Chapter 21, Problem 47PP

(a)

Interpretation Introduction

Interpretation:

To identify which of the given set of compounds have pK_a value lesser than 20 and to identify the acidic proton in the same

Concept introduction:

pKa is negative base-10 logarithm of the dissociation constant of acid (Ka) of a solution. ${\text{pK}}_{\text{a}}{\text{= -log}}_{\text{10}}{\text{K}}_{\text{a}}$ pKa value of acid is inversely related to the strength of acid.  If pKa value is lower it means the acid is stronger and vice-versa.

pKa is used is to describe acid dissociation because it is expressed in small decimal numbers.

If pKa value is below 20 the compound is said to have acidic proton and vice versa

To identify : The compound pK_a value and the most acidic proton

Answer to Problem 47PP

The pK_a value of (a) is just below 20 and it has acidic proton next to carbonyl

Explanation of Solution

Deprotonation of the highlighted hydrogen and resonance stabilization

 From the above scheme we can infer that the highlighted hydrogen is removed resulting in formation of a conjugate base.  The conjugate base is resonance stabilized as the negative charge is delocalized on oxygen atom also.  Therefore the highlighted proton is most acidic (pK_a value below 20) as the removal of it leads to a stabilized conjugate base.

(b)

Interpretation Introduction

Interpretation:

To identify which of the given set of compounds have pK_a value lesser than 20 and to identify the acidic proton in the same

Concept introduction:

pKa is negative base-10 logarithm of the dissociation constant of acid (Ka) of a solution. ${\text{pK}}_{\text{a}}{\text{= -log}}_{\text{10}}{\text{K}}_{\text{a}}$ pKa value of acid is inversely related to the strength of acid.  If pKa value is lower it means the acid is stronger and vice-versa.

pKa is used is to describe acid dissociation because it is expressed in small decimal numbers.

If pKa value is below 20 the compound is said to have acidic proton and vice versa

To identify : The compound pK_a value and the most acidic proton

Answer to Problem 47PP

The pK_a value of (b) is just above 20 and it does not have acidic proton

No acidic proton is present

Explanation of Solution

The above given compound does not have an acidic proton that can undergo deprotonation.  Hence this compound is expected to have pK_a above 20.

(c)

Interpretation Introduction

Interpretation:

To identify which of the given set of compounds have pK_a value lesser than 20 and to identify the acidic proton in the same

Concept introduction:

pKa is negative base-10 logarithm of the dissociation constant of acid (Ka) of a solution. ${\text{pK}}_{\text{a}}{\text{= -log}}_{\text{10}}{\text{K}}_{\text{a}}$ pKa value of acid is inversely related to the strength of acid.  If pKa value is lower it means the acid is stronger and vice-versa.

pKa is used is to describe acid dissociation because it is expressed in small decimal numbers.

If pKa value is below 20 the compound is said to have acidic proton and vice versa

To identify : The compound pK_a value and the most acidic proton

Answer to Problem 47PP

The pK_a value of (c) is just below 20 and it has acidic proton next to carbonyl

Explanation of Solution

Deprotonation of the highlighted hydrogen and resonance stabilization

From the above scheme we can infer that the highlighted hydrogen is removed resulting in formation of a conjugate base.  The conjugate base is resonance stabilized as the negative charge is delocalized on oxygen atom also.  Therefore the highlighted proton is most acidic (pK_a value below 20) as the removal of it leads to a stabilized conjugate base.

(d)

Interpretation Introduction

Interpretation:

To identify which of the given set of compounds have pK_a value lesser than 20 and to identify the acidic proton in the same

Concept introduction:

pKa is negative base-10 logarithm of the dissociation constant of acid (Ka) of a solution. ${\text{pK}}_{\text{a}}{\text{= -log}}_{\text{10}}{\text{K}}_{\text{a}}$ pKa value of acid is inversely related to the strength of acid.  If pKa value is lower it means the acid is stronger and vice-versa.

pKa is used is to describe acid dissociation because it is expressed in small decimal numbers.

If pKa value is below 20 the compound is said to have acidic proton and vice versa

To identify : The compound pK_a value and the most acidic proton

Answer to Problem 47PP

The pK_a value of (d) is just below 20 and it has acidic proton next to carbonyl

Explanation of Solution

Deprotonation of the highlighted hydrogen and resonance stabilization

From the above scheme we can infer that the highlighted hydrogen is removed resulting in formation of a conjugate base.  The conjugate base is resonance stabilized as the negative charge is delocalized on two oxygen atoms (doubly stabilized enolate ion).  Therefore the highlighted proton is most acidic (pK_a value below 20) as the removal of it leads to a stabilized conjugate base.

(e)

Interpretation Introduction

Interpretation:

To identify which of the given set of compounds have pK_a value lesser than 20 and to identify the acidic proton in the same

Concept introduction:

pKa is negative base-10 logarithm of the dissociation constant of acid (Ka) of a solution. ${\text{pK}}_{\text{a}}{\text{= -log}}_{\text{10}}{\text{K}}_{\text{a}}$ pKa value of acid is inversely related to the strength of acid.  If pKa value is lower it means the acid is stronger and vice-versa.

pKa is used is to describe acid dissociation because it is expressed in small decimal numbers.

If pKa value is below 20 the compound is said to have acidic proton and vice versa

To identify : The compound pK_a value and the most acidic proton

Answer to Problem 47PP

The pK_a value of (e) is just below 20 and it has acidic proton next to carbonyl

Explanation of Solution

Deprotonation of the highlighted hydrogen

From the above scheme we can infer that the highlighted hydrogen is removed resulting in formation of an alokoxide ion.  As such the compound is expected to have pK_a lower than 20.