Chapter 19, Problem 19.37P

Which compound in each pair has the lower $\text{p}{K}_{\text{a}}$? Which compound in each pair has the stronger conjugate base?

a. c.

b. ${\text{ClCH}}_{\text{2}}\text{COOH}$ or ${\text{FCH}}_{\text{2}}\text{COOH}$ d. ${\text{CNCH}}_{\text{2}}\text{COOH}$ or ${\text{CH}}_3\text{COOH}$

Interpretation Introduction

(a)

Interpretation: The compound in the given pair that has lower $\text{p}{K}_{\text{a}}$ is to be predicted. The compound in the given pair that has the stronger conjugate base is to be stated.

Concept introduction: An acid is the substance that has the ability to donate a proton in the chemical reaction and base is a substance that accepts a proton. The species that do not possess hydrogen ions and formed from an acid are known as the conjugate base. The major factors that contribute to the acidic strength are inductive effect and polarity.

Answer to Problem 19.37P

The compound benzoic acid has lower $\text{p}{K}_{\text{a}}$ value and cyclohexylmethanol forms a strong conjugate base.

Explanation of Solution

The given compounds are,

Figure.1

The strength of an acid depends upon the stability of conjugate base. Stable is the conjugate base, higher is the acidity of acid. The anion formed in benzoic acid is resonance stabilized and the negative charge is delocalized on two highly electronegative oxygen atom. Thus, the conjugate base formed by the benzoic acid is more stable than that formed by cyclohexyl methanol. The compounds which have lower $\text{p}{K}_{\text{a}}$ value and the compound which is a strong conjugate base are shown below.

Figure 2

The resonance stabilization of benzoic acid is shown below.

Figure 3

Therefore, the compound benzoic acid has lower $\text{p}{K}_{\text{a}}$ value and cyclohexylmethanol forms a strong conjugate base.

Conclusion

The compound benzoic acid has lower $\text{p}{K}_{\text{a}}$ value and cyclohexylmethanol forms a strong conjugate base.

Interpretation Introduction

(b)

Interpretation: The compound in the given pair that has lower $\text{p}{K}_{\text{a}}$ is to be predicted. The compound in the given pair that has the stronger conjugate base is to be stated.

Concept introduction: An acid is the substance that has the ability to donate a proton in the chemical reaction and base is a substance that accepts a proton. The species that do not possess hydrogen ions and formed from an acid are known as the conjugate base. The major factors that contribute to the acidic strength are inductive effect and polarity.

Answer to Problem 19.37P

The compound $2\text{-}$ fluoroacetic acid has lower $\text{p}{K}_{\text{a}}$ value and $2\text{-}$ chloroacetic acid forms a strong conjugate base.

Explanation of Solution

The given compounds are ${\text{ClCH}}_{\text{2}}\text{COOH}$ and ${\text{FCH}}_{\text{2}}\text{COOH}$.

The electron withdrawing groups increase the acidic strength of the compound. Fluorine is more electronegative than chlorine. Due to this, the acidic strength of $2\text{-}$ fluoroacetic acid is higher as compared to the acidic strength of $2\text{-}$ chloroacetic acid.

The $\text{p}{K}_{\text{a}}$ value is inversely related to the acidic strength of the compound. The strong acids produce weak conjugate bases. The compounds which have lower $\text{p}{K}_{\text{a}}$ value and the compound which is a strong conjugate base are shown below.

Figure 4

Therefore, the compound $2\text{-}$ fluoroacetic acid has lower $\text{p}{K}_{\text{a}}$ value and $2\text{-}$ chloroacetic acid forms a strong conjugate base.

Conclusion

The compound $2\text{-}$ fluoroacetic acid has lower $\text{p}{K}_{\text{a}}$ value and $2\text{-}$ chloroacetic acid forms a strong conjugate base.

Interpretation Introduction

(c)

Interpretation: The compound in the given pair that has lower $\text{p}{K}_{\text{a}}$ is to be predicted. The compound in the given pair that has the stronger conjugate base is to be stated.

Concept introduction: An acid is the substance that has the ability to donate a proton in the chemical reaction and base is a substance that accepts a proton. The species that do not possess hydrogen ions and formed from an acid are known as the conjugate base. The major factors that contribute to the acidic strength are inductive effect and polarity.

Answer to Problem 19.37P

The compound $4\text{-}$ chlorobenzoic acid has lower $\text{p}{K}_{\text{a}}$ value and $4\text{-}$ methylbenzoic acid forms a strong conjugate base.

Explanation of Solution

The given compounds are shown below.

Figure 5

The electron withdrawing groups increase the acidic strength of the compound. Chlorine is electron withdrawing group, whereas methyl is electron donating group. Due to this, the acidic strength of $4\text{-}$ chlorobenzoic acid is higher as compared to the acidic strength of $4\text{-}$ methylbenzoic acid.

The $\text{p}{K}_{\text{a}}$ value is inversely related to the acidic strength of the compound. The strong acids produce weak conjugate bases. The compounds which have lower $\text{p}{K}_{\text{a}}$ value and the compound which is a strong conjugate base are shown below.

Figure 6

Therefore, the compound $4\text{-}$ chlorobenzoic acid has lower $\text{p}{K}_{\text{a}}$ value and $4\text{-}$ methylbenzoic acid forms a strong conjugate base.

Conclusion

The compound $4\text{-}$ chlorobenzoic acid has lower $\text{p}{K}_{\text{a}}$ value and $4\text{-}$ methylbenzoic acid forms a strong conjugate base.

Interpretation Introduction

(d)

Interpretation: The compound in the given pair that has lower $\text{p}{K}_{\text{a}}$ is to be predicted. The compound in the given pair that has the stronger conjugate base is to be stated.

Concept introduction: An acid is the substance that has the ability to donate a proton in the chemical reaction and base is a substance that accepts a proton. The species that do not possess hydrogen ions and formed from an acid are known as conjugate base. The major factors that contribute to the acidic strength are $\text{p}{K}_{\text{a}}$ value, inductive effect and polarity.

Answer to Problem 19.37P

The compound $\text{2-}$ cyanoacetic acid has lower $\text{p}{K}_{\text{a}}$ value and acetic acid generates a strong conjugate base.

Explanation of Solution

The given compounds are shown below.

Figure 7

The electron withdrawing groups increase the acidic strength of the compound. Cyano is an electron withdrawing group. Due to this, the acidic strength of $\text{2-}$ cyanoacetic acid is higher as compared to the acidic strength of acetic acid.

The $\text{p}{K}_{\text{a}}$ value is inversely related to the acidic strength of the compound. The strong acids produce weak conjugate bases. The compounds which have lower $\text{p}{K}_{\text{a}}$ value and the compound which is the strong conjugate base are shown below.

Figure 8

Therefore, the compound $\text{2-}$ cyanoacetic acid has lower $\text{p}{K}_{\text{a}}$ value and acetic acid generates strong conjugate base.

Conclusion

The compound $\text{2-}$ cyanoacetic acid has lower $\text{p}{K}_{\text{a}}$ value and acetic acid produces strong conjugate base.