3. Acids and Bases

A medicinal chemist needed to deprotonate acetylene ( HC≡CH ) for use in a coupling reaction. Among the options given, which base(s) could be used for this process?

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.

Rank the following compounds in the order of increasing acidity.

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

Why is the $pKa$ of cyclopentane so much higher than the $pKa$ of cyclopentadiene?

Give the pK_a value of the following compounds.

b.

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?

Estimate the K_eq for the following reactions based on the stability of the anions involved.

(a)

Arrange the following compounds in order of decreasing acidity.

CH₃COOH, CH₃OH, CH₃CH₃, CH₃SO₃H, CH₃NH₂, CH₃SH, CH₃C≡CH

Given the structure of ascorbic acid (vitamin C):

(a) Is ascorbic acid a carboxylic acid?

(b) Compare the acid strength of ascorbic acid (pK_a = 4.71) with acetic acid.

What is the approximate $pKa$ value of acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$)?

You are planning to carry out a reaction that produces protons. The reaction will be buffered at pH = 10.5. Would it be better to use a protonated methylamine/methylamine buffer or a protonated ethylamine/ethylamine buffer? (pK_a of protonated methylamine = 10.7; pK_a of protonated ethylamine = 11.0)