Chapter 3, Problem 3.29P

Interpretation Introduction

(a)

Interpretation:

An explanation regarding the arrangement of the compounds in order of their decreasing $\text{p}{K}_{\text{a}}$ is to be stated.

Concept introduction:

The $\text{p}{K}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). It indicates the acidity of a compound. The acidity of a compound increases as the $p{\text{K}}_{\text{a}}$ value decreases.

Answer to Problem 3.29P

The arrangement of the given compounds in decreasing order of their $\text{p}{K}_{\text{a}}$ value is shown below.

$\begin{array}{l}{\text{CH}}_3{\text{CH}}_2\text{OH}{\text{ClCH}}_2{\text{CH}}_2\text{SH}{\text{ClCH}}_2{\text{CH}}_2\text{OH}\\ \text{C}>\text{A}>\text{B}\end{array}$

Explanation of Solution

The given compounds are shown below.

$\begin{array}{l}{\text{ClCH}}_2{\text{CH}}_2\text{SH}{\text{ClCH}}_2{\text{CH}}_2\text{OH}{\text{CH}}_3{\text{CH}}_2\text{OH}\\ \text{A}\text{B}\text{C}\end{array}$

Figure 1

The acidity of an atom is directly proportional to the electronegativity of an atom. The $\text{O}-\text{H}$ bond can attract more electrons as compared to the $\text{S}-\text{H}$ bond. Therefore, compound B is more acidic than compound A.

Compound C contains ${\text{CH}}_2{\text{CH}}_{\text{3}}$ group which is an electron-donating group. Due to $+\text{I}$ effect of ${\text{CH}}_2{\text{CH}}_{\text{3}}$ group, the acidity of compound C decreases.

Compound A contains sulfur atom which is less electronegative than oxygen atom but it contains a chlorine atom. Chlorine atom is electronegative in nature due to which acidity of compound A increases.

Therefore, the order of acidity of the compounds is shown below.

$\begin{array}{l}{\text{ClCH}}_2{\text{CH}}_2\text{OH}{\text{ClCH}}_2{\text{CH}}_2\text{SH}{\text{CH}}_3{\text{CH}}_2\text{OH}\\ \text{B}>\text{A}>\text{C}\end{array}$

Figure 2

The $\text{p}{K}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). Therefore, on increasing acidity, $\text{p}{K}_{\text{a}}$ value decreases.

Therefore, the order of decreasing $p{\text{K}}_{\text{a}}$ value of the compounds is shown below.

$\begin{array}{l}{\text{CH}}_3{\text{CH}}_2\text{OH}{\text{ClCH}}_2{\text{CH}}_2\text{SH}{\text{ClCH}}_2{\text{CH}}_2\text{OH}\\ \text{C}>\text{A}>\text{B}\end{array}$

Figure 3

The order of decreasing $p{\text{K}}_{\text{a}}$ value of the compounds is $\text{C}>\text{A}>\text{B}$.

Conclusion

The arrangement of the given compounds in decreasing order of their $\text{p}{K}_{\text{a}}$ value is shown in Figure 3.

Interpretation Introduction

(b)

Interpretation:

An explanation regarding the arrangement of the compounds in order of their decreasing $\text{p}{K}_{\text{a}}$ is to be stated.

Concept introduction:

The $\text{p}{K}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). It indicates the acidity of a compound. The acidity of a compound increases as the $\text{p}{K}_{\text{a}}$ value decreases.

Answer to Problem 3.29P

The arrangement of the order of decreasing $p{\text{K}}_{\text{a}}$ value of the compounds is shown below.

Explanation of Solution

The given compounds are shown below.

Figure 4

The acidity of an atom is directly proportional to the electronegativity of an atom. Compound C contains two ${\text{OCH}}_{\text{3}}$ groups which are electron-withdrawing in nature that means it can attract the shared pair of electrons towards itself. Thus, the acidity of compound C increases.

Compound A contains only one ${\text{OCH}}_{\text{3}}$ group which is an electron-withdrawing group that means it can attract the shared pair of electrons towards itself but the effect is less than compound C. Thus, the acidity of compound A is less than compound C.

Compound B contains ${\text{CH}}_{\text{3}}$ group which is an electron-donating group. Due to $+\text{I}$ effect of ${\text{CH}}_{\text{3}}$ group the acidity of compound B decreases

Therefore, the order of acidity of the compounds is shown below.

Figure 5

The $\text{p}{K}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). Thus, on increasing acidity, $\text{p}{K}_{\text{a}}$ value decreases.

Therefore, the order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is shown below.

Figure 6

The order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is $\text{B}>\text{A}>\text{C}$.

Conclusion

The arrangement of the order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is shown in Figure 6.

Interpretation Introduction

(c)

Interpretation:

An explanation regarding the arrangement of the compounds in order of their decreasing $\text{p}{K}_{\text{a}}$ is to be stated.

Concept introduction:

The $\text{p}{K}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). It indicates the acidity of a compound. The acidity of a compound increases as a result $p{\text{K}}_{\text{a}}$ value decreases.

Answer to Problem 3.29P

The arrangement of the order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is shown below.

Explanation of Solution

The given compounds are shown below.

Figure 7

The conjugate bases of the compounds are shown below.

Figure 8

Compound C shown in Figure 8 is more basic than compound A and B due he presence of carboxylate ion. Compound B contains one chlorine atom which is an electron-withdrawing group. Thus, due to $-\text{I}$ effect of a chlorine atom, compound B is least basic.

The order of basicity of the compounds is shown below.

Figure 9

The order of basicity is directly proportional to the $p{\text{K}}_{\text{a}}$ value of a compound as the $p{\text{K}}_{\text{a}}$ value of an acid is the negative logarithm of ${\text{K}}_{\text{a}}$ (acid dissociation constant). On increasing acidity, the basicity of a compound decreases. Thus, on increasing basicity, $p{\text{K}}_{\text{a}}$ value increases.

Therefore, the order of decreasing $p{\text{K}}_{\text{a}}$ value of the compounds is shown below.

Figure 9

The order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is $\text{C}>\text{A}>\text{B}$.

Conclusion

The arrangement of the order of decreasing $\text{p}{K}_{\text{a}}$ value of the compounds is shown in Figure 9.