3. Acids and Bases

Which of the following compounds has the lowest $pKa$ value?

Which of the following molecules contains a hydrogen atom with a $pKa$ value between $20$ and $25$?

In the context of acid-base chemistry, what does the $p$ in $p{K}_a$ represent?

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

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

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.