Chapter 19, Problem 19.50P

Draw all resonance structures of the conjugate bases formed by removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in $\text{cyclohexane}-1,3-\text{dione}$ and acetanilide. For each compound, rank these protons in order of increasing acidity and explain the order you chose.

a. b.

Interpretation Introduction

(a)

Interpretation: The resonance structure of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in $\text{cyclohexane}-1,3-\text{dione}$ is to be drawn. The increasing order of acidity is to be ranked.

Concept introduction: Most of the organic structures cannot be represented using single Lewis structure. Therefore, there exists more than one Lewis structure for representing a molecule or ion. These structures are known as resonance structures. These are the hypothetical structures and do not specify the exact structure. These resonance structure combine together to give resonance hybrid that is lower in energy and is the most stable structure.

The delocalization of electrons results in the formation resonance structure.

Answer to Problem 19.50P

The resonance structure of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in $\text{cyclohexane}-1,3-\text{dione}$ is,

Figure 1

The increasing order of acidity of protons is ${\text{H}}_{\text{b}}<{\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}$.

Explanation of Solution

The given compound is,

Figure 2

The conjugate base formed by the removal of ${\text{H}}_{\text{c}}$ is shown below.

Figure 3

Therefore, the resonance structure of the conjugate base formed by the removal of the ${\text{H}}_{\text{c}}$ is,

Figure 4

The conjugate base formed by the removal of ${\text{H}}_{\text{a}}$ is shown below.

Figure 5

Therefore, the resonance structure of the conjugate base formed by the removal of the ${\text{H}}_{\text{c}}$ is,

Figure 6

The stability of resonance structures depends upon the following rules:

• The stability of resonance structure is more if it possesses more bonds and fewer charges.

• Resonance structure will be more stable if atoms of the resonance structure have complete octet.

• The stability of resonance structure is more if negative charge is on more electronegative atom

The removal of ${\text{H}}_{\text{b}}$ does not give resonance structure.

Figure 7

Proton acidity depends upon the stability of conjugated bases. More the number of resonance structures more will be the stability of molecule. Therefore, the increasing order of acidity of protons is ${\text{H}}_{\text{b}}<{\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}$.

Conclusion

The resonance structures of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in $\text{cyclohexane}-1,3-\text{dione}$ are rightfully drawn. The increasing order of acidity of protons is ${\text{H}}_{\text{b}}<{\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}$.

Interpretation Introduction

(b)

Interpretation: The resonance structure of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in acetanilide is to be drawn. The increasing order of acidity is to be ranked.

Concept introduction: Most of the organic structures cannot be represented using single Lewis structure. Therefore, there exists more than one Lewis structure for representing a molecule or ion. These structures are known as resonance structures. These are the hypothetical structures and do not specify the exact structure. These resonance structure combine together to give resonance hybrid that is lower in energy and is the most stable structure.

The delocalization of electrons results in the formation resonance structure.

Answer to Problem 19.50P

The resonance structure of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in acetanilide is,

Figure 8

The increasing order of acidity of protons is ${\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}<{\text{H}}_{\text{b}}$.

Explanation of Solution

The given compound is,

Figure 9

The conjugate base formed by the removal of ${\text{H}}_{\text{b}}$ is shown below.

Figure 10

Therefore, the resonance structure of the conjugate base formed by the removal of the ${\text{H}}_{\text{c}}$ is,

Figure 11

Similarly, the resonance structure of the conjugate base formed by the removal of the ${\text{H}}_{\text{c}}$ is,

Figure 12

The stability of resonance structures depends upon the following rules:

• The stability of resonance structure is more if it possesses more bonds and fewer charges.

• Resonance structure will be more stable if atoms of the resonance structure have complete octet.

• The stability of resonance structure is more if negative charge is on more electronegative atom

The removal of ${\text{H}}_{\text{a}}$ does not give resonance structure.

Figure 13

Proton acidity depends upon the stability of conjugated bases. More the number of resonance structures more will be the stability of molecule. Therefore, the increasing order of acidity of protons is ${\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}<{\text{H}}_{\text{b}}$.

Conclusion

The resonance structures of the conjugate bases formed by the removal of the labeled protons $\left({\text{H}}_{\text{a}}\text{,}{\text{H}}_{\text{b}}\text{,}\text{and}{\text{H}}_{\text{c}}\right)$ in acetanilide are rightfully drawn. The increasing order of acidity of protons is ${\text{H}}_{\text{a}}<{\text{H}}_{\text{c}}<{\text{H}}_{\text{b}}$.