Question

Estimate the ${\mathbf{pK}}_{\mathbf{a}}$of each of the indicated bonds.

a.

b.

c.

Short answer

a. 38

b. 16

c.2.9

Step-by-step solution

Dissociation constant of acid

The dissociation constant of acid $\left(K_a\right)$quantifies the acid’s strength in a solution. The ${\mathbf{pK}}_{\mathbf{a}}$ value is less if the acid is strong and vice-versa.

Relation between the conjugate base and acidic strength

The acid’s strength is inversely correlated to the strength of its conjugate base. If the acid is weak then the conjugate base will be strong.

Estimation of pKa of bonds

a. The ${\mathrm{pK}}_{\mathrm{a}}$of the nitrogen-hydrogen bond in ${\mathrm{H}}_2\mathrm{N}-\mathrm{H}$is 38. Thus, the estimated ${\mathrm{pK}}_{\mathrm{a}}$ value of the nitrogen-hydrogen bond, that is, a primary amine,is also 38.

b. The ${\mathrm{pK}}_{\mathrm{a}}$ of the oxygen-hydrogen bond in ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{O}-\mathrm{H}$is 16. Thus, the estimated ${\mathrm{pK}}_{\mathrm{a}}$value of the oxygen-hydrogen bond is also 16.

c. The ${\mathrm{pK}}_{\mathrm{a}}$ of the oxygen-hydrogen bond in $\mathrm{HCOO}-\mathrm{H}$is 3.75. The addition of bromine tends to decrease the ${\mathrm{pK}}_{\mathrm{a}}$value to some extent. Thus, the estimated ${\mathrm{pK}}_{\mathrm{a}}$ value of the oxygen-hydrogen bond in bromoacetic acid is also 2.9.