3. Acids and Bases

Rank the following organic compounds in the order of increasing pKa.

Rank the following compounds in the order of increasing acidity.

Rank the following species in order of increasing basicity. Explain your reasons for ordering them as you do.

NH₃ CH₃O^– H₂O CH₃COO^– NaOH NH₂^– HSO₄^–

The following compounds can all react as bases.

c. Rank the original compounds in order, from strongest base to weakest base.

Compare the relative acidity of 1-molar aqueous solutions of the following acids.

Estimate the pKa values of the following compounds:

c. CH₃CH₂COOH

d. CH₃CH₂CH₂N⁺H₃

a. Which is a stronger acid: one with a pK_a of 5.2 or one with a pK_a of 5.8?

b. Which is a stronger acid: one with an acid dissociation constant of 3.4 × 10⁻³ or one with an acid dissociation constant of 2.1 × 10⁻⁴?

Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pK_a = −2.4, while the other has pK_a = −8.0.

a. Show the reaction of each compound with water.

b. Match each structure with its pK_a, and explain your choice.

Acetic acid can also react as a very weak base (pK_b = 20). Two different sites on acetic acid might become protonated to give the conjugate acid. Draw both of these possible conjugate acids, and explain (resonance) why the correct one is more stable. Calculate the pK_a of this conjugate acid.

Given the data in Problem 47:

b. What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?

Butyric acid, the compound responsible for the unpleasant odor and taste of sour milk, has a pK_a value of 4.82. What is its Ka value? Is it a stronger acid or a weaker acid than vitamin C?

For each indicated proton, suggest an approximate pK_a value from Table 4.5. Rationalize your choice.

(d)