Acidity and Basicity

We need to know the acidity characteristics of some representative organic/inorganic molecules. Please learn/memorize the following structures and their pKa’s…

There is an extensive discussion of acids and bases and trends in acidity with structure in the regular chem 231 tutorial section named
2_acids_bases_Ka_delta_G_trends

Acid-Base Problems

Write the Ka expression and reaction with water for the following acids:

1) H2S 2) NH4+

3) H3PO4 (only the first deprotonation) 4) NH3

Kb expressions are the base forms of Ka expressions, where a base deprotonates water to make hydroxide. Write the Kb expressions for the following bases:

5) NH3 6) F

For the pairs of molecules below, predict which is the stronger acid, and justify your choice with an explanation.

9) 10) H3PO4 H2PO4

11) 12) CH4 HC(CN)3

13) 14)

15) 16)

For questions 17 through 20, you’ll need to know the following information… write the reaction and the expression for the equilibrium constant and determine the equilibrium constant.

17) The reaction of (phenoxide) with H2S.

18) The reaction of fluoride (F) with (acetic acid).

19) The reaction of NH4+ (ammonium) with .

20) The reaction of HS with HF.

21) This question requires some thought. Shown below is imidazole:

Under acidic conditions, imidazole undergoes protonation to the conjugate acid. Determine which nitrogen receives the H+. Give the structures of the two conjugate acids and determine which is the most stable.

Answers

1) H2S + H2O HS + H3O+

2) NH4+ + H2O NH3 + H3O+

3) H3PO4 + H2O H2PO4 + H3O+

4) NH3 + H2O NH2 + H3O+

5) NH3 + H2O NH4+ + HO

6) F + H2O HF + HO

9) Now, we can’t use atom electronegativity since the charge is on carbon after deprotonation in both cases; also, there is no resonance in either case. Ethylene, the molecule on the right, will have a lone pair in an sp2 orbital; acetylene, the molecule on the right, will have a lone pair in an sp orbital. An sp orbital is smaller (50% s for sp versus 33% s for sp2) so the electrons are more stabilized since they are closer to the nucleus. So acetylene (pKa = ~25) will be more acidic than ethylene (pKa = ~44).

In general, negative charges are better stabilized in orbitals that have more s character (meaning a higher % of s in the hybrid orbital).

10) Intuitively, it will be easier to deprotonate a neutral molecule compared to an anion, if the structures are similar. Here they are since the anion on the right is the conjugate base of the molecule on the left, so the molecule on the left is deprotonated once to give the anion on the right. Indeed, H3PO4 has a pKa of 2.2 and H2PO4 has a pKa of 7.2.

11) In both cases, the negative charge is located on an O after deprotonation, and we can stabilize that negative charge via resonance. There are three possible resonance forms in the anion formed from deprotonation of HNO3, and two possible resonance forms in the anion formed from HONO. So HNO3 (pKa = -1.4) is a stronger acid than HONO (pKa = 3.3).

So the more we can delocalize a negative charge via resonance (i.e. more resonance stcuctures), the more stable the anion is.

12) The negative charge on methane is stuck after the deprotonation. :CH3 has a negative charge localized on the C with no possibility for resonance. Deprotonating HC(CN)3 gives
. There are two additional resonance structures not shown that put the negative charge on the other two N atoms. As a result, HC(CN)3 has a pKa of 1.0 while methane has a pKa of ~49.

13) The nitro group (NO2) has a plus charge on the N adjacent to the ring, so it’s logical to assume it will pull electron density from the ring after deprotonation, making the least acidic molecule the regular (non-nitro phenol). Between the other molecules, we need to deprotonate and see if we can use the NO2 group to stabilize the positive charge. This can