3. Acids and Bases

Citrus fruits are rich in citric acid, a compound with three COOH groups. Explain the following:

a. The first pK_a (for the COOH group in the center of the molecule) is lower than the pK_a of acetic acid.

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]

(b)

Given the following compounds: ${\mathrm{CH}}_3{\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid), ${\mathrm{CH}}_3{\mathrm{CH}}_3{\mathrm{CH}}_3\mathrm{COH}$ (tert-butanol), and ${\mathrm{CH}}_3{\mathrm{CH}}_3\mathrm{NH}$ (dimethylamine), rank them from most acidic to least acidic.

Choose the more basic member of each pair of isomers, and show why the base you chose is more basic.

c.

d.

If reacted with a strong base, which of the labeled protons would you expect to be removed first?

Which of the following correctly ranks the compounds $\mathrm{CH}{3}_3\mathrm{COH}$, $\mathrm{CH}{3}_2\mathrm{COH}$, and $\mathrm{CH}{3}_1\mathrm{COH}$ in order of decreasing electrophilic strength (most electrophilic to least electrophilic)?

Ethanol, methylamine, and acetic acid are all amphoteric, reacting as either acids or bases depending on the conditions.

b. Rank ethanol, methylamine, and acetic acid in decreasing order of basicity. In each case, show the equation for the reaction with a generic acid (HA) to give the conjugate acid.

Choose the more basic member of each pair of isomers, and show why the base you chose is more basic.

(a)

(b)

Which of the following compounds contains the most acidic hydrogen atom?

The pK_a values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:

The relative pK_a values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic. Explain these relative acidities.

c. NH₂: meta > para > ortho

Without using pK_a values, pick out the more reactive (least stable) base in each pair. Explain your answer.

(a)

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

d. Which is more electronegative: an sp³ oxygen or an sp² oxygen?

e. Which compounds demonstrate that the relative electronegativities of a hybridized nitrogen atom are sp > sp² > sp³?

Which anion in each pair would you expect to react more quickly with H⁺?

(a)

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]

(h)

For each of the following compounds, indicate the atom that is protonated when an acid is added to a solution of the compound.

a.

b.