3. Acids and Bases

Given the structure of ascorbic acid (vitamin C):

(c) Predict which proton in ascorbic acid is the most acidic.

(d) Draw the form of ascorbic acid that is present in the body (aqueous solution, pH = 7.4)

For each compound, indicate the atom that is most apt to be protonated.

a.

b.

c.

Any base whose conjugate acid has a pK_a greater than ______ can remove a proton from a terminal alkyne to form an acetylide ion (in a reaction that favors products).

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

a. Which is a stronger base: CH₃COO⁻ or HCOO⁻? (The pK_a of CH₃COOH is 4.8; the pK_a of HCOOH is 3.8.)

b. Which is a stronger base: HO⁻ or ^-NH₂? (The pK_a of H2O is 15.7; the pK_a of NH3 is 36.)

c. Which is a stronger base: H₂O or CH₃OH? (The pK_a of H3O+ is −1.7; the pK_a of CH₃O⁺H₂ is −2.5.)

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(b)

Estimate the pKa values of the following compounds:

c. CH₃CH₂COOH

d. CH₃CH₂CH₂N⁺H₃

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^–

Using the table of pK_a values given in [Appendix I], answer the following:

a. Which is the most acidic organic compound in the table?

b. Which is the least acidic organic compound in the table?

c. Which is the most acidic carboxylic acid in the table?

Arrange each group of compounds in order of increasing basicity.

c. sodium benzoate, sodium ethoxide, and sodium phenoxide

d. pyridine, sodium ethoxide, and sodium acetate

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(c)

Hydrogen peroxide (HOOH) has a pK_a of 11.6, making it roughly 10,000 times as strong an acid as water (pK_a = 15.7). Explain why H₂O₂ is a stronger acid than H₂O.

Rank the following from strongest base to weakest base:

b.

Which of the following bases can remove a proton from a terminal alkyne in a reaction that favors products?

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(e)