3. Acids and Bases

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

Rank the compounds in each set in order of increasing acid strength.

(c)

Rank the compounds in each set in order of increasing acid strength.

(b) CH₃CH₂CH₂CHBrCOOH, CH₃CH₂CHBrCH₂COOH, CH₃CHBrCH₂CH₂COOH

Rank the compounds in each set in order of increasing acid strength.

(a) CH₃CH₂COOH, CH₃CHBrCOOH, CH₃CBr₂COOH

Explain why the C-3 OH group of vitamin C is more acidic than the C-2 OH group.

Explain the difference in the pK_a values of the carboxyl groups of alanine, serine, and cysteine