3. Acids and Bases

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

c.

d.

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

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

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

(d)

(e)

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.

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

a.

Why is the pK_a value of protonated hydroxylamine (6.0) so much lower than the value of a protonated primary amine such as protonated methylamine (10.7)?

Rank the indicated hydrogen in the following compounds from most acidic to least acidic:

Which is a stronger acid?

c. CH₃OCH₂CH₂CH₂OHorCH₃CH₂OCH₂CH₂OH

d.

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

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

Rank the following compounds from easiest to hardest at removing a proton from its methyl substituent:

Explain why a base can remove a proton from the α-carbon of N,N-dimethylethanamide but not from the α-carbon of either N-methylethanamide or ethanamide.

Imidazole has two potentially basic sites (a and b). Of the two, where would you expect a proton to add preferentially?

Which is a stronger base?

a.

b.