3. Acids and Bases

Explain why the pK_a of p-nitrophenol is 7.14, whereas the pK_a of m-nitrophenol is 8.39.

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a K_eq greater than, equal to, or less than 1?

(a)

In the compound ${\mathrm{CH}}_3{\mathrm{COCH}}_2\mathrm{COOH}$ (acetoacetic acid), which hydrogen is the most acidic?

Rank the following amines in order of their basicity (strongest base = 1 ; weakest base = 6).

In each pair, choose the most acidic compound. Justify your answer. The most acidic proton in each compound has been indicated.

(b)

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the base or the conjugate base?

(ii) Which side of the reaction is favored?

(iii) Would you expect a K_eq greater than or less than 1?

(c)

Which of the following indicated atoms would you expect to be most basic?

(a)

Write equations for the following acid–base reactions. Label the conjugate acids and bases, and show any inductive stabilization. Predict whether the equilibrium favors the reactants or products. Try to do this without using a table of pKa values, but if you need a hint, you can consult Appendix 4.

a. CH₃CH₂OH + CH₃NH⁻

b. F₃CCOONa + Br₃C—COOH

c. CH₃OH + H₂SO₄

Identify the stronger base in each pair. Explain your choice. Citing pK_a values is not an acceptable answer.

(c)

Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.

(c)

Explain why the $\alpha$-hydrogen of an N,N-disubstituted amide is less acidic (pK_a = 30) than the $\alpha$-hydrogen of an ester (pK_a = 25).

Rank the compounds in each of the following groups from strongest acid to weakest acid:

c.

Rationalize the difference in pKₐ values for the two hydroxyl groups.

The pK_a of ascorbic acid (vitamin C, page 55) is 4.17, showing that it is slightly more acidic than acetic acid (CH₃COOH, pK_a 4.74).

c. Compare the most stable conjugate base of ascorbic acid with the conjugate base of acetic acid, and suggest why these two compounds have similar acidities, even though ascorbic acid lacks the carboxylic acid (COOH) group.

Which is the most acidic compound in each pair?

(c)