3. Acids and Bases

For each of the following substituents, indicate whether it withdraws electrons inductively, donates electrons by hyperconjugation, withdraws electrons by resonance, or donates electrons by resonance.

a. Br

b. CH₂CH₃

c.

Which loses a proton more readily: a methyl group bonded to cyclohexane or a methyl group bonded to benzene?

A nitro group (–NO₂) effectively stabilizes a negative charge on an adjacent carbon atom through resonance:

Two of the following nitrophenols are much more acidic than phenol itself. The third compound is only slightly more acidic than phenol. Use resonance structures of the appropriate phenoxide ions to show why two of these anions should be unusually stable.

In each pair, choose the most basic compound. Justify your answer.

(d)

In each pair, choose the most acidic compound. Justify your answer. The most acidic proton in each compound has been indicated.

(b)

Which of the following indicated atoms would you expect to be most basic?

(c)

Rationalize the difference in pKₐ values for the two hydroxyl groups.

Rank the compounds in each of the following groups from strongest acid to weakest acid:

a.

Explain why the $\alpha$-hydrogen of an N,N-disubstituted amide is less acidic (pK_a = 30) than the $\alpha$-hydrogen of an ester (pK_a = 25).

Rank the compounds in each of the following groups from strongest acid to weakest acid:

c.

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.

a. Cl: ortho > meta > para

Why is protonated pyrimidine (pK_a = 1.0) more acidic than protonated pyridine (pKa = 5.2)?

Why is the conjugate acid of morpholine more acidic than the conjugate acid of piperidine?

Explain why pyrrole (pK_a ~ 17) is a much stronger acid than ammonia (pK_a = 36).

Predict the products (if any) of the following acid–base reactions.

(d) α-bromopropionic acid + sodium propionate

(e) benzoic acid + sodium phenoxide