3. Acids and Bases

Using pK_a values, calculate an approximate K_eq value for the following substitution reaction.

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Which reaction in Problem 23 has the smallest equilibrium constant?

Given that pH + pOH = 14 and that the concentration of water in a solution of water is 55.5 M, show that the pK_a of water is 15.7. (Hint: pOH=−log [HO−])

Calculate K_eq for the acid–base reaction shown. Which side is favored and why?

As long as the pH is not less than ___________, at least 50% of a protonated amine with a pK_a value of 10.4 will be in its neutral, nonprotonated form.

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

3. pH = 7

4. pH = 10

5. pH = 13

b. Determine the exact pKa values, using a calculator.

c. Which is the strongest acid?

1. nitrous acid (HNO₂), K_a = 4.0 × 10⁻⁴

2. nitric acid (HNO₃), K_a = 22

3. bicarbonate (HCO₃⁻), K_a = 6.3 × 10⁻¹¹

4. hydrogen cyanide (HCN), K_a = 7.9 × 10⁻¹⁰

5. formic acid (HCOOH), K_a = 2.0 × 10⁻⁴

6. phosphoric acid (H₃PO₄), K_a = 2.1

Calculate the equilibrium constant for each of the acid–base reactions shown.

b.

For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH = 5.5:

a. CH₃COOH(pK_a = 4.76)

b. CH₃CH₂N⁺H₃ (pK_a = 11.0)

Parts (a)–(d) of this assessment assist in the development of what will become a common theme in organic reactions and should be worked in order. [Think carefully about how each question relates to the others.]

(c) Without worrying about the mechanism of the reaction, estimate an equilibrium constant for the following carbonyl addition reaction based on the relative stability of the Lewis bases.

For the following acid–base reaction, (c) calculate the ratio of butan-2-ol to 2-butoxide.

For each of the following compounds, draw the form that predominates at pH = 3, pH = 6, pH = 10, and pH = 14:

a. CH₃COOH (pK_a = 4.8)

Given the K_a values, estimate the pK_a value of each of the following acids without using a calculator (that is, is it between 3 and 4, between 9 and 10, and so on?):

1. nitrous acid (HNO₂), K_a = 4.0 × 10⁻⁴

2. nitric acid (HNO₃), K_a = 22

3. bicarbonate (HCO₃⁻), K_a = 6.3 × 10⁻¹¹

4. hydrogen cyanide (HCN), K_a = 7.9 × 10⁻¹⁰

5. formic acid (HCOOH), K_a = 2.0 × 10⁻⁴

6. phosphoric acid (H₃PO₄), K_a = 2.1

Calculate the equilibrium constant for the reaction of ethylamine and propanoic acid. Which side is favored?