Chemistry Reasoning Questions - 3

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.

Today's Question is related to Organic Chemistry Reasoning Questions-1

  Why o-nitrophenol is much more volitile than p-nitrophenol?

Answer:

Dear readers, we have seen before, Hydrogen-bonding leads to an increase in intermolecular ‘aggregation’ forces and is manifested particularly in the boiling point and solubility of the organic compound.

But, Intramolecular hydrogen-bonds can also be formed, and and are of particular significance when the resulting ring is five- or six- membered. This is called chelation.

Since chelation does not rise to intermolecular aggregation forces, chelated compounds have normal boiling point. para-nitrophenol forms intermolecular hydrogen-bonds.

Organic Chemistry Reasoning Questions - 2

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.

 

Often resonance effect based questions are raised in all chemistry compettive exams.

For example:

Give reason: phenanthrene have more stabilization energy than anthracene?

Answer:

In general, for compounds containing equal numbers of benzene rings, that for which the greatest number of Kekule structure can be drawn has the largest stabilization energy.



Anthracene Resonance Strectures



Phenanthrene Resonance Strectures

Due to higher number of phenanthren resonance forms, it has more stabilization energy.

 Tomorrow we will see some other reasoning questions.

Organic Chemistry Reasoning Questions - 1

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.

Today's Question:

2,4-pentanedione(A) in hexane exit 8% of keto tautomer and 92% of the enol tautomer.

Q1. Why (A) prefer to form B??

Q2. Give Reason: In water 2,4-penatanedione consist of 84% of keto  and 16% enol tautomer.

Answer:

A1. Hydrogen Bonding.

A2. In water, The Keto tautomer is stabilized by hydrogen-bonding of its two carbonyl-oxygen atoms with the solvent, effectively nullifying one of the stabilizing influence in the enol tautomer.

The boiling point of ethanol is very much higher than that of its isomer, dimethyl ether. Why?

Answer:

Hydrogen Bonding. Hydrogen-bonding leads to an increase in intermolecular ‘aggregation’ forcesand is manifested particularly in the boiling point and solubility of organic compound. The boiling point is raised because energy is required to separate H –bonded molecules. That’s why ethanol boiling point is higher than that of dimethyl ether.

We can see other questions tomorrow.